54753 A_54753A A 54753A

User Manual: A_54753A

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User’s Guide

Publication number 54753-97015
Third edition, May 2000

For Safety information, Warranties, and Regulatory information, see pages
behind the index
©

Agilent 54753A and 54754A
TDR Plug-in Modules

Agilent 54753A and 54754A Plug-in Modules

The Agilent 54753A and 54754A TDR plug-in modules provide you with
TDR and TDT measurement features. In addition to the TDR and TDT
measurement features, the TDR plug-ins provide two accurate
oscilloscope measurement channels with user selectable bandwidths of
12.4 or 18 GHz. The lower bandwidth mode provides excellent
oscilloscope noise performance for accurate measurement of small
signals. The high bandwidth mode provides high-fidelity display and
measurement of very high-speed waveforms.
The Agilent 54753A TDR plug-in module provides:
• Automatic and manual single-ended TDR and TDT measurement
capability
• Automatic and manual waveform, histogram, FFT, waveform math,
eye pattern measurements, statistical measurements, and limit
testing capabilities.
• User selectable 12.4 or 18 GHz bandwidth (Channel 1).
• User selectable 12.4 or 20 GHz bandwidth (Channel 2).
• 2.5 GHz bandwidth trigger channel.
• 3.5 mm (m) connectors.
The Agilent 54754A TDR plug-in module provides:
• Automatic and manual single-ended and differential TDR and TDT
measurement capability.
• Automatic and manual waveform, histogram, FFT, waveform math,
eye pattern measurements, statistical measurements, and limit
testing capabilities.
• User selectable 12.4 or 18 GHz bandwidth.
• 2.5 GHz bandwidth trigger channel.
• 3.5 mm (m) connectors.

Accessories Supplied
The following accessories are supplied with the TDR plug-in modules:
One 50 ohm SMA (m) terminator, Agilent part number 1250-2153
Two SMA shorts (m), Agilent part number 0960-0055
TDR Demo Board, Agilent part number 54754-66503
One User’s Guide
One Programmer’s Guide
One Service Guide

Accessories Available
The following accessories are available for use with the TDR plug-in
modules.
Options
Option 0B1 Additional set of user documentation
Option 001 Agilent 83480A mainframe operating system upgrade
Option 002 Agilent 54750A mainframe operating system upgrade
Option 003 Delete demo board
Agilent 54755A TDR option for Agilent 83480A mainframe operating system
upgrade
Optional Accessories
Agilent 10086A ECL terminator
Agilent 54006A 6 GHz divider probe
Agilent 54007A accessory kit
Agilent 54008A 22 ns delay line
Agilent 54118A 500 MHz to 18 GHz trigger
Agilent 54701A 2.5 GHz Active Probe with Option 001
Connection Devices
SMA (f-f) adapter, Agilent part number 1250-1158
APC 3.5 (f-f) adapter, Agilent part number 1250-1749

iii

In This Book

This book is the operating manual for the Agilent 54753A and 54754A TDR plugin modules, and contains 13 chapters.
General Information Chapter 1 contains overview information, menu
and front panel key information, and trigger information. Chapter 2
contains important information on the care of the TDR plug-in connectors.
TDR Front Panel and Menu Keys Chapter 3, 4, 5 and 6 describe the
front panel keys and all the menu keys.
Task Oriented Examples Chapter 7 contains example single-ended
TDR measurements using a demo board included with each TDR plug-in
module. Chapter 8 contains example differential TDR measurements.
TDR Theory Chapters 9, 10, and 11 contain in-depth theory of TDR
transmission lines and how to use TDR in designing systems.
Specifications and Characteristics Chapter 12 contains the
specifications and characteristic for the TDR plug-in modules.
Problems and Error Messages Chapter 13 contains troubleshooting
information and error messages.

Contents

The Instrument at a Glance 1-1
Menu and Key Conventions 1-3
The Agilent 54753A, 54754A TDR Plug-In Modules 1-4
Plug-in Module Purpose 1-4
Front Panel of the Plug-in Module 1-4
Getting the Best Performance 1-5
Installing a Plug-in Module 1-6
Trigger 1-6

Care and Handling of Precision Connectors 2-1
3.5 mm Connector Care 2-3
Connector Wear 2-3
Operator Skill 2-3
Device Specifications 2-3
Accuracy Considerations 2-6
Visual Inspection 2-8
Mechanical Inspection 2-8
Connecting the Devices 2-16

Setup Channel Menu 3-1
Displaying the Setup Channel menu 3-4
Display 3-4
Scale 3-4
Offset 3-5
Bandwidth. . . 3-6
Alternate scale. . . 3-6
Calibrate . . . 3-8

Contents-1

Contents

Calibration Procedures 3-11
Performing a Plug-in Module Vertical Calibration 3-12
Calibrating Voltage Probes 3-12

Agilent 54753A TDR/TDT Setup Menu 4-1
Displaying the TDR/TDT Setup Menu 4-4
Stimulus 4-4
TDT 1 dest 4-4
TDR 1 dest 4-5
Normalize response . . . 4-5
TDR rate automatic . . . (250 kHz) 4-8
Preset TDR/TDT 4-8

Agilent 54754A TDR/TDT Setup Menu 5-1
Displaying the TDR/TDT Setup Menu 5-7
Stimulus 5-7
TDT 1 dest 5-8
Normalize response . . . 5-8
TDR rate automatic . . . (250 kHz) 5-11
Preset TDR/TDT 5-12
TDT 2 dest 5-13
Normalize response . . . 5-13
TDR rate automatic . . . (250 kHz) 5-16
Preset TDR/TDT 5-17
TDT 1 dest 5-18
TDT 2 dest 5-18
Normalize 1 response . . .
Normalize 2 response . . . 5-19

Contents-2

Contents

TDR rate automatic . . . (250 kHz) 5-21
Preset TDR/TDT 5-22
TDR/TDT 5-23
TDR response 1 5-23
TDR response 2 5-24
TDT response 1 5-25
TDT response 2 5-25
Establish ref plane 5-26
TDR rate automatic . . . (250 kHz) 5-27
Preset TDR/TDT 5-28
TDT 1 dest 5-30
TDR 1 dest 5-30
Normalize response . . . 5-31
TDR rate automatic . . . (250 kHz) 5-34
Preset TDR/TDT 5-34

Measure and Other TDR Specific Menus 6-1
TDR/TDT Measure Menu 6-4
Marker Menu 6-9
Reference 6-9
Marker units . . . 6-9
Response Menu Items 6-10

Single-ended TDR Measurements 7-1
Single-ended TDR Features 7-3
Establishing the Reference Plane and Normalizing 7-8
Measuring Transmission Line Impedance 7-19
Measuring Transmission Line Percent Reflection 7-25

Contents-3

Contents

Measuring Excess L/C 7-32
Measuring the Distance to a Discontinuity 7-37

Differential TDR Measurements 8-1
Differential TDR Features 8-3
Measuring Differential and Common Mode Impedance 8-5
Making Differential TDT Measurements 8-15

TDR Fundamentals 9-1
Propagation on a Transmission Line 9-4
Step Reflection Testing 9-6
Instrument Configuration 9-23

Improving Time Domain Network Measurements 10-1
Sources of Measurement Error 10-3
Removing Measurement Errors 10-6

Transmission Line Theory Applied to Digital Systems 11-1
Transmission Line Design 11-2
Signal Propagation Delay for Microstrip and Strip Lines with Distributed or Lumped
Loads 11-15
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements 11-21
References 11-38

Contents-4

Contents

Specifications and Characteristics 12-1
Specifications 12-3
Vertical Specifications 12-4
Environmental Specifications 12-5
Power Requirements 12-7
Weight 12-7
Characteristics 12-7
Trigger Input Characteristics 12-7
Product Regulations 12-8

In Case of Difficulty 13-1
If You Have Problems 13-3
If the Mainframe Does Not Operate 13-3
If the Plug-in Does Not Operate 13-4
Error Messages 13-5

Contents-5

Contents

Contents-6

The Instrument at a Glance

Operating the Instrument

What you’ll find in this chapter
This chapter describes:
• the key conventions used in this manual
• the front panel, rear panel and keys that do not display menus on the screen
Understanding the information in this chapter will help you successfully operate
the instrument.

CAUTION

1-2

The Instrument at a Glance
Menu and Key Conventions

Menu and Key Conventions
The keys labeled Trigger, Disk, and Run are all examples of front-panel keys.
Pressing some front-panel keys accesses menus of functions that are displayed
along the right side of the display screen. These menus are called softkey
menus.
Softkey menus list functions other than those accessed directly by the frontpanel keys. To activate a function on the softkey menu, press the unlabeled key
immediately next to the annotation on the screen. The unlabeled keys next to
the annotation on the display are called softkeys.
Additional functions are listed in blue type above and below some of the frontpanel keys. These functions are called shifted functions. To activate a shifted
function, press the blue front-panel Shift key and the front-panel key next to
the desired function.
Throughout this manual front-panel keys are indicated by bold lettering of the
key label, for example, Time base. Softkeys are indicated by italic lettering of
the key label, for example, Scale. The softkeys displayed depend on the frontpanel key pressed and which menu is selected. Shifted functions are indicated
by the front-panel Shift key followed by the shaded shifted function, for example
the Local function (above the Stop/Single front-panel key) will be shown as Shift,
Local.
A softkey with On and Off in its label can be used to turn the softkey’s function
on or off. To turn the function on, press the softkey so On is highlighted. To
turn the function off, press the softkey so Off is highlighted. An On or Off softkey
function will be indicated throughout this manual as: Test On.
A softkey such as Sweep Triggered Freerun offers you a choice of functions. In
this case you could choose Triggered by pressing the softkey until Triggered is
highlighted, or choose Freerun by pressing the softkey until Freerun is
highlighted. Softkey choices will be indicated throughout this manual as: Sweep
Triggered Freerun Triggered.
When some softkeys, such as Calibrate Probe, are pressed the first time, a
calibration will be made. Some softkeys, such as Offset require the entry of a
numeric value. To enter or change the value, use the general purpose knob
located below the front-panel Measure section.

1-3

The Instrument at a Glance
The Agilent 54753A, 54754A TDR Plug-In Modules

The Agilent 54753A, 54754A TDR Plug-In Modules
The TDR plug-in modules are two of several plug-in modules available for the
Agilent 83480A and Agilent 54750A mainframes.

Plug-in Module Purpose
The purpose of the plug-in module is to provide measurement channels,
including sampling, for the mainframe. The plug-in module scales the input
signal, sets the bandwidth of the system, and allows the offset to be adjusted so
the signal can be viewed. The output of the plug-in module is an analog signal
that is applied to the ADCs on the acquisition boards inside the mainframe. The
plug-in module also provides a trigger signal input to the time base/trigger board
inside the mainframe.

Front Panel of the Plug-in Module
The plug-in module takes up two, of the four, mainframe slots. The front panel
of the plug-in module has two channel inputs and an external trigger input. The
front panel also has two probe power connectors for Agilent 54700-series
probes, an auxiliary power connector for general purpose use, and a key for
each channel that displays the softkey menu. The softkey menu allows you to
access the channel setup features of the plug-in module for the selected input.

1-4

The Instrument at a Glance
Getting the Best Performance

Figure 1-1

Front panel of the plug-in module.

Getting the Best Performance
To ensure you obtain the specified accuracy, you must perform a plug-in module
vertical calibration. The calibration must also be performed when you move a
plug-in module from one slot to another or to a different mainframe. Refer to
“Performing a Plug-in Module Vertical Calibration” on page 3-12 for information
on performing a plug-in module vertical calibration.

1-5

The Instrument at a Glance
Installing a Plug-in Module

Installing a Plug-in Module
You do not need to turn off the mainframe to install or remove a plug-in module.
The plug-in module can be installed in slots 1 and 2 or 3 and 4 on the
Agilent 83480A, 54750A mainframe. The plug-in module will not function if it
is installed in slots 2 and 3.
To make sure the instrument meets all of the published specifications, there
must be a good ground connection from the plug-in module to the mainframe.
The RF connectors on the rear of the plug-in module are spring loaded, so fingertighten the knurled screw on the front panel of the plug-in module to make sure
the plug-in is securely seated in the mainframe.
CAUTION

Do not use extender cables to operate the plug-in module outside of the
mainframe. The plug-in module and/or mainframe can be damaged by
improper grounding when using extender cables.

Trigger
The external trigger level range for this plug-in module is ±1 V. The trigger
source selection follows the slots the plug-in module is installed in. For example,
if the plug-in module is installed in slots 1 and 2, then the trigger source is listed
as trigger 2. If it is installed in slots 3 and 4, then the trigger source is listed as
trigger 4.
CAUTION

CAUTION

The maximum safe input voltage is ±2 V + peak ac (+16 dBm). Therefore, to
avoid damaging the trigger input circuitry, do not apply any voltage outside
this range.
The input circuits can be damaged by electrostatic discharge (ESD).
Therefore, avoid applying static discharges to the front-panel input connectors.
Before connecting any coaxial cable to the connectors, momentarily short the
center and outer conductors of the cable together. Avoid touching the frontpanel input connectors without first touching the frame of the instrument. Be
sure the instrument is properly earth-grounded to prevent buildup of static
charge.

1-6

Care and Handling of Precision
Connectors

The Care and Handling of Precision
Connectors

What you’ll find in this chapter
This chapter describes:
•
•
•
•
•
•
•

3.5 mm connector care
connector wear
device specifications
accuracy considerations
visual inspection
mechanical inspection
connecting devices

Understanding the information in this chapter will help you successfully operate
the instrument.

CAUTION

2-2

Care and Handling of Precision Connectors
3.5 mm Connector Care

3.5 mm Connector Care
This chapter shows you how to take care of 3.5 mm connectors so that you can
maintain high levels of accuracy, repeatability, and system performance. Taking
appropriate care of your connectors will also extend their service life. Most of
the information can also be applied to 2.4 mm connectors. For additional
information on 2.4 mm connectors, refer to operating note "2.4 mm Adapters
and Calibration Accessories" Agilent part number 11900-90903.

Connector Wear
Connector wear will eventually degrade performance. The calibration devices,
which are typically used only a few times each day, should have a very long life.
However, because the connectors often undergo many connections a day, they
wear rapidly. Therefore, it is essential that all connectors on the Agilent 54753A
or 54754A TDR plug-in modules be inspected regularly, both visually (with a
magnifying glass) and mechanically (with a connector gage), and replaced as
necessary. Procedures for visual and mechanical inspection are given in the next
section of this manual. It is easier and cheaper to replace a worn adapter than
a worn channel connector.

Operator Skill
Operator skill in making good connections is essential. The mechanical
tolerances of the precision 3.5 mm connectors used in the Agilent 54007A kit
are two or three times better than the tolerances in regular 3.5 mm connectors.
Slight errors in operator technique that would go unnoticed with regular
connectors often appear with precision connectors in the calibration kit.
Incorrect operator technique can often result in lack of repeatability. Carefully
study and practice the connection procedures that are explained later in this
manual until your calibration measurements are consistently repeatable.

Device Specifications
Electrical specifications depend upon several mechanical conditions. A 3.5 mm
connector is a precision connector dedicated to very specific tolerances. SMA
connectors are not precision mechanical devices. They are not designed for
repeated connections and disconnections and are very susceptible to

2-3

Care and Handling of Precision Connectors
Device Specifications

mechanical wear. They are often found, upon assembly, to be out of
specification. This makes them potentially destructive to any precision 3.5 mm
connectors to which they might be mated.
Use extreme caution when mating SMA connectors with 3.5 mm precision
connectors. Prevent accidental damage due to worn or out-of-specification
SMA connectors. Such connectors can destroy a precision 3.5 mm connector,
even on the first connection.
Agilent Technologies recommends that you keep three points clearly in mind
when you mate SMA and precision 3.5 mm connectors: SMA inspection,
alignment, and mechanical mismatch.
SMA Inspection
Before mating an SMA connector (even a new one) with a precision 3.5 mm
connector, carefully inspect the SMA connector, both visually and mechanically
with a precision connector gauge designed to measure SMA connectors. A male
SMA connector pin that is too long can smash or break the delicate fingers on
the precision 3.5 mm female connector. Gauging SMA connectors is the most
important step you can take to prevent damaging your equipment.
Alignment
Be careful when aligning the connectors. Push the two connectors together with
the male contact pin precisely concentric with the female. Do not overtighten
or rotate either center conductor. Turn only the outer nut of the male connector
and use a torque wrench (5 lb.in., 60 N-cm) for the final connection. Note that
this torque is less than that when mating precision 3.5 mm connectors with each
other. A torque wrench suitable for SMA connectors preset to 5 lb.in. is available
(Agilent part number 8710-1582, CD 0).
The TDR plug-in modules come with adaptors already installed to prevent
damage to the channel connectors. Then, if accidental damage does occur, the
adapter is all that needs to be replaced. It is easier and cheaper to replace a
damaged adapter than a channel connector. SMA connectors can then be mated
with precision 3.5 mm connectors without difficulty or fear of expensive and
time-consuming repairs.
Mechanical Mismatch
Significant structural and dimensional differences exist between these two
types of connectors. Precision 3.5 mm connectors, also known as APC-3.5
connectors, are air-dielectric devices. Only air exists between the center and
outer conductors. The male or female center conductor is supported by a plastic
"bead" within the connector. In SMA connectors, a plastic dielectric supports

2-4

Care and Handling of Precision Connectors
Device Specifications

the entire length of the center conductor. In addition, the diameter of both the
center and outer conductors of an SMA connectors differ from that of a precision
3.5 mm connector.
If these precautions and recommendations are followed, SMA connectors can
be mated with 3.5 mm precision connectors without fear of expensive and time
consuming repairs.
Figure 2-1

SMA and a Precision 3.5 mm Connectors

When an SMA connector is mated with a precision 3.5 mm connector, the
connection exhibits a continuity mismatch (SWR), typically about 1.10 at
20 GHz. This mismatch is less than when precision 3.5 mm connectors are
mated. Keep this fact in mind when making measurements on SMA and
precision 3.5 mm coupled junctions.

2-5

Care and Handling of Precision Connectors
Accuracy Considerations

Figure 2-2

Typical SWR of SMA and Precision 3.5 mm Connectors

Accuracy Considerations
Accuracy requires that 3.5 mm precision connectors be used. However, SMA
connectors can be used if special care is taken when mating the two, and all
connectors are undamaged and clean. Before each use, the mechanical
dimensions of all connectors must be checked with a connector gauge to make
sure that the center conductors are positioned correctly. All connections must
be made for consistent and repeatable mechanical (and therefore electrical)
contact between the connector mating surfaces.
Carefully study and practice all procedures in this chapter until you can
successfully perform them repeatedly. Accuracy and repeatability are critical
for good high frequency measurements. Note that the device connection
procedures differ in several important ways from traditional procedures used
in the industry. Agilent Technologies procedures have been developed through
careful experimentation.

2-6

Care and Handling of Precision Connectors
Accuracy Considerations

Handling Precision 3.5 mm Connectors
• Precision 3.5 mm connectors must be handled carefully if accurate
calibrations and measurements are to be obtained.
• Store the devices in the foam-lined storage case when not in use.
• Avoid bumping or scratching any part of the mating surfaces.
• Be careful to align the center connectors.
• Check the alignment carefully before tightening the connector nuts.
• Use a torque wrench for all final connections in order to avoid
overtightening.
• Support the devices being used in order to avoid vertical or lateral force on
any connectors. This precaution is critical when using the airline, 6 cm "L",
or cables.
When Disconnecting Devices:
• Do not rock or bend any connections.
• Pull the connector straight out without unscrewing or twisting.
• Before storage, screw the connector nut all the way out to help protect the
surfaces, and use the plastic caps provided. These plastic caps can be taken
off easily by unscrewing, rather than pulling.
CAUTION

Do not use a damaged or defective connector. It will damage any good
connector to which it is attached. Throw the connector away or have it
repaired.
A connector is bad if it fails either the visual or mechanical examinations or
when an experienced operator cannot make repeatable connections. The time
and expense involved in replacing channel connectors warrants considerable
caution when any connector might be less than perfect.
If any doubts exist about a connector, call your Agilent Technologies
representative. Agilent Technologies field offices offer limited professional
advice and have access to the factory for information.

2-7

Care and Handling of Precision Connectors
Visual Inspection

Visual Inspection
Always begin a calibration with a careful visual inspection of the connectors,
including the test set connectors to make sure they are and undamaged.
CAUTION

Make sure that you and your equipment are grounded before touching any
center conductor so you won't cause static electricity and create a potential
for electrostatic discharge. When using or cleaning connectors, be aware that
you are touching exposed center connectors that are connected directly to the
internal circuits of the oscilloscope. Touching the center conductor, especially
with a wiping or brushing motion, can cause an electrostatic discharge (ESD)
and severely damage these sensitive circuits.
Use an illuminated, 4-power magnifying glass for visual inspection.

1
2
3

Before you begin, make sure you and any equipment you are using are
grounded to prevent electrostatic discharge.
Examine the connectors first for obvious problems, such as deformed
threads, contamination, or corrosion.
Next concentrate on the mating surfaces of each connector. Look for
scratches, rounded shoulders, misalignment, or any other signs of wear
or damage.
Make sure that the surfaces are clean, free of dust and solvent residues.
Dirt or damage visible with a 4-power magnifying glass can cause
degraded electrical performance and possible connector damage. All
connectors should be repaired or discarded immediately.

Mechanical Inspection
Mechanical inspection of the connectors is the next step. This inspection
consists of using the appropriate male or female precision 3.5 mm connector
gauge to check the mechanical dimensions of all connectors, including those on
the test set. The purpose of doing this is to make sure that perfect mating will
occur between the connector surfaces. Perfect mating assures a good electrical
match and is very important mechanically to avoid damaging the connectors
themselves, especially on the oscilloscope.

2-8

Care and Handling of Precision Connectors
Mechanical Inspection

Center Conductor
The critical dimension to be measured is the recession of the center conductor.
This dimension is shown as MP and FP in Figure 2-3 and Figure 2-4. No
protrusion of the center conductor's shoulder is allowable on any connector.
The maximum allowable recession of the center conductor shoulder is 0.003 in.
(0.08 mm) on all connectors, except those on the channel connectors.
On the channel connectors, not only is no protrusion allowable, the shoulder of
the center conductor must be recessed at least 0.0002 in. (0.005 mm). The
maximum allowable recession of the center conductor shoulder on the channel
connectors is 0.0021 in. (0.056 mm).

2-9

Care and Handling of Precision Connectors
Mechanical Inspection

Figure 2-3

D = inside diameter of the outer conductor
d = diameter of male/female center connector
A = outside diameter of outer conductor at the mating plane
r = corner relief for male connector
B = protrusion of the male contact pin tip beyond the outer conductor mating plane
C = recession of the outer conductor mating plane behind outer face of connector
MP = recession of male contact pin shoulder behind outer conductor mating plane
Male Connectors
inches
millimeters
D = 0.1378 ± 0.0005
3.500 ± 0.013
d = 0.0598 ± 0.0003
1.519 ± 0.008
A = 0.1803 + 0.000
4.580 + 0.00
- 0.002
- 0.05
r = 0.003
0.08
B = 0.085 +0.005
2.16 + 0.13
- 0.015
- 0.38
C = 0.120 ± 0.015
3.05 ± 0.38
Mp = 0.00 + 0.003
0.000 + 0.08
- 0.000
- 0.00
dm = 0.037 + 0.000
0.94 + 0.00
- 0.001
- 0.03
Mechanical Dimensions of Connector Faces

2-10

Female Connectors
inches
millimeters
D = 0.1378 ± 0.0005
3.500 ± 0.013
d = 0.0598 ± 0.0003
1.519 ± 0.008
A = 0.1807 + 0.002
4.590 + 0.05
- 0.000
- 0.00
r = 0.003
0.08
N/A
C = 0.176 ± 0.002
Fp = 0.000 + 0.003
- 0.00
N/A

1.93 ± 0.05
0.000 + 0.08
- 0.00

Care and Handling of Precision Connectors
Mechanical Inspection

Figure 2-4

Mechanical Dimensions of the Short Circuit

Outer Conductor
If any contact protrudes beyond the outer conductor mating plane, the contact
is out of tolerance and must be replaced. If the center conductor is not recessed
at least 0.0002 in. (0.005 mm), it is out of tolerance and must be replaced. In
both cases the out-of-tolerance connector will permanently damage any
connector attached to it. Destructive electrical interference will also result due
to buckling of the female contact fingers. This is often noticeable as a power
hole several dB deep occurring at about 22 GHz.
If any contact is recessed too far behind the outer conductor mating plane
(0.0021 in. 0.056 mm, except in test sets), poor electrical contact will result,
causing high electrical reflections. Careful gauging of all connectors will help
prevent this condition.
Before using the connector gauge to measure the connectors, visually inspect
the end of the gauge and the calibration block in the same way that you
inspected the connectors. Dirty or damaged gauge facings can cause dirty or
damaged connectors. Two connector gauges are available from Agilent
Technologies, one for each connector type, male and female. Refer to
Figure 2-5. A single gauge calibration block is used to zero both gauges; one end
protrudes for zeroing the male connector gauge. The part number for both
gauges, as well as the calibration block is 85052-80010.
Figure 2-5 to Figure 2-8 show how to use the connector gauges. Zero the gauge
with the calibration block. Refer to Figure 2-5. It is recommended that you zero
both gauges first, then measure each of the terminations and/or adapters that
will be used. Then, as the last step, measure the channel connectors.
Figure 2-7 and Figure 2-8 show how to measure precision 3.5 mm connectors.
Note that a plus (+) reading on the gauge indicates recession of the center
conductor and a minus (-) reading indicates protrusion. Since no protrusion of
either connector is allowable, readings for connectors within the allowable

2-11

Care and Handling of Precision Connectors
Mechanical Inspection

range will be on the plus (+) scale of the gauge. Also note that the allowable
tolerance range for the test set connectors is different from the range for other
connectors. Both ranges are shown in Figure 2-7 and Figure 2-8. Before
measuring test set connectors, be sure that the power to the test set is off and
that you and your equipment are grounded to prevent electrostatic discharge.
Figure 2-5

Precision 3.5 mm Connector Gauges

2-12

Care and Handling of Precision Connectors
Mechanical Inspection

Figure 2-6

Zeroing Precision 3.5 mm Connector Gauge

2-13

Care and Handling of Precision Connectors
Mechanical Inspection

Figure 2-7

Measuring Precision 3.5 mm Male Connectors

2-14

Care and Handling of Precision Connectors
Mechanical Inspection

Figure 2-8

Measuring Precision 3.5 mm Female Connectors

2-15

Care and Handling of Precision Connectors
Connecting the Devices

Connecting the Devices
Figure 2-9 and Figure 2-10 illustrate Agilent Technologies’ recommended
procedures for making connections with the calibration devices. Notice that
these recommended procedures differ from traditional procedures used in the
microwave industry, especially the counter-rotation technique and procedure
for connecting the airline.
The counter-rotation technique, recommended here, involves a slight rotation
of the termination or adapter just before the final tightening of the connector
nut. This eliminates the very small air wedge between the outer conductors that
frequently occurs when the body is held stationary during tightening, as it is in
the traditional procedure. The Agilent 54753A or Agilent 54754A plug-in
modules will detect the reflections caused by such small wedges.
The counter-rotation technique does not harm the connectors. The gold plating
on the outer conductor surface will become burnished in time. This is normal,
and as long as the surface remains smooth, the connector is still good. After
much use the gold plating may eventually wear through and expose the
beryllium-copper substratum. This too is normal, and if it is smooth the
connector is still good, although the beryllium-copper surface may oxidize if the
connector is used infrequently.
If the burnished surface is rough, scratched, rippled, or has other irregularities,
too much tightening force is being used. If the roughness is severe, the
connector is ruined and should not be used.
CAUTION

Damage can result if SMA connectors are overtightened to precision 3.5 mm
connectors. Use a torque wrench designed for SMA connectors, set to a 5 in lb
(60 N/cm). A torque wrench suitable for SMA connectors is available, Agilent
part number 8710-1582.

2-16

Care and Handling of Precision Connectors
Connecting the Devices

Counter-Rotation Technique
The recommended Agilent Technologies counter-rotation technique is for
precision 3.5 mm connectors. Before making any connections to the channel
connectors, ground yourself with a grounded wrist strap. Also, it is good practice
to grasp the outer shell of the test port before you make any connections to the
channel connectors in order to discharge any static electricity on your body.
This is the most effective single safeguard to prevent ESD damage to your
instruments.

If the device has a retractable connector nut, fully retract the nut before
mating the connectors. Carefully align the male and female contact pins
and slide the connectors straight together until the center and the two
outer conductors meet. Be careful not to twist or bend the contact pins.
You should feel a slight resistance as the connectors mate.
Make the preliminary connection by attaching the connector nut of the
male connector to the female. The male connector is held stationary as
the female connector is tightened and draws the male pin into the
female connector. Refer to Figure 2-9. Any other method used may cause
the male pin to damage the female connector. Support the body of the
device and turn the connector nut until the mating surfaces make light
contact. Do not overtighten. All you want is a connection of the outer
conductors with gentle contact at all points of both mating surfaces.

Figure 2-9

Connecting 3.5 mm Devices

2-17

Care and Handling of Precision Connectors
Connecting the Devices

When you are satisfied with this preliminary connection, use the
following counter-rotation technique to eliminate air wedges between
the mating planes. Refer to Figure 2-10. If the calibration device is male,
hold the connector nut firmly. Very slowly rotate the body of the device
about 10-20 ° counterclockwise. Note that this slight rotation or
backwiping is sufficient. Greater rotation does not improve electrical
performance and increases wear on the connector surfaces.

Figure 2-10

Counter-rotation Technique

If the calibration device is female (the connector nut is on the TDR plug-in
module), very slowly rotate both the connector nut and the body of the device
clockwise 10-20 ° (counterclockwise rotation will loosen the connection).
Light, smooth frictional resistance felt during the counter-rotation indicates you
have made the preliminary connection correctly and that the counter-rotation
technique has been successful. Roughness felt during counter-rotation
indicates either that the connectors are damaged or that there is roughness in
the connector nut/thread contact. Inspect both connectors again before
proceeding, to make sure that the roughness is due to roughness in the
connector nut interface rather than on the connector mating planes.

Tighten the connector nut finger tight, allowing the device to turn with
the nut if it tends to do so. A small rotation of the body of the device at
this point is acceptable and tends to occur naturally.
Use a torque wrench to make the final connection. Use of the torque
wrench assures the final connection will be tight enough for optimum
electrical performance, but not so tight as to distort or damage the
connectors.

2-18

Care and Handling of Precision Connectors
Connecting the Devices

To disconnect, follow this procedure:

1
2

Loosen the connector nut on the male connector with the torque
wrench. Leave the connection finger tight.
While supporting the calibration device, gently unfasten the connectors
and pull the calibration device straight out of the channel connector.
Do not twist either the center conductor or the outer conductor housing
or exert lateral or vertical (bending) force on the connection.
Some precision 3.5 mm female connector fingers are very tight and can pull the
center pin of their mates out past specifications as they are disconnected. If such
a male pin is inserted into a female connector, it can cause considerable damage
by pushing the female center conductor back too far. Be aware of this possibility
and check all connectors before mating them again.

2-19

2-20

Setup Channel Menu

What you’ll find in this chapter
This chapter describes the Setup Channel menu. A key tree and description of the
available functions are included.

CAUTION

The input circuits can be damaged by electrostatic discharge (ESD). Therefore,
avoid applying static discharges to the front-panel input connectors. Before
connecting any coaxial cable to the connectors, momentarily short the center
and outer conductors of the cable together. Avoid touching the front-panel
input connectors without first touching the frame of the instrument. Be sure
the instrument is properly earth-grounded to prevent buildup of static charge.
The top left keys of the plug-in module are the Channel keys. These keys give
you access to the Setup Channel menu for each input. The Setup Channel menu
is displayed on the right side of the screen when the Channel key is pressed.
There are several types of softkeys available. A description of the different
softkeys and their functions is provided in the Agilent 83480A, 54750A User’s
Quick Start Guide supplied with the mainframe.

3-2

Setup Channel Menu

Figure 3-1

Electrical Setup Channel menu.

3-3

Setup Channel Menu
Displaying the Setup Channel menu

Displaying the Setup Channel menu
To display the Setup Channel menu, press the Channel key.

Display
The Display softkey turns the channel display off and on. When the channel
display is on, a waveform is displayed for that channel, unless the offset is
adjusted so the waveform is clipped off of the display or the instrument is not
triggering.
The channel number, vertical scaling, and offset are displayed at the bottom left
of the waveform area. They remain on the display until the channel is turned
off, or an automatic measurement is performed. The automatic measurement
results share the same area of the display as the channel setups.
When the channel display is off, the waveform display for that channel is turned
off, pulse parameter measurements are stopped and acquisition on that channel
is stopped, unless it is needed as an operand for waveform math functions or
TDR/TDT responses.
Even though the channel display is off, you can still use the plug-in as a function
source in the Math menu or as a source for four normalize, differential, or
common mode responses. However, the instrument will not trigger unless one
or more of the other channel displays are turned on, or unless a math function
or TDR/TDT response is using one of the channels.
Key Path

Channel Display

Scale
The Scale softkey controls the vertical scaling of the waveform. If the fine mode
is off, then the knob and arrow keys change the vertical scaling in a 1-2-5
sequence. When fine mode is on, the knob and arrow keys change the vertical
scaling in 1 mV increments. You can also use the keypad to enter values in 1 mV
increments, independent of the fine mode selection.
The units the scale is displayed in depend on the unit of measure selected with
the Units softkey. The choices for units are volts, watts, amperes, ohms, %
reflect, gain, or unknown.

3-4

Setup Channel Menu
Offset

When the ohm, % reflect, or gain units are selected, the control changes to
Magnify scale. In this mode of operation, the TDR plug-in’s hardware scale
behaves as it would when the units mode is selected for all stimulus except for
differential or common mode.
If the TDR/TDT stimulus is set to differential or common mode, the control also
changes to Magnify scale. However, the hardware scale is set so that the
displayed waveform is never clipped. The Magnify scale control is a software
scaling control.
Key Path

Channel Scale

Offset
The Offset softkey moves the waveform vertically. It is similar to the position
control on analog oscilloscopes. The advantage of digital offset is that it is
calibrated. The offset voltage is the voltage at the center of the graticule area,
and the range of offset is 500 mV. You can use the knob, arrow keys, or keypad
to change the offset setting. The fine mode also works with offset.
When an Agilent 54700-series active probe is used with the plug-in module and
is connected to the probe power connector adjacent to the channel input, the
offset control adjusts the external scale factor and offset of the hybrid inside
the active probe. A probe connected to the auxiliary power connector adjacent
to the trigger input will function, but the channel scale factor will not be adjusted
automatically.
The units the offset is displayed in depend on the unit of measure selected with
the Units softkey. The choices for units are volts, watts, amperes, ohms, %
reflect, gain, or unknown.
When the ohm, % reflect or gain units are selected, the control changes to
Magnify offset. In this mode of operation, the TDR plug-in’s hardware offset
behaves as it would when the units mode is selected for all stimulus except for
differential or common mode.
If the TDR/TDT stimulus is set to differential or common mode, the control also
changes to Magnify offset, however, the harrower scale is set so that the
displayed waveform is never clipped. The Magnify offset control is a software
offset control.
Key Path

Channel Offset

3-5

Setup Channel Menu
Bandwidth. . .

Bandwidth. . .
You can use the Bandwidth function to select either 12.4 GHz or 18 GHz
bandwidth. For the Agilent 54753A TDR plug-in module, channel 2 can be either
12.4 GHz or 20 GHz bandwidth.
Key Path

Channel Bandwidth. . .

Alternate scale. . .
The Alternate Scale function allows you to change the units used to label the
vertical scale of the display. It also allows you to select the attenuation units
and the attenuation factor.
Key Path

Channel Alternate scale . . .
Atten units
The Atten Units function lets you select how you want the probe attenuation
factor represented. The choices are either decibel or ratio. The formula for
calculating decibels is:
V out
P out
- or 10 log --------20 log --------V in
Pin

The Atten Units function is not available when the units are set to ohm, % reflect,
or gain.
Attenuation
The Attenuation function lets you select an attenuation that matches the device
connected to the instrument. When the attenuation is set correctly, the
instrument maintains the current scale factors, if possible. All marker values
and voltage or wattage measurements will reflect the actual signal at the input
to the external device.

3-6

Setup Channel Menu
Alternate scale. . .

The attenuation range is from 0.0001:1 to 1,000,000:1. When you connect a
compatible active probe to the probe power connector, adjacent to the
corresponding channel input, the instrument automatically sets the
attenuation. For all other devices, set the probe attenuation with the knob,
arrow keys, or keypad.
Refer to “Calibrating Voltage Probes” on page 3-12 for information on calibrating
to the tip of the probe.
The Attenuation function is not available when ohm, % reflect, or gain units are
selected.
Key Path

Channel Alternate scale . . . Attenuation
Units
The Units function lets you select the unit of measure appended to the channel
scale, offset, trigger level, and vertical measurement values. For the plug-in
module, the units are volts, watts, amperes, ohms, % reflect, gain, or unknown.
Use volt for voltage probes, ampere for current probes, watt for optical-toelectrical (O/E) converters, and unknown when there is no unit of measure or
when the unit of measure is not one of the available choices. The gain selection
is only available when the channel has been chosen as a TDT destination.
The two additional choices, ohms and % reflect are selectable once a TDR/TDT
normalization and reference plane have been established (see the 54753A Setup
Menu or 54754A Setup Menu chapter under Establish normalization & ref plane
for more information). Use Ohms when TDR/TDT vertical scale units of ohms/
div are required for making measurements. Use % reflect when TDR/TDT
percentage of reflection units are required.

Key Path

Channel Alternate scale . . . Units
Ext gain and Ext offset
When you select ampere, watt, or unknown, two additional functions become
available: External Gain and External Offset. These two additional functions
allow you to compensate for the actual characteristics of the probe rather than
its ideal characteristics. For example, you might have an amplified lightwave
converter with ideal characteristics of 300 V/W with 0 V offset. But, its actual
characteristics are 324 V/W with 1 mV of output offset. Therefore, set the
External Gain to 324 V/W and the External Offset to 1 mV.

3-7

Setup Channel Menu
Calibrate . . .

Key Path

Channel External scale . . . Units Volt Ext gain or Ext Offset
Channel External scale . . . Units Watt Ext gain or Ext Offset
Channel External scale . . . Units Unknown Ext gain or Ext Offset

Calibrate . . .
The calibrate menu allows you to null out any skew between probes or cables
and to check the present calibration status of the instrument.
Key Path

Channel Calibrate . . .
Skew
The Skew function changes the horizontal position of a waveform on the display.
The Skew function has a range of approximately 100 µs. You can use skew to
compensate for differences in cable or probe lengths. It also allows you to place
the triggered edge at the center of the display when you are using a power
splitter connected between the channel and trigger inputs. Another use for
skew is when you are comparing two waveforms that have a timing difference
between them. If you are more interested in comparing the shapes of two
waveforms rather than the actual timing difference between them, you can use
Skew to overlay one waveform on top of the other waveform.
To adjust the skew on two channels
1. Turn both channels on and overlay the signals vertically.
2. Expand the time base, so the rising edges are about a 45 degree angle.
3. Adjust the skew on one of channels, so that the rising edges overlap at the
50 percent points.

Key Path

Channel Calibrate . . . Skew

3-8

Setup Channel Menu
Calibrate . . .

TDR Skew
The TDR Skew function changes the position of the TDR step. The TDR Skew
function has a range of ≅ ±400 pS. The units of the function are shown in % of
the maximum range, or ±100%. The Skew function and the TDR Skew function
differ in that Skew moves the acquired waveform with respect to the trigger,
while the TDR Skew function moves the TDR step with respect to the trigger.
TDR Skew can be used to align the TDR steps of the two TDR channels for more
accurate differential TDR measurements when cable or probe lengths are
different.
To deskew the two TDR channels
1. Turn on both channels and overlay the signals vertically with no cables attached.
2. Expand the time base so the rising edges are about a 45 degree angle.
3. Adjust the TDR skew on one of the channels, so that the rising edges overlap at
the 50 percent points.
4. Attach the cables. If the cables differ in length, then the waveforms will not
overlay each other.
5. Using the ∆Time auto measurement or manual markers, measure the ∆time (the
skew) between the TDR channels.
6. Adjust the channel Skew for the channel whose waveform is to the right of the
other channels waveform until the skew is ½ of the measured ∆time.
7. Adjust the TDR Skew for the right most waveform until the remaining skew is ≅ 0.
Key Path

Channel Calibrate . . . TDR Skew

3-9

Setup Channel Menu
Calibrate . . .

Cal status
The Cal Status function displays a screen similar to Figure 3-8.
Key Path

Channel Calibrate Cal Status

Figure 3-8

A typical Cal Status display.

Current Date This is the current date and time. You can compare this to
the last plug-in module calibration time. That way you will know how long
it has been since the last plug-in module calibration was performed.
Current Frame ∆Temp This is the temperature change on the inside of
the instrument since the last mainframe calibration was performed. A
positive number indicates how many degrees warmer the mainframe is
currently as compared to the temperature of the mainframe at the last
mainframe calibration.

3-10

Setup Channel Menu
Calibration Procedures

Channel 1 Calibration Status The instrument displays Calibrated
when the plug-in module has been calibrated in the current mainframe slot
otherwise the instrument displays Uncalibrated. Once a plug-in is
calibrated, the temperature difference (∆Temp) between when the plug-in
was calibrate and the current temperature is displayed. The plug-in module
will met dc accuracy specifications as long as the ∆Temp is within the range
of -5 °C to +5 °C. Also displayed, is a list of the plug-in module’s model
number, serial number, date of last calibration, and time of last calibration.
Calibrate probe
Connect a voltage probe to the plug-in and then press:
Calibrate probe
Continue
The instrument calibrates to the tip of the probe by setting the probe
attenuation to the actual attenuation ratio of the probe. The instrument also
automatically compensates for any offset that the probe may introduce.
Key Path

Channel Calibrate Calibrate probe Continue

Calibration Procedures
What you’ll find in this section
This sections contains procedures for performing:
• a plug-in module vertical calibration
• calibration of voltage probes
• calibration of other devices

3-11

Setup Channel Menu
Performing a Plug-in Module Vertical Calibration

Performing a Plug-in Module Vertical Calibration
1

To perform a plug-in module vertical calibration, press:
Utility
Calibrate . . .
Calibrate plug-in. . .

Select the plug-in module to be calibrated by pressing:
1 and 2 or 3 and 4

Start the calibration procedure by pressing:
Start cal

Follow the on screen instructions.

Calibrating Voltage Probes
Because the mainframe’s CAL signal is a voltage source, you can let the
instrument compensate for the actual characteristics of your probe by letting
the instrument calibrate to the tip of the probe.
Performing the Calibration
To calibrate a voltage probe to the probe tip, set the instrument as follows:
Atten units ratio
Units Voltage
Calibrate Probe
The instrument automatically calibrates to the tip of the probe, sets the probe
attenuation and compensates for any probe offset.
Calibrating Other Devices
Because the mainframe’s CAL signal is a voltage source, it cannot be used to
calibrate to the probe tip when the units are set to ampere, watt, or unknown.
Instead, set the external gain and external offset to compensate for the actual
characteristics of the probe or device. If you do not know the actual
characteristics, you can refer to the typical specifications that came with the
probe or device.

3-12

Setup Channel Menu
Calibrating Voltage Probes

Performing the Calibration
To compensate for the actual characteristics of the probe or device, set the
instrument as follows:
Atten units ratio
Attenuation 1:1
Units Ampere (Watt or unknown)
Ext gain actual gain characteristics of the probe or device.
Ext offset offset introduced by the probe or device.

3-13

3-14

Agilent 54753A TDR/TDT Setup Menu

What you’ll find in this chapter
This chapter describes the TDR/TDT Setup menu. A key tree and description of the
available functions is included.

CAUTION

The input circuits can be damaged by electrostatic discharge (ESD).
Therefore, avoid applying static discharges to the front-panel input connectors.
Before connecting any coaxial cable to the connectors, momentarily short the
center and outer conductors of the cable together. Avoid touching the frontpanel input connectors without first touching the frame of the instrument. Be
sure the instrument is properly earth-grounded to prevent buildup of static
charge.
The top right key of the plug-in module is the TDR/TDT Setup key. This key gives
you access to the TDR/TDT Setup menu. The TDR/TDT Setup menu is displayed
on the right side of the screen when the TDR/TDT Setup key is pressed. There
are several types of softkeys available.

4-2

Agilent 54753A TDR/TDT Setup Menu

Figure 4-1
TDR/TDT Setup

Stimulus

Off
On
External
Cancel
Enter

TDR/TDT help . . .

Single ended TDR/TDT
Differential common mode TDR/TDT
Done

TDT 1 dest

channel 2
none
Cancel
Enter

Normalize response . . .

Risetime
TDR/TDT

TDR
TDT

TDR normalize

off
on

Establish normalization & ref plane
Done

TDR rate automatic . . . (250 kHz)

automatic
manual

Preset TDR/TDT

TDR rate

Done

TDR/TDT Setup Menus

4-3

Agilent 54753A TDR/TDT Setup Menu
Displaying the TDR/TDT Setup Menu

Displaying the TDR/TDT Setup Menu
To display the TDR/TDT Setup menu, press the TDR/TDT Setup key on the TDR
plug-in module.

Stimulus
Pressing the Stimulus softkey produces a pull-down menu used to turn on or
turn off the TDR step. The Agilent 54753A is a single-ended TDR plug-in and
has one TDR stimulus channel. The following table contains a list of the available
stimulus menu choices and their descriptions.
Table 4-1
Stimulus Menu Choices

Key Path

Stimulus

Description

off

Turns the TDR step off and disables the TDR measurement system.

Turns the TDR step on and enables the TDR measurement system.

external

This setup provides control for and requires an external step
generator before measurements can be made.

TDR/TDT Setup Stimulus

TDT 1 dest
The TDT 1 dest softkey only appears when the Stimulus is set to on or external.
Pressing the TDT 1 dest softkey produces a pull-down menu used to select the
channel used as the destination channel for TDT measurements. The choices
available for this pull-down menu depend on the other TDR or electrical plugin, if any, in the mainframe.
Any electrical channel is a valid TDT destination channel. If external stimulus
is selected, the TDT destination may not be set to the currently defined TDR
destination channel.

4-4

Agilent 54753A TDR/TDT Setup Menu
TDR 1 dest

If no other valid TDT destination channels are available, then "none" is the only
choice. If a TDT destination other than none is selected, then the Preset
TDR/TDT control will turn on and preset the TDT destination channel.
Key Path

TDR/TDT Setup TDT 1 dest

TDR 1 dest
The TDR 1 dest softkey only appears when the stimulus is set to external.
Pressing the TDR 1 dest softkey produces a pull-down menu used to select the
channel used as the destination channel for TDR measurements. Any electrical
channel is a valid TDR destination channel. The TDR destination may not be
set to the currently defined TDT destination.
Key Path

TDR/TDT Setup TDR 1 dest

Normalize response . . .
The Normalize response function allows you to change the risetime of the
normalized step, to select TDR and TDT normalization, to turn on or off the
display of the normalized TDR or TDT trace, to change the scaling of the
normalized trace, and to establish the normalization filter values and reference
plane.
Risetime
The Risetime function allows you to change the normalized step’s risetime from
a minimum of
10 ps
or
time per division (s/div) × 10 divisions
min = 8 points × --------------------------------------------------------------------------------------------record length

whichever is greater, to a maximum of
max = 5 × time per division (s/div)

4-5

Agilent 54753A TDR/TDT Setup Menu
Normalize response . . .

While the TDR step’s risetime applied to the system under test is fixed, the
measured response has a set of mathematical operations applied to it. These
mathematical operations effectively change the displayed response to the
system just as if a different TDR step risetime had actually been applied. This
allows you to select a risetime for TDR/TDT measurements that is close to the
actual risetime used in your system. This risetime value applies to both TDR
and TDT normalized channels. For more information on normalization, see the
chapter titled Improving Time Domain Network Measurements.
Key Path

TDR/TDT Setup Normalize response . . . Risetime
TDR/TDT
The TDR/TDT function is used to select between TDR and TDT for normalization.
Both TDR and TDT channels can be normalized. This control selects which
normalized trace is referred to for the following controls. Before TDT
normalization can be done, you must select a TDT destination (TDT 1 dest).

Key Path

TDR/TDT Setup Normalize response . . . TDR/TDT
TDR or TDT normalize
The TDR normalize function is available if the TDR mode is selected by using the
previous control; otherwise, the TDT normalize function is available. In either
case, this function turns on or off the display of the normalized trace. The TDR
and TDT normalization functions can be on at the same time.

Key Path

TDR/TDT Setup Normalize response . . . TDR normalize
Normalize scaling . . .
The Normalize scaling function is used when vertical and horizontal scaling of
the normalized response is required that is independent from that of the source
channel. This function only appears when TDR or TDT normalize is set to on.

Key Path

TDR/TDT Setup Normalize response . . . Normalize scaling . . .
Vertical There are two choices for vertical mode: track source and manual.
The track source mode sets the control of the vertical scaling for the
normalized trace to that of the source channel. When manual mode is
selected, the Y Scale and Y Offset menus appear allowing you to
independently change the vertical scale and offset of the normalized trace.

Key Path

TDR/TDT Setup Normalize response . . .Normalize scaling . . . Vertical

4-6

Agilent 54753A TDR/TDT Setup Menu
Normalize response . . .

Horizontal There are two choices for horizontal mode: track source and
manual. The track source mode sets the control of the horizontal scaling
for the normalized trace to that of the source channel. When manual mode
is selected, the Y Scale and Y Position menus appear allowing you to
independently change the horizontal scale and position for the normalized
trace.
Key Path

TDR/TDT Setup Normalize response . . .Normalize scaling . . . Horizontal
Establish normalization & ref plane
The Establish normalization & ref plane function establishes the filter values used
to normalize a channel and to set the reference plane for TDR and TDT
measurements. This function must be performed separately for TDR and TDT
modes. The normalization and reference plane must be re-established when
power is lost or when the instrument is turned off. However, the values used for
normalization and reference plane calculations can be stored to and re-loaded
from disk. (See the Disk Menu chapter in the Agilent 83480A, Agilent 54750
User’s Guide for more information.)
The normalization filter values and reference plane become invalidated when the
timebase scale, record length or channel bandwidth is changed. Also, the
normalization process will not be able to remove small synchronous noise in the
channels baseline if the timebase position is changed. For most measurements
this error is very small.
The reference plane must be established before ohm, % reflect or gain units are
selectable for the channel. Also, the Reference ref plane function is not
available in the Marker menu.
The function steps you through the normalization and reference plane
procedure for the selected measurement type. The procedure steps are
displayed at the top of the screen. The items required for calibration are shown
in Table 4-2.

4-7

Agilent 54753A TDR/TDT Setup Menu
TDR rate automatic . . . (250 kHz)

Table 4-2
Establish normalization & ref plane Hardware Requirements
Measurement

Requirements

TDR

50 ohm 3.5 mm SMA terminator
3.5 mm SMA short

TDT

1 or 2 each 3.5 mm SMA cables
3.5 mm barrel connector

Key Path

TDR/TDT Setup Normalize response . . . Establish normalization & ref plane . . .

TDR rate automatic . . . (250 kHz)
The TDR rate automatic . . . (250 kHz) function allows you to manually or
automatically select the repetition rate of the TDR step. The range of values
for manual mode selection are from 50 Hz to 250 kHz repetition rate using a
1-2-5 sequence. When this function is set to automatic, the TDR step repetition
rate varies automatically as the Time base Scale is changed to keep multiple
steps off screen. As the TDR rate decreases, TDR measurements can be made
on longer transmission lines.
Key Path

TDR/TDT Setup TDR rate automatic

Key Path

TDR/TDT Setup TDR rate manual

Preset TDR/TDT
The Preset TDR/TDT function prepares the oscilloscope for making TDR/TDT
measurements by automatically setting several menu fields. The TDR preset
feature appears in the TDR/TDT Setup menu once a stimulus has been selected.
The menus that are affect by this feature are shown in Table 4-3.

4-8

Agilent 54753A TDR/TDT Setup Menu
Preset TDR/TDT

Table 4-3
Preset TDR/TDT Configuration
Menu

Menu Item

Set To

Acquisition

Averaging

Best

Flatness

Time Base

Scale

500.0 ps/div

Position

Set to a value which places the
incident edge on screen.

Display

Scale

100 mV/div

Offset

200.0 mV (for Stimulus on)
0.0 mV (form Stimulus external)

Bandwidth

12.4 GHz (for Stimulus on)
18.0 GHz (for Stimulus external)

Attenuation units

ratio

Attenuation

1.000 : 1

Channel 1 Setup

Key Path

TDR/TDT Setup Preset TDR/TDT

4-9

4-10

Agilent 54754A TDR/TDT Setup Menu

What you’ll find in this chapter
This chapter describes the Agilent 54754A TDR/TDT Setup menu. A key tree and
description of the available functions is included.

CAUTION

The input circuits can be damaged by electrostatic discharge (ESD).
Therefore, avoid applying static discharges to the front-panel input connectors.
Before connecting any coaxial cable to the connectors, momentarily short the
center and outer conductors of the cable together. Avoid touching the frontpanel input connectors without first touching the frame of the instrument. Be
sure the instrument is properly earth-grounded to prevent buildup of static
charge.
The top right key of the plug-in module is the TDR/TDT Setup key. This key gives
you access to the TDR/TDT Setup menu. The TDR/TDT Setup menu is
displayed on the right side of the screen when the TDR/TDT Setup key is pressed.
There are several types of softkeys available.

5-2

Agilent 54754A TDR/TDT Setup Menu

Figure 5-1
TDR/TDT Setup

Stimulus

Off
1 only
2 only
1 and 2
differential
common mode
external
Cancel
Enter

TDR/TDT help . . .

Single ended TDR/TDT
Differential common mode TDR/TDT
Done

TDT 1 dest

channel 2
none
Cancel
Enter

TDT 2 dest

channel 1
none
Cancel
Enter

TDR 1 dest

channel 2
none
Cancel
Enter

5-3

Agilent 54754A TDR/TDT Setup Menu

Normalize response . . .

Risetime
TDR/TDT

TDR
TDT

TDR normalize

off
on

Establish normalization & ref plane
Done

Normalize 1 response . . .

Risetime
TDR/TDT

TDR
TDT

TDR normalize

off
on

Establish normalization & ref plane
Done

Normalize 2 response . . .

Risetime
TDR/TDT

TDR
TDT

TDR normalize

off
on

Establish normalization & ref plane
Done

5-4

Agilent 54754A TDR/TDT Setup Menu

TDR response 1 . . .

Response

off
differential
common mode
Cancel
Enter

Response scaling . . .

Vertical

track source
manual

Y Scale
Y Offset

Horizontal

track source
manual

X Scale
X Position

Done

TDR response 2 . . .

Response

off
differential
common mode
Cancel
Enter

Response scaling . . .

Vertical

track source
manual

Establish ref plane

Y Scale
Y Offset

Horizontal

track source
manual

X Scale
X Position

Done

5-5

Agilent 54754A TDR/TDT Setup Menu

TDR rate automatic . . . (250 kHz)

automatic
manual

Preset TDR/TDT

TDR/TDT Setup Menu.

5-6

Done

TDR rate

Agilent 54754A TDR/TDT Setup Menu
Displaying the TDR/TDT Setup Menu

Displaying the TDR/TDT Setup Menu
To display the TDR/TDT Setup menu, press the TDR/TDT Setup key on the TDR
plug-in module.

Stimulus
Pressing the Stimulus softkey produces a pull-down menu used to turn on or
turn off the TDR step. The differential TDR plug-in has two TDR channels. The
following table contains a list of the available stimulus menu choices and their
descriptions.
Table 5-1
Stimulus Menu Choices

Key Path

Stimulus

Description

off

Turns the TDR steps off for both TDR channels and the TDR
measurement system off.

1 only

Turns the TDR step on for channel 1 only and enables the TDR
measurement system.

2 only

Turns the TDR step on for channel 2 only and enables the TDR
measurement system.

1 and 2

Turns the TDR steps on for both channels. This mode is used to
make two independent single-ended TDR measurements.

differential

Turns the differential TDR steps on for both channels. The step for
channel 1 is a positive going step while the step for channel 2 is
effectively a negative going step. This mode is used to make
differential TDR measurements

common mode

Turns the common mode TDR steps on for both channels. Both
steps are positive going steps. Unlike the 1 and 2 mode, this mode
is used to make common mode measurements.

external

This setup provides control for and requires an external step
generator before measurements can be made.

TDR/TDT Setup Stimulus

5-7

Agilent 54754A TDR/TDT Setup Menu
TDT 1 dest

1 Only Stimulus Menus
This section describes the menus that are available when the stimulus is set to
1 only.

TDT 1 dest
Pressing the TDT 1 dest softkey produces a pull-down menu used to select the
channel used as the destination channel for TDT measurements. The choices
available for this pull-down menu depend on the other TDR or electrical plugin, if any, in the mainframe.
Any electrical channel is potentially a valid TDT destination channel. The TDT
destination may not be assigned to a channel already assigned as a TDT
destination.
If no other valid TDT destination channels are available, then "none" is the only
choice. If a TDT destination other than none is selected, then the Preset
TDR/TDT control will turn on and preset the TDT destination channel.
Key Path

TDR/TDT Setup TDT 1 dest

Normalize response . . .
The Normalize response function is available for all the Stimulus types except
differential and common mode. The Normalize response function allows you to
change the risetime of the normalized step, to select TDR and TDT
normalization, to turn on or off the display of the normalized TDR or TDT trace,
to change the scaling of the normalized trace, and to establish the normalization
filter values and reference plane.
Risetime
The Risetime function allows you to change the normalized step’s risetime from
a minimum of
10 ps
or
time per division (s/div) × 10 divisions
min = 8 points × --------------------------------------------------------------------------------------------record length

5-8

Agilent 54754A TDR/TDT Setup Menu
Normalize response . . .

which ever is greater, to a maximum of
max = 5 × time per division (s/div)

While the TDR step’s risetime applied to the system under test is fixed, the
measured response has a set of mathematical operations applied to it. These
mathematical operations effectively change the displayed response to the
system just as if a different TDR step risetime had actually been applied. This
allows you to select a risetime for TDR/TDT measurements that is close to the
actual risetime used in your system. This risetime value applies to both TDR
and TDT normalized channels. For more information on normalization, see
Chapter 10, ”Improving Time Domain Network Measurements”.
Key Path

Key Path

TDR/TDT Setup Normalize response . . . TDR/TDT
TDR or TDT normalize
The TDR normalize function is available if the TDR mode is selected by using
the previous menu otherwise the TDT normalize function is available. In either
case, this function turns on or off the display of the normalized trace. The TDR
and TDT normalization functions can be on at the same time.

Key Path

TDR/TDT Setup Normalize response . . . Normalize scaling . . .

5-9

Agilent 54754A TDR/TDT Setup Menu
Normalize response . . .

Vertical There are two choices for vertical mode: track source and manual.
The track source mode sets the control of the vertical scaling for the
normalized trace to that of the source channel. When manual mode is
selected, the Y Scale and Y Offset menus appear allowing you to
independently change the vertical scale and offset of the normalized trace.
Key Path

TDR/TDT Setup Normalize response . . .Normalize scaling . . . Vertical
Horizontal There are two choices for horizontal mode: track source and
manual. The track source mode sets the control of the horizontal scaling
for the normalized trace to that of the source channel. When manual mode
is selected, the Y Scale and Y Position menus appear allowing you to
independently change the horizontal scale and position for the normalized
trace.

Key Path

TDR/TDT Setup Normalize response . . .Normalize scaling . . . Horizontal
Establish normalization & ref plane
The Establish normalization & ref plane function establishes the filter values used
to normalize a channel and to set the reference plane for TDR and TDT
measurements. This function must be performed separately for TDR and TDT
modes. The normalization and reference plane must be re-established when
power is lost or when the instrument is turned off. However, the values used for
normalization and reference plane calculations can be stored to and re-loaded
from disk. (See the Disk Menu chapter in the Agilent 83480A, Agilent 54750
User’s Guide for more information.)
The normalization filter values and reference plane are invalidated when the
timebase scale, record length or channel bandwidth is changed. Also, the
normalization process will not be able to remove small synchronous noise in the
channels baseline if the timebase position is changed. For most measurements
this error is very small.
The reference plane must be established before ohm, % reflect or gain units are
selectable for the channel. Also, the Reference ref plane function is not
available in the Marker menu.
The function steps you through the normalization and reference plane
procedure for the selected measurement type. The procedure steps are
displayed at the top of the screen. The items required for calibration are shown
in Table 5-2 on page 5-11.

5-10

Agilent 54754A TDR/TDT Setup Menu
TDR rate automatic . . . (250 kHz)

Table 5-2
Establish normalization & ref plane Hardware Requirements
Measurement

Requirements

TDR

50 ohm 3.5 mm SMA terminator
3.5 mm SMA short

TDT

1 or 2 each 3.5 mm SMA cables
3.5 mm barrel connector

Key Path

TDR/TDT Setup Normalize response . . . Establish normalization & ref plane . . .

TDR/TDT Setup TDR rate automatic

Key Path

TDR/TDT Setup TDR rate manual

5-11

Agilent 54754A TDR/TDT Setup Menu
Preset TDR/TDT

Menu Item

Set To

Acquisition

Averaging

Best

Flatness

Time Base

Scale

500.0 ps/div

Position

Set to a value which places the
incident edge on screen.

Display

Scale

100 mV/div

Offset

200.0 mV

Bandwidth

12.4 GHz

Attenuation units

ratio

Attenuation

1.000 : 1

Channel 1 Setup

Key Path

TDR/TDT Setup Preset TDR/TDT

5-12

2 Only Stimulus Menus
This section describes the menus that are available when the stimulus is set to
2 only.

TDT 2 dest
Pressing the TDT 2 dest softkey produces a pull-down menu used to select the
channel used as the destination channel for TDT measurements. The choices
available for this pull-down menu depend on the other TDR or electrical plugin, if any, in the mainframe.
Any electrical channel is potentially a valid TDT destination channel. The TDT
destination may not be assigned to a channel already assigned as a TDT
destination.
If no other valid TDT destination channels are available then "none" is the only
choice. If a TDT destination other than none is selected then the preset TDR/
TDT control will turn on and preset the TDT destination channel.
Key Path

TDR/TDT Setup TDT 2 dest

which ever is greater, to a maximum of
max = 5 × time per division (s/div)

5-13

Agilent 54754A TDR/TDT Setup Menu
Normalize response . . .

system just as if a different TDR step risetime had actually been applied. This
allows you to select a risetime for TDR/TDT measurements that is close to the
actual risetime used in your system. This risetime value applies to both TDR
and TDT normalized channels. For more information on normalization, see
Chapter 10, ”Improving Time Domain Network Measurements”.
Key Path

Key Path

TDR/TDT Setup Normalize response . . . Normalize scaling . . .
Vertical There are two choices for vertical mode: track source and manual.
The track source mode sets the control of the vertical scaling for the
normalized trace to that of the source channel. When manual mode is
selected, the Scale and Offset menus appear allowing you to independently
change the vertical scale and offset of the normalized trace.

Key Path

5-14

Agilent 54754A TDR/TDT Setup Menu
Normalize response . . .

independently change the horizontal scale and position for the normalized
trace.
Key Path

TDR/TDT Setup Normalize response . . .Normalize scaling . . . Horizontal
Establish normalization & ref plane
The Establish normalization & ref plane function establishes the filter values used
to normalize a channel and to set the reference plane for TDR and TDT
measurements. This function must be performed separately for TDR and TDT
modes. The normalization and reference plane must be re-established when
power is lost or when the instrument is turned off. However, the values used for
normalization and reference plane calculations can be stored to and re-loaded
from disk. (See the Disk Menu chapter in the Agilent 83480A, Agilent 54750
User’s Guide for more information.)
The normalization filter values and reference plane are invalidate when the
timebase scale, record length or channel bandwidth is changed. Also, the
normalization process will not be able to remove small synchronous noise in the
channels baseline if the timebase position is changed. For most measurements
this error is very small.
The reference plane must be established before ohm, % reflect or gain units are
selectable for the channel. Also, the Reference ref plane function is not
available in the Marker menu.

5-15

Agilent 54754A TDR/TDT Setup Menu
TDR rate automatic . . . (250 kHz)

The function steps you through the normalization and reference plane
procedure for the selected measurement type. The procedure steps are
displayed at the top of the screen. The items required for calibration are shown
in Table 5-4.
Table 5-4
Establish normalization & ref plane Hardware Requirements
Measurement

Requirements

TDR

50 ohm 3.5 mm SMA terminator
3.5 mm SMA short

TDT

1 or 2 each 3.5 mm SMA cables
3.5 mm barrel connector

Key Path

TDR/TDT Setup Normalize response . . . Establish normalization & ref plane . . .

TDR/TDT Setup TDR rate automatic

Key Path

TDR/TDT Setup TDR rate manual

5-16

Agilent 54754A TDR/TDT Setup Menu
Preset TDR/TDT

Menu Item

Set To

Acquisition

Averaging

Best

Flatness

Time Base

Scale

500.0 ps/div

Position

Set to a value which places the
incident edge on screen.

Display

Scale

100 mV/div

Offset

200.0 mV

Bandwidth

12.4 GHz

Attenuation units

ratio

Attenuation

1.000 : 1

Channel 2 Setup

Key Path

TDR/TDT Setup Preset TDR/TDT

5-17

1 and 2 Stimulus Menus
This section describes the menus that are available when the stimulus is set to
1 and 2.

TDT 1 dest
Pressing the TDT 1 dest softkey produces a pull-down menu used to select the
channel used as the destination channel for channel 1 TDT measurements. The
choices available for this pull-down menu depend on the other TDR or electrical
plug-in, if any, in the mainframe.
Any electrical channel is potentially a valid TDT destination channel. The TDT
destination may not be assigned to a channel already assigned as a TDT
destination.
If no other valid TDT destination channels are available then "none" is the only
choice. If a TDT destination other than none is selected then the preset TDR/
TDT control will turn on and preset the TDT destination channel.
Key Path

TDR/TDT Setup TDT 1 dest

TDT 2 dest
Pressing the TDT 2 dest softkey produces a pull-down menu used to select the
channel used as the destination channel for channel 2 TDT measurements. The
choices available for this pull-down menu depend on the other TDR or electrical
plug-in, if any, in the mainframe.
Any electrical channel is potentially a valid TDT destination channel. The TDT
destination may not be assigned to a channel already assigned as a TDT
destination.
If no other valid TDT destination channels are available then "none" is the only
choice. If a TDT destination other than none is selected then the preset TDR/
TDT control will turn on and preset the TDT destination channel.
Key Path

TDR/TDT Setup TDT 2 dest

5-18

Agilent 54754A TDR/TDT Setup Menu
Normalize 1 response . . . Normalize 2 response . . .

Normalize 1 response . . .
Normalize 2 response . . .
When the Stimulus is 1 and 2, there are two menus, Normalize 1 response and
Normalize 2 response, which are used to normalize the two independent TDR
channels separately. The functions allow you to change the risetime of the
normalized step, to select TDR and TDT normalization, to turn on or off the
display of the normalized TDR or TDT trace, to change the scaling of the
normalized trace, and to establish the normalization filter values and reference
plane.
Risetime
The Risetime function allows you to change the normalized step’s risetime from
a minimum of
10 ps
or
time per division (s/div) × 10 divisions
min = 8 points × --------------------------------------------------------------------------------------------record length

which ever is greater, to a maximum of
max = 5 × time per division (s/div)

While the TDR step’s risetime applied to the system under test is fixed, the
measured response has a set of mathematical operations applied to it. These
mathematical operations effectively change the displayed response to the
system just as if a different TDR step risetime had actually been applied. This
allows you to select a risetime for TDR/TDT measurements that is close to the
actual risetime used in your system. This risetime value applies to both TDR
and TDT normalized channels. For more information on normalization, see
Chapter 10, ”Improving Time Domain Network Measurements”.
Key Path

Key Path

TDR/TDT Setup Normalize response . . . TDR/TDT

5-19

Agilent 54754A TDR/TDT Setup Menu
Normalize 1 response . . . Normalize 2 response . . .

TDR or TDT normalize
The TDR normalize function is available if the TDR mode is selected by using
the previous menu otherwise the TDT normalize function is available. In either
case, this function turns on or off the display of the normalized trace. The TDR
and TDT normalization functions can be on at the same time.
Key Path

Key Path

TDR/TDT Setup Normalize response . . . Normalize scaling . . .
Vertical There are two choices for vertical mode: track source and manual.
The track source mode sets the control of the vertical scaling for the
normalized trace to that of the source channel. When manual mode is
selected, the Scale and Offset menus appear allowing you to independently
change the vertical scale and offset of the normalized trace.

Key Path

TDR/TDT Setup Normalize response . . .Normalize scaling . . . Vertical
Horizontal There are two choices for horizontal mode: track source and
manual. The track source mode sets the control of the horizontal scaling
for the normalized trace to that of the source channel. When manual mode
is selected, the Scale and Position menus appear allowing you to
independently change the horizontal scale and position for the normalized
trace.

Key Path

5-20

Agilent 54754A TDR/TDT Setup Menu
TDR rate automatic . . . (250 kHz)

normalization and reference plane calculations can be stored to and re-loaded
from disk. (See the Disk Menu chapter in the Agilent 83480A, Agilent 54750
User’s Guide for more information.)
The normalization filter values and reference plane are invalidate when the
timebase scale, record length or channel bandwidth is changed. Also, the
normalization process will not be able to remove small synchronous noise in the
channels baseline if the timebase position is changed. For most measurements
this error is very small..
The reference plane must be established before ohm, % reflect or gain units are
selectable for the channel. Also, the Reference ref plane function is not
available in the Marker menu.
The function steps you through the normalization and reference plane
procedure for the selected measurement type. The procedure steps are
displayed at the top of the screen. The items required for calibration are shown
in Table 5-6.
Table 5-6
Establish normalization & ref plane Hardware Requirements
Measurement
TDR

Requirements
50 ohm 3.5 mm SMA terminator
3.5 mm SMA short

TDT

1 or 2 each 3.5 mm SMA cables
3.5 mm barrel connector

Key Path

TDR/TDT Setup Normalize response . . . Establish normalization & ref plane . . .

5-21

Agilent 54754A TDR/TDT Setup Menu
Preset TDR/TDT

rate varies automatically as the Time base Scale is changed to keep multiple
steps off screen. As the TDR rate decreases, TDR measurements can be made
on longer transmission lines.
Key Path

TDR/TDT Setup TDR rate automatic

Key Path

TDR/TDT Setup TDR rate manual

Key Path

Menu Item

Set To

Acquisition

Averaging

Best

Flatness

Time Base

Scale

500.0 ps/div

Position

Set to a value which places the
incident edge on screen.

Channel 1 Setup and Display

Channel 2 Setup

Scale

100 mV/div

Offset

200.0 mV

Bandwidth

12.4 GHz

Attenuation units

ratio

Attenuation

1.000 : 1

TDR/TDT Setup Preset TDR/TDT

5-22

Differential and Common Mode Stimulus
Menus
This section describes the menus that are available when the stimulus is set to
differential or common mode.

TDR/TDT
This TDR/TDT function allows you to select either TDR measurements or TDT
measurements. The TDT measurement capability requires that an additional
TDR or electrical plug-in module be installed in the mainframe.
Key Path

TDR/TDT Setup TDR/TDT

TDR response 1
The TDR response 1 function is used to enable or disable the display of the
differential or common mode TDR response 1. The choices available are off,
differential, or common mode.
Key Path

TDR/TDT Setup TDR response 1
Response scaling . . .
The Response scaling function is used when vertical and horizontal scaling of
the normalized response is required that is independent from that of the source
channel. This function only appears when TDR response 1 is set to differential
or common mode.

Key Path

TDR/TDT Setup TDR response 1 Response scaling . . .
Vertical There are two choices for vertical mode: track source and manual.
The track source mode sets the control of the vertical scaling for the
normalized trace to that of the source channel. When manual mode is
selected, the Scale and Offset menus appear allowing you to independently
change the vertical scale and offset of the normalized trace.

Key Path

TDR/TDT Setup TDR response 1 . . .Response scaling . . . Vertical

5-23

Agilent 54754A TDR/TDT Setup Menu
TDR response 2

Horizontal There are two choices for horizontal mode: track source and
manual. The track source mode sets the control of the horizontal scaling
for the normalized trace to that of the source channel. When manual mode
is selected, the Scale and Position menus appear allowing you to
independently change the horizontal scale and position for the normalized
trace.
Key Path

TDR/TDT Setup TDR response 1 . . .Response scaling . . . Horizontal

TDR response 2
The TDR response 2 function is used to enable or disable the display of the
differential or common mode TDT response 2. The choices available are off,
differential, or common mode.
Key Path

TDR/TDT Setup TDR response 2
Response scaling . . .
The Response scaling function is used when vertical and horizontal scaling of
the normalized response is required that is independent from that of the source
channel. This function only appears when TDR response 2 is set to differential
or common mode.

Key Path

TDR/TDT Setup TDR response 2 Response scaling . . .
Vertical There are two choices for vertical mode: track source and manual.
The track source mode sets the control of the vertical scaling for the
normalized trace to that of the source channel. When manual mode is
selected, the Scale and Offset menus appear allowing you to independently
change the vertical scale and offset of the normalized trace.

Key Path

TDR/TDT Setup TDR response 2 . . .Response scaling . . . Vertical
Horizontal There are two choices for horizontal mode: track source and
manual. The track source mode sets the control of the horizontal scaling
for the normalized trace to that of the source channel. When manual mode
is selected, the Scale and Position menus appear allowing you to
independently change the horizontal scale and position for the normalized
trace.

Key Path

TDR/TDT Setup TDR response 2 . . .Response scaling . . . Horizontal

5-24

Agilent 54754A TDR/TDT Setup Menu
TDT response 1

TDT response 1
The TDT response 1 function is used to enable or disable the display of the
differential or common mode response 1. The choices available are off,
differential, or common mode.
Key Path

TDR/TDT Setup TDT response 1
Response scaling . . .
The Response scaling function is used when vertical and horizontal scaling of
the normalized response is required that is independent from that of the source
channel. This function only appears when TDT response 1 is set to differential
or common mode.

Key Path

TDR/TDT Setup TDT response 1 Response scaling . . .
Vertical There are two choices for vertical mode: track source and manual.
The track source mode sets the control of the vertical scaling for the
normalized trace to that of the source channel. When manual mode is
selected, the Scale and Offset menus appear allowing you to independently
change the vertical scale and offset of the normalized trace.

Key Path

TDR/TDT Setup TDT response 1 . . .Response scaling . . . Vertical
Horizontal There are two choices for horizontal mode: track source and
manual. The track source mode sets the control of the horizontal scaling
for the normalized trace to that of the source channel. When manual mode
is selected, the Scale and Position menus appear allowing you to
independently change the horizontal scale and position for the normalized
trace.

Key Path

TDR/TDT Setup TDT response 1 . . .Response scaling . . . Horizontal

TDT response 2
The TDT response 2 function is used to enable or disable the display of the
differential or common mode response 2. The choices available are off,
differential, or common mode.
Key Path

TDR/TDT Setup TDT response 2

5-25

Agilent 54754A TDR/TDT Setup Menu
Establish ref plane

Response scaling . . .
The Response scaling function is used when vertical and horizontal scaling of
the normalized response is required that is independent from that of the source
channel. This function only appears when TDT response 2 is set to differential
or common mode.
Key Path

TDR/TDT Setup TDT response 2 Response scaling . . .
Vertical There are two choices for vertical mode: track source and manual.
The track source mode sets the control of the vertical scaling for the
normalized trace to that of the source channel. When manual mode is
selected, the Scale and Offset menus appear allowing you to independently
change the vertical scale and offset of the normalized trace.

Key Path

TDR/TDT Setup TDT response 2 . . .Response scaling . . . Vertical
Horizontal There are two choices for horizontal mode: track source and
manual. The track source mode sets the control of the horizontal scaling
for the normalized trace to that of the source channel. When manual mode
is selected, the Scale and Position menus appear allowing you to
independently change the horizontal scale and position for the normalized
trace.

Key Path

TDR/TDT Setup TDT response 2 . . .Response scaling . . . Horizontal

Establish ref plane
The Establish ref plane function is used to set the reference plane for TDR and
TDT measurements for differential or common mode stimulus. The function
steps you through the calibration procedure for the selected measurement type.
The steps are displayed at the top of the screen. The items required for
calibration are shown in Table 5-8 on page 5-27.

5-26

Agilent 54754A TDR/TDT Setup Menu
TDR rate automatic . . . (250 kHz)

Table 5-8
Establish ref plane Hardware Requirements
Measurement

Requirements

TDR

50 ohm 3.5 mm SMA terminator
3.5 mm SMA short

TDT

1 or 2 each 3.5 mm SMA cables
3.5 mm barrel connector

The reference plane is invalidated when the timebase scale, record length or
channel bandwidth is changed.
Before establishing a reference plane for common mode and differential TDR,
the test setup must be deskewed. See Chapter 8, ”Differential TDR
Measurements” for information on how to deskew a test setup.
The reference plane must be established before ohm, % reflect or gain units are
selectable for the channel. Also, the Reference ref plane function is not
available in the Marker menu.
Key Path

TDR/TDT Setup Normalize response . . . Establish ref plane . . .

TDR/TDT Setup TDR rate automatic

Key Path

TDR/TDT Setup TDR rate manual

5-27

Agilent 54754A TDR/TDT Setup Menu
Preset TDR/TDT

Menu Item

Set To

Acquisition

Averaging

Best

Flatness

Time Base

Scale

500.0 ps/div

Position

Set to a value which places the
incident edge on screen.

Display

Scale

100 mV/div

Offset

200.0 mV

Bandwidth

12.4 GHz

Attenuation units

ratio

Attenuation

1.000 : 1

Scale

100 mV/div

Offset

-200 mV

Channel 1 Setup

Channel 2 Setup

5-28

Agilent 54754A TDR/TDT Setup Menu
Preset TDR/TDT

Table 5-10
Common Mode Stimulus Preset TDR/TDT Configuration
Menu

Menu Item

Set To

Acquisition

Averaging

Best

Flatness

Time Base

Scale

500.0 ps/div

Position

Set to a value which places the
incident edge on screen.

Display

Scale

100 mV/div

Offset

200.0 mV

Bandwidth

12.4 GHz (for Stimulus on)
18.0 GHz (for Stimulus external)

Channel 1 Setup

Channel 2 Setup

Key Path

Attenuation units

ratio

Attenuation

1.000 : 1

Scale

100 mV/div

Offset

200.0 mV

TDR/TDT Setup Preset TDR/TDT

5-29

External Stimulus Menus
This section describes the menus that are available when the stimulus is set to
external.

TDT 1 dest
Pressing the TDT 1 dest softkey produces a pull-down menu used to select the
channel used as the destination channel for TDT measurements. The choices
available for this pull-down menu depend on the other TDR or electrical plugin, if any, in the mainframe.
Any electrical channel is potentially a valid TDT destination channel. The TDT
destination may not be assigned to a channel already assigned as a TDT
destination. The TDT destination may not be set to the currently defined TDR
destination channel.
If no other valid TDT destination channels are available then "none" is the only
choice. If a TDT destination other than none is selected then the preset TDR/
TDT control will turn on and preset the TDT destination channel.
Key Path

TDR/TDT Setup TDT 1 dest

TDR 1 dest
Pressing the TDR 1 dest softkey produces a pull-down menu used to select the
channel used as the destination channel for TDR measurements. Any electrical
channel is a valid TDR destination channel. The TDR destination may not be
set to the currently defined TDT destination.
Key Path

TDR/TDT Setup TDR 1 dest

5-30

Agilent 54754A TDR/TDT Setup Menu
Normalize response . . .

which ever is greater, to a maximum of
max = 5 × time per division (s/div)

While the TDR step’s risetime applied to the system under test is fixed, the
measured response has a set of mathematical operations applied to it that
effectively displays the response to the system as if a different TDR step risetime
had actually been applied. This allows you to select a risetime for TDR/TDT
measurements that is close to the actual risetime used in your system. This
risetime value applies to both TDR and TDT normalized channels. For more
information on normalization, see Chapter 10, ”Improving Time Domain
Network Measurements”.
Key Path

Key Path

TDR/TDT Setup Normalize response . . . TDR/TDT

5-31

Agilent 54754A TDR/TDT Setup Menu
Normalize response . . .

Key Path

TDR/TDT Setup Normalize response . . . Normalize scaling . . .
Vertical There are two choices for vertical mode: track source and manual.
The track source mode sets the control of the vertical scaling for the
normalized trace to that of the source channel. When manual mode is
selected, the Scale and Offset menus appear allowing you to independently
change the vertical scale and offset of the normalized trace.

Key Path

TDR/TDT Setup Normalize response . . .Normalize scaling . . . Vertical
Horizontal There are two choices for horizontal mode: track source and
manual. The track source mode sets the control of the horizontal scaling
for the normalized trace to that of the source channel. When manual mode
is selected, the Scale and Position menus appear allowing you to
independently change the horizontal scale and position for the normalized
trace.

Key Path

5-32

Agilent 54754A TDR/TDT Setup Menu
Normalize response . . .

normalization and reference plane calculations can be stored to and re-loaded
from disk. (See the Disk Menu chapter in the Agilent 83480A, Agilent 54750
User’s Guide for more information.)
The normalization filter values and reference plane are invalidate when the
timebase scale, record length or channel bandwidth is changed. Also, the
normalization process will not be able to fully remove step non-flatness if the
timebase position is changed since the filter values have been established.
The reference plane must be established before ohm, % reflect or gain units are
selectable for the channel. Also, the Reference ref plane function is not
available in the Marker menu.
The function steps you through the normalization and reference plane
procedure for the selected measurement type. The procedure steps are
displayed at the top of the screen. The items required for calibration are shown
in Table 5-11.
Table 5-11
Establish normalization & ref plane Hardware Requirements
Measurement
TDR

Requirements
50 ohm 3.5 mm SMA terminator
3.5 mm SMA short

TDT

1 or 2 each 3.5 mm SMA cables
3.5 mm barrel connector

Key Path

TDR/TDT Setup Normalize response . . . Establish normalization & ref plane . . .

5-33

Agilent 54754A TDR/TDT Setup Menu
TDR rate automatic . . . (250 kHz)

TDR/TDT Setup TDR rate automatic

Key Path

TDR/TDT Setup TDR rate manual

Channel 1 Setup

Key Path

Menu Item
Averaging

Best

Flatness

Scale

500.0 ps/div

Position

Set to a value which places the incident
edge on screen.

Display

Scale

100 mV/div

Offset

0.0 mV

Bandwidth

18.0 GHz

Attenuation units

ratio

Attenuation

1.000 : 1

TDR/TDT Setup Preset TDR/TDT

5-34

Set To

Measure and Other TDR Specific
Menus

Measure and Other TDR Specific Menus

What you’ll find in this chapter
This chapter describes the Measure menu and the mainframe menu changes that
occur when a TDR plug-in module is in the mainframe.

CAUTION

The input circuits can be damaged by electrostatic discharge (ESD).
Therefore, avoid applying static discharges to the front-panel input connectors.
Before connecting any coaxial cable to the connectors, momentarily short the
center and outer conductors of the cable together. Avoid touching the frontpanel input connectors without first touching the frame of the instrument. Be
sure the instrument is properly earth-grounded to prevent buildup of static
charge.
There are several mainframe menus which change when a TDR plug-in module
is inserted into the mainframe menus. This chapter will cover only the changes
that occur to the mainframe menus and will not discuss the menu functions
which are already documented in the Agilent 83480A, 54750A User’s Guide.

6-2

Measure and Other TDR Specific Menus

Figure 6-1
Blue Key

TDR/TDT Measure

TDR minimum reflection

channel #
response #
Cancel
Enter

TDR maximum reflection

channel #
response #
Cancel
Enter

TDT propagation delay

channel #
response #
Cancel
Enter

TDT gain

channel #
response #
Cancel
Enter

TDR Setup menu.

6-3

Measure and Other TDR Specific Menus
TDR/TDT Measure Menu

TDR/TDT Measure Menu
To display the Measure menu, press the blue key immediately followed by the
TDR/TDT Measure key on the TDR plug-in module. There are four automated
measurements which may be selected: TDR minimum reflection, TDR maximum
reflection, TDT propagation delay, and TDT gain.
TDR Minimum Reflection
Pressing the TDR minimum reflection softkey display the automatically calculated
minimum percent reflection value at the bottom of the display under the
waveform graticule.
The TDR plug-in module must be calibrated and a reference plane established
before this measurement can be selected.
The calculation of the value is dependant on the type of response and stimulus
that are selected.
For differential response to differential stimulus and common mode response
to common mode stimulus, the following formula is used:
Vmin – ( V1ref50Ω + V 2ref50Ω )
Minimum reflection = 100 -----------------------------------------------------------------------------------------------------------( V 1ref50Ω – V1ref0Ω ) + ( V2ref50Ω – V 2ref0Ω )

6-4

Measure and Other TDR Specific Menus
TDR/TDT Measure Menu

For differential response to common mode stimulus and common mode
response to differential stimulus, the following formula is used.
Vmin – ( V1ref50Ω + V 2ref50Ω )
----------------------------------------------------------------------2
Minimum reflection = 100 ------------------------------------------------------------------------------------------------------------( V 1ref50Ω – V1ref0Ω ) + ( V2ref50Ω – V 2ref0Ω )
------------------------------------------------------------------------------------------------------------2

where:
Vmin = the minimum voltage value along the waveform
V1ref50Ω = the channel 1 reference plane voltage into a 50 ohm load
V1ref0Ω = the channel 1 reference plane voltage into a short
V2ref50Ω = the channel 2 reference plane voltage into a 50 ohm load
V2ref0Ω = the channel 2 reference plane voltage into a short
For all other stimulus, the following formula is used.
V min – Vref 50Ω 
Minimum reflection = 100  ---------------------------------------V

–V
ref 50Ω

ref0Ω

where:
Vmin = the minimum voltage value along the waveform
Vref50Ω = the reference plane voltage into a 50 ohm load
Vref0Ω = the reference plane voltage into a short
Key Path

blue key TDR/TDT Measure TDR minimum reflection

6-5

Measure and Other TDR Specific Menus
TDR/TDT Measure Menu

TDR Maximum Reflection
Pressing the TDR maximum reflection softkey display the automatically calculated
maximum percent reflection value at the bottom of the display under the
waveform graticule.
The TDR plug-in module must be calibrated and a reference plane established
before this measurement can be selected.
The calculated value is dependant on the type of response and stimulus that
are selected.
For differential response to differential stimulus and common mode response
to common mode stimulus, the following formula is used:
V max – ( V1ref50Ω + V 2ref50Ω )
Maximum reflection = 100 -----------------------------------------------------------------------------------------------------------( V 1ref50Ω – V 1ref0Ω ) + ( V2ref50Ω – V2ref0Ω )

where:
Vmax = the maximum voltage value along the waveform
V1ref50Ω = the channel 1 reference plane voltage into a 50 ohm load
V1ref0Ω = the channel 1 reference plane voltage into a short
V2ref50Ω = the channel 2 reference plane voltage into a 50 ohm load
V2ref0Ω = the channel 2 reference plane voltage into a short
For differential response to common mode stimulus and common mode
response to differential stimulus, the following formula is used.
V max – ( V1ref50Ω + V 2ref50Ω )
----------------------------------------------------------------------2
Maximum reflection = 100 ------------------------------------------------------------------------------------------------------------( V 1ref50Ω – V 1ref0Ω ) + ( V2ref50Ω – V2ref0Ω )
------------------------------------------------------------------------------------------------------------2

6-6

Measure and Other TDR Specific Menus
TDR/TDT Measure Menu

For all other stimulus, the following formula is used.
V max – Vref 50Ω 
Maximum reflection = 100  ---------------------------------------V

–V
ref50Ω

ref0Ω

where:
Vmax = the maximum voltage value along the waveform
Vref50Ω = the reference plane voltage into a 50 ohm load
Vref0Ω = the reference plane voltage into a short
Key Path

blue key TDR/TDT Measure TDR maximum reflection
TDT Propagation Delay
Pressing the TDT propagation delay softkey display the automatically calculated
propagation delay value at the bottom of the display under the waveform
graticule.
The TDR plug-in module must be calibrated and a reference plane established
before this measurement can be selected.
The value is calculated using the following formula.
TDT propagation delay = T edge – T ref

where:
Tedge = the time value of the reflected edge
Tref = the time value of the reference plane
Key Path

blue key TDR/TDT Measure TDT propagation delay
TDT Gain
Pressing the TDT gain softkey display the automatically calculated value at the
bottom of the display under the waveform graticule.
The TDR plug-in module must be calibrated and a reference plane established
before this measurement can be selected.
The calculated value is dependant on the type of response and stimulus that
are selected.

6-7

Measure and Other TDR Specific Menus
TDR/TDT Measure Menu

For differential response to differential stimulus and common mode response
to common mode stimulus, the following formula is used:
V max – Vmin
TDT gain = 100 -----------------------------------------------------------------------------------------------------------( V 1ref50Ω – V1ref0Ω ) + ( V2ref50Ω – V 2ref0Ω )

where:
Vmax = the maximum voltage value along the waveform
Vmin = the minimum voltage value along the waveform
V1ref50Ω = the channel 1 reference plane voltage into a 50 ohm load
V1ref0Ω = the channel 1 reference plane voltage into a short
V2ref50Ω = the channel 2 reference plane voltage into a 50 ohm load
V2ref0Ω = the channel 2 reference plane voltage into a short
For differential response to common mode stimulus and common mode
response to differential stimulus, the following formula is used.
V max – Vmin
TDT gain = 100 -----------------------------------------------------------------------------------------------------------( V 1ref50Ω – V1ref0Ω ) + ( V2ref50Ω – V 2ref0Ω )
------------------------------------------------------------------------------------------------------------2

ref0Ω

where:
Vmax = the maximum voltage value along the waveform
Vmin = the minimum voltage value along the waveform
Vref50Ω = the reference plane voltage into a 50 ohm load
Vref0Ω = the reference plane voltage into a short
Key Path

blue key TDR/TDT Measure TDT gain

6-8

Measure and Other TDR Specific Menus
Marker Menu

Marker Menu
To display the Marker menu, press the SETUP Marker key. There is a marker
mode, the TDR/TDT marker mode, that is affected by the presence of a TDR plugin module in the mainframe. Selecting the TDR/TDT mode produces the + Source,
+ Position, X Source, and X Position just like other plug-in modules. However,
when a TDR plug-in module is present and one of the TDR channels is selected
as + or X source, the Reference menu has an additional choice called ref plane.

Reference
Pressing the Reference softkey allows selection of either trigger or ref
plane. When trigger is selected, the marker positions are displayed with
respect to the trigger point. Choosing ref plane displays the marker positions
with respect to the reference plane. The ref plane selection requires
establishing a reference plane before the markers will display information.
Key Path

Marker Reference

Marker units . . .
Pressing the Marker units softkey produces two menus: a Horiz units menu and
a Vertical units menu. The Horiz units menu is used to set the X marker units
located at the bottom of the display. The unit choices are second, meter, and
feet.
When either meter or feet is selected, two additional menus are displayed:
Propagation and Dielectric. The Propagation softkey is used to select the
propagation constant’s unit of measure: dielectric constant, velocity in meters
per second, or velocity in feet per second.
When one of the velocity propagation constants is selected, the Dielectric menu
changes to Velocity. You should set the propagation constant to the value of the
device under test’s propagation constant. This constant is used for distance
calculations.
The Vertical units menu is used to set the Y marker units located at the bottom
of the display. The unit choices are Volt, Ohm, and % reflect.
Key Path

Marker Reference ref plane Marker units . . .

6-9

Measure and Other TDR Specific Menus
Response Menu Items

Response Menu Items
When a TDR plug-in module is present in the mainframe, response menu
choices will appear in many of the mainframe menus. The following is a list of
the mainframe menus which will contain response menu choices.
• Disk store waveform From waveform
• Display Graph
• Histogram Window
• Limit Test Fail Action
• Marker + Source
• Marker X Source
• Mask Fail Action
• Mask Scale Automask
• Math Define function . . . Operand 1
• Math Define function . . . Operand 2
• Measure Source
• Waveform From waveform

6-10

Single-ended TDR Measurements

What you’ll find in this chapter
This chapter describes how to make singe-ended TDR measurements and
describes the reason for the processes required to make these measurements.

CAUTION

7-2

Single-ended TDR Measurements
Single-ended TDR Features

Single-ended TDR Features
The Agilent 54753A and Agilent 54754A TDR plug-in modules are both capable
of performing singe-ended TDR measurements. These measurements include
characterizing microstrip lines, PC board traces, and coaxial cables. Because
TDR measurements are complex, the TDR plug-in modules have several
features which make measurements easier.
The Preset TDR/TDT Feature
The Preset TDR/TDT feature prepares the oscilloscope for making Time Domain
Reflectometry (TDR) and Time Domain Transmission (TDT) measurements by
automatically setting several menu fields. The Preset TDR/TDT feature appears
in the TDR/TDT Setup menu once a stimulus has been selected.
TDR Establish Normalization and Reference Plane Feature
This feature performs the following:
• Establishes the Reference Plane.
• Measures the negative going step reflected from a short.
• Builds a normalization filter.
• Measures the response to a 50 ohm terminator.
The Reference Plane is defined as the point in time that coincides with the 50%
point of the negative going step that is reflected from a short connected to a 50
ohm line. Once the Reference Plane is established, cursor measurements can
be made with respect to this point in time rather than to the trigger point.
Typically, the short is connected at the end of a cable which will be connected
to the device under test. This effectively establishes this end of the cable as
the Reference Plane.
The next stage in the Establish Normalization and Reference Plane process
involves measuring the negative going step reflected from the short at the end
of the cable. This allows the oscilloscope to base the percent reflection and
ohms measurements on the actual measured step height rather than the
nominal step height of 200 mV.
Also, from this information, the oscilloscope builds a normalization filter which
can be applied to any reflected signal. When applied to the short, the filter
produces a step which has no preshoot or overshoot and has an impulse
response which is approximately Gaussian. The risetime of this filtered step can
be selected when making TDR measurements of systems using a range of
risetimes. This filter removes any losses or discontinuities from the TDR plug-

7-3

Single-ended TDR Measurements
Single-ended TDR Features

in to the shorted end of the cable. Also, it allows the TDR response to be
measured as though it had been stimulated by a step with no preshoot or
overshoot rather than the actual step.
The final stage in Establishing Normalization is measuring the response of the
cable when a 50 ohm terminator is connected to the end of the cable in place
of the short. This response will be subtracted from all TDR measurements as
long as the Time base position has not been changed since the reference plane
was established. The normalization filter is applied to the resultant waveform.
This process removes the incident step from the normalized waveform and any
small synchronous noise that may be present.
TDT Establish Normalization and Reference Plane Feature
This feature performs the following:
• Establishes the Reference Plane.
• Measures the step transmitted to the TDT destination channel.
• Builds a normalization filter.
The Reference Plane is defined as the point in time that coincides with the 50%
point of the step that is transmitted when the cables used to connect to a device
under test (DUT) are connected together. Once the Reference Plane is
established, cursor measurements can be made in terms of propagation delay
rather than referenced to the trigger point.
The next stage in establishing the TDT Normalization process involves
measuring the step transmitted when the cables used to connect to the DUT
are connected together. As long as the transmitted step is allowed to settle to
its final value, this step height is equal to the incident step height. The sweep
speed must be slow enough to allow any slow tails caused by cable losses to
settle. This allows the oscilloscope to base its calculations of gain on the actual
measured height of the step instead of the nominal step height of 200 mV.
Also, from this information the oscilloscope builds a normalization filter which
can be applied to any transmitted signal. When applied to the step transmitted
through the cables used for TDT measurements, this filter removes any losses
or discontinuities caused by the cables. It also allows the TDT response of a
DUT to be measured as if stimulated by a perfect step rather than by the actual
step of the oscilloscope. The risetime of the perfect step is selectable allowing
TDT measurements of systems to be made at several different risetime values.

7-4

Single-ended TDR Measurements
Single-ended TDR Features

Bandwidth Limit
This feature, which is located under waveform math, allows any waveform on
screen to be filtered by a digital low pass filter. This filter is a 4th order BesselThompson filter. It has no pre-shoot and very little overshoot. The inherent
risetime of the TDR plug-in modules is 35 pS. For many real world systems,
this risetime is much too fast.
Often, it is necessary to test the response of a DUT using a step with a slower
risetime. One way of doing this is to use a TDR normalization filter. This is the
best way because it removes cable losses and discontinuities from the measured
values. It does, however, require that a normalization calibration be performed
at the reference plane. In cases where doing a normalization calibration is
difficult or where the highest degree of accuracy is not needed, the Bandwidth
Limit filter can be used to simulate the DUT with a slower risetime step,
providing good measurement results. In fact, if good quality cables and
connectors are used to connect to the DUT, this method can produce very
similar results to normalization, especially for slower risetimes.

7-5

Single-ended TDR Measurements
Single-ended TDR Features

Excess L/C
The most common discontinuities seen on an TDR waveform are due to series
inductances or shunt capacitances. Some causes of series inductances are wire
bonds or traces that are too narrow. Some causes of shunt capacitance are wire
bond pads or traces that are too wide. A series inductance is seen as a positive
bump while a shunt capacitance is seen as a negative bump in the oscilloscope
waveform (Figure 7-1).
Figure 7-1

Inductance

Capacitance

Series Inductance and Shunt Capacitance

A feature called Excess L/C can be used to calculate the excess inductance (L)
or excess capacitance (C) between the x and + markers. The Excess L/C feature
is enabled by setting the Marker Mode menu to TDR/TDT and the Reference
menu to ref plane. If a reference plane has been established by selecting
the Establish normalization & ref plane in the TDR/TDT Setup Normalize response...
menu, then a readout appears at the bottom of the screen called "Excess L/C.”
The excess L or C in this case is defined as the equivalent amount of series L
or shunt C that would cause a discontinuity with equal area to the discontinuity
between the x and + markers.

7-6

Single-ended TDR Measurements
Single-ended TDR Features

In cases where a discontinuity is due to a lumped L or C, the Excess L/C can be
used to directly measure the L or C value. This is done by placing one marker
just to the left of the discontinuity and the other marker just to the right of the
disconinuity. The oscilloscope will calculate the excess L/C by integrating the
% reflection between the markers.
When measuring discontinuities on lines whose impedance is not 50 ohms, the
Excess L/C measurement is still valid as long as the 50 ohm measurement system
is connected the non-50 ohm line without using matching resistors.
It is also possible measure the Excess L/C of shunt inductors and series
capacitors using TDT measurements. A TDT measurement of a 50 ohm system
with a shunt inductance produces a negative bump in the transmitted response
while a series capacitance produces a positive bump in the transmitted
response.
Alternate Channel Scales
Many times when making TDR measurements, you may want to view a TDR
waveform in units of % reflection or ohms as the vertical scale. This can be done
using the Alternate Scale . . . control in the SETUP Channel menu. You must
establish normalization and reference plane values before you can select either
of these two scales.
When making TDT measurements, channels may be viewed in units of gain,
volts, watts, amperes, and unknown.
The reference plane calibrations as well as normalization calibrations are
volatile and are lost when power is cycled.

7-7

Single-ended TDR Measurements
Establishing the Reference Plane and Normalizing

Establishing the Reference Plane and Normalizing
Establishing a reference plane allows you to effectively change the launch point
of the TDR step from the TDR plug-in module’s connector to the input of the
device under test (DUT). Typically, a cable is connected between a DUT and
the TDR plug-in module. Establishing a reference plane and normalizing
removes any effects caused by the cable.
Normalization produces a TDR step which has no preshoot or overshoot and
has an impulse response that is approximately Gaussian. Establishing
normalization and reference plane increases the accuracy of TDR and TDT
measurements.
Normalization requires the TDR step to be on screen and not clipped.
To perform the tasks in this section, you need the following:
• 1 good quality SMA cable one meter in length, such as the Agilent 8120-4948
cable.
• 1 SMA short found on the Agilent 54754A plug-in module.
• 1 SMA 50 ohm load found on the Agilent 54754A plug-in module.
• 1 Agilent 54754A or Agilent 54753A TDR Module.
Performing TDR Normalization
The purpose of this section is to show the process used to normalize a coaxial
cable for TDR measurements. The following procedure shows how to perform
TDR normalization.

1
2
3
4
5
6
7

Connect a 1 m SMA coaxial cable to channel 1 of the TDR plug-in
module.
Press the STORAGE Setup menu key above the display.
Press the Default setup softkey.
Press TDR/TDT Setup on the TDR plug-in module.
Press the Stimulus softkey and select 1 only (on for the Agilent 54753A).
Press the Enter softkey.
Press the Preset TDR/TDT softkey.
These steps set the oscilloscope to a known condition and activates the TDR
step on channel 1. You should see a display similar to the one shown in
Figure 7-2.

7-8

Single-ended TDR Measurements
Establishing the Reference Plane and Normalizing

Figure 7-2

7-9

Single-ended TDR Measurements
Establishing the Reference Plane and Normalizing

1
2

Press the SETUP Time base key located below the display.
Change the Scale until you see two positive going edges on screen
(Figure 7-3).
The left-most edge is the incident step and starts at 0 mV and goes to 200 mV.
The right-most edge is the reflected step that has traveled from the TDR step
generator to the end of the cable and back to the TDR sampler. This step starts
at 200 mV and goes to 400 mV.

Figure 7-3

Reflected Step

Incident Step

1
2

Change the Scale and Position to approximately center the incident step
in the middle of the display and to move the reflected step off screen.
Press the blue key followed by the 7 key to turn the automated risetime
measurement on and select channel 1.

7-10

Single-ended TDR Measurements
Establishing the Reference Plane and Normalizing

Press the Enter softkey.
You should see a display similar to the one in Figure 7-4. Note that the value of
Risetime is 40 ps.

Figure 7-4

7-11

Single-ended TDR Measurements
Establishing the Reference Plane and Normalizing

Change the Position until the reflected edge is displayed at the
approximate center of the display.
You should see a display similar to the one shown in Figure 7-5, however, the
risetime will depend on the quality of cable being used.

Figure 7-5

Note that the risetime of the reflected step is greater than the 40 ps of the
incident step. This difference is due to the losses in the cable and connectors.

1
2
3
4

Press the blue key followed by the 8 key to turn on the automated
Falltime measurement and select channel 1.
Press the Enter softkey.
Connect an SMA short to the end of the cable.
Press the Clear display key. Whenever an external connection is changed,
Clear display should be pressed to reset averaging.
You should see a display similar to the one in Figure 7-6.

7-12

Single-ended TDR Measurements
Establishing the Reference Plane and Normalizing

Figure 7-6

The falltime of the reflected step is also greater than the risetime of the incident
step. The next set of steps will establish normalization and the reference plane.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15

Change the Position until the negative edge is at the third graticule from
the left side of the display.
Press the blue key followed by the Clr key.
Press the TDR/TDT Setup key.
Press the Normalize response . . . softkey.
Press the Establish normalization & ref plane softkey.
Press the Continue softkey.
Replace the SMA short with a SMA 50 ohm load.
Press the Continue softkey.
Press the TDR normalize softkey to turn on the normalized trace.
Set the Risetime to 39 ps.
Remove the 50 ohm load from the end of the cable.
Press the Clear display key. Whenever an external connection is changed,
Clear display should be pressed to reset averaging.
Press the SETUP Channel 1/3 key.
Press the Display softkey to turn off the channel 1 display.
Press the blue key followed by the 7 key to turn on the automated
risetime measurement and select response 1.

7-13

Single-ended TDR Measurements
Establishing the Reference Plane and Normalizing

Press the Enter softkey.

Figure 7-7

The risetime of the normalized step is now approximately equal to the risetime
of the incident step.

1
2
3
4

Press the blue key followed by the 8 key to turn on the automated
measurement and select response 1.
Press the Enter softkey.
Connect an SMA short to the end of the cable.
Press the Clear display key. Whenever an external connection is changed,
Clear display should be pressed to reset averaging.
The falltime is now approximately equal to the risetime of the incident step.
Therefore, the normalization has removed cable loss effects by boosting the
higher frequencies.

7-14

Single-ended TDR Measurements
Establishing the Reference Plane and Normalizing

Performing TDT Normalization
The purpose of this section is to show the process used to normalize a coaxial
cable for TDT measurements. The following procedure shows how to perform
TDT normalization.

1
2
3
4
5
6
7
8
9
10
11
12

Connect a 1 m SMA coaxial cable from channel 1 to channel 2 of the
TDR plug-in module.
Press the STORAGE Setup menu key located above the display.
Press the Default setup softkey.
Press TDR/TDT Setup on the TDR plug-in module.
Press the Stimulus softkey and select 1 only (on for the
Agilent 54753A) in.
Press the Enter softkey.
Press the TDT 1 dest softkey and select channel 2. This selects the
destination channel for the TDT measurements.
Press the Enter softkey.
Press the Preset TDR/TDT softkey.
Press the SETUP Channel 2/4 key.
Change the Offset to 50 mV/div.
Press the SETUP Time base key located below the display.

7-15

Single-ended TDR Measurements
Establishing the Reference Plane and Normalizing

Change the Scale until you see two positive going edges on screen
(Figure 7-8).

Figure 7-8

1
2
3
4
5
6

Press the blue key followed by the 7 key to turn on the automated
risetime measurement.
Select channel 1 to turn on the automated risetime measurement for
channel 1.
Press the Enter softkey.
Press the blue key followed by the 7 key.
Select channel 2 to turn on the automated risetime measurement for
channel 2.
Press the Enter softkey (Figure 7-9).

7-16

Single-ended TDR Measurements
Establishing the Reference Plane and Normalizing

Figure 7-9

The risetime for channel 1 (incident step) is approximately 40 ps while the
risetime for channel 2 (received step) is greater than 40 ps. This is due to cable
and connector losses. We will now perform a TDT normalization.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15

Change the Scale to 500.0 ps/div.
Change the Position until the positive step on channel 2 is positioned at
the 3rd graticule from the left side of the display.
Press the TDR/TDT Setup key.
Press the Normalize response . . . softkey.
Press the TDR/TDT softkey to select TDT.
Press the Establish normalization & ref plane softkey.
Press the Continue softkey.
Press the TDT normalize softkey to select on.
Press the SETUP Channel 2/4 key.
Press the Display softkey to turn off channel 2 display.
Press the blue key followed by the Clr key.
Press the blue key followed by the 7 key and select response 2.
Press the Enter softkey.
Press the TDR/TDT Setup key.
Press the Normalize response . . . softkey.

7-17

Single-ended TDR Measurements
Establishing the Reference Plane and Normalizing

Set the Risetime to 39 ps.

Figure 7-10

The risetime of the normalized TDT step is approximately the same as the
risetime of the incident step. This shows that the effects due to cable and
connector losses are removed.

7-18

Single-ended TDR Measurements
Measuring Transmission Line Impedance

Measuring Transmission Line Impedance
This section shows how to measure transmission line impedance. To perform
the tasks in this section, you need the following:
• 1 good quality SMA cables one meter in length, such as the Agilent 8120-4948
cable.
• 1 demo board (54754-66503) supplied with the TDR plug-in.
• 1 SMA short supplied with the TDR plug-in.
• 1 SMA 50 ohm load supplied with the TDR plug-in.
• 1 Agilent 54754A or Agilent 54753A TDR Module.
The following procedure shows how to perform an impedance measurement.

1
2

Connect a 1 m SMA cable to channel 1 of the TDR plug-in module.
Connect the other end of the cable to the demo board’s single
transmission line connector that is closest to the narrow trace
(Figure 7-11).

Figure 7-11

7-19

Single-ended TDR Measurements
Measuring Transmission Line Impedance

1
2
3
4
5
6
7
8
9
10

Press the STORAGE Setup menu key above the display.
Press the Default setup softkey.
Press TDR/TDT Setup on the TDR plug-in module.
Press the Stimulus softkey and select 1 only (on for the Agilent 54753A).
Press the Enter softkey.
Press the Preset TDR/TDT softkey.
Press the SETUP Channel 1/3 key.
Change the Scale to 50.0 mV/div.
Press the Time base key.
Change the Position until you see a display similar to Figure 7-12.

Figure 7-12

Narrow Trace
Discontinuity
End of cable

Wide Trace
Discontinuity

The narrow trace discontinuity is more inductive than the nominal 50 ohm
transmission line while the wide trace discontinuity is more capacitive than the
nominal 50 ohm line. The end of the cable is the TDR step launch point into
the transmission line. If the cable was not good quality cable and had major
discontinuities of its own, it would be difficult to find the discontinuities of the
transmission line.

7-20

Single-ended TDR Measurements
Measuring Transmission Line Impedance

We will now measure the impedance of the narrow trace using the measured
voltage along the discontinuity (vd1) of the transmission line and the following
equation:
1+p
Z = 50Ω × -----------1–p

where:
– 200 mVp = vd1
---------------------------------200 mV

The 50 ohms is the impedance of the transmission line up to the narrow trace
and the 200 mV is the nominal height of the TDR step.
We will use the markers to measure the voltage as follows.

1
2

Press the SETUP Channel 1/3 key.
Change the Scale to 100.0 mV/div.
Before making any marker measurements, the on screen waveform must not be
clipped. Therefore, you must always adjust the channel scale until the waveform
is not clipped.

3
4
5
6
7
8
9
10

Press the SETUP Marker key.
Press the Mode softkey and select TDR/TDT from the list.
Press the Enter softkey.
Press the Reference softkey to select ref plane.
Press the Marker units . . . softkey.
Press the Vertical units softkey to select Volt.
Press the Done softkey.
Change the + Position until the + marker is over the narrow trace
discontinuity.

7-21

Single-ended TDR Measurements
Measuring Transmission Line Impedance

At the bottom of the display, read the number of volts for the narrow
trace discontinuity (Figure 7-13).

Figure 7-13

Narrow Trace
Voltage

In this case, the voltage, vd1, is approximately equal to 238.834 mV.
Substituting and solving for p we have:
238.834 – 200
p = ---------------------------------- = 0.19417
200

therefore,
1 + 0.19417
Z = 50Ω × ---------------------------- = 74.09565Ω
1 – 0.19417

Instead of calculating the impedance from the measured voltage, we can have
the oscilloscope calculate the impedance. Before we can do this we must first
establish normalization and the reference plane at the end of the cable
connected to the demo board. This requires a very accurate low reflection
26.5 GHz 50 ohm load, such as the Agilent 909D.

1
2
3
4

Press the TDR/TDT Setup key.
Press Normalize response . . . softkey.
Disconnect the cable from the demo board.
Press the Establish normalization & ref plane softkey.

7-22

Single-ended TDR Measurements
Measuring Transmission Line Impedance

5
6
7
8
9
10

Connect an SMA short to the end of the cable.
Press the Continue softkey.
Remove the SMA short and connect an SMA 50 ohm load to the end of
the cable.
Press the Continue softkey.
Remove the 50 ohm load and re-connect the cable to the demo board.
Press the Clear display key. Whenever an external connection is changed,
Clear display should be pressed to reset averaging.
Next we will use the oscilloscopes impedance measurement feature to measure
the impedance of the narrow trace discontinuity.

1
2
3
4
5
6
7
8
9
10

Press the SETUP Channel 1/3 key.
Press the Alternate scale . . . softkey.
Press the Units softkey and select Ohm.
Press the Enter softkey.
Press the Done softkey.
Press the SETUP Marker key.
Press the Reference softkey to select ref plane.
Press the Marker units . . . softkey.
Press the Vertical units softkey to select Ohm.
Press the Done softkey.

7-23

Single-ended TDR Measurements
Measuring Transmission Line Impedance

Change the + Position until the + marker is over the peak of the narrow
trace discontinuity (Figure 7-14).

Figure 7-14

The automated impedance measurement shows a value of 78.094 ohms which
agrees approximately with our manually calculated value. However, the
automated measurement is more accurate since it measured the actual step
height instead of assuming a 200 mV step.

7-24

Single-ended TDR Measurements
Measuring Transmission Line Percent Reflection

Measuring Transmission Line Percent Reflection
This section shows how to measure transmission line percent reflection with a
step risetime of 500 ps. To perform the tasks in this section, you need the
following:
• 1 good quality SMA cable one meter in length, such as the Agilent 8120-4948
cable.
• 1 each demo board (54754-66503) supplied with the TDR plug-in.
• 1 SMA short.
• 1 SMA 50 ohm load.
• 1 Agilent 54754A or Agilent 54753A TDR Module.
The following procedure shows how to perform a percent reflection
measurement.

1
2

Connect a 1 m SMA cable to channel 1 of the TDR plug-in module.
Connect the other end of the cable to the demo board’s single
transmission line connector that is closest to the wide trace
(Figure 7-15).

7-25

Single-ended TDR Measurements
Measuring Transmission Line Percent Reflection

Figure 7-15

1
2
3
4
5
6
7
8

Press the STORAGE Setup menu key above the display.
Press the Default setup softkey.
Press TDR/TDT Setup on the TDR plug-in module.
Press the Stimulus softkey and select 1 only (on for the Agilent 54753A).
Press the Enter softkey.
Press the Preset TDR/TDT softkey.
Press the Time base key.
Change the Position until the display is similar to Figure 7-16.

7-26

Single-ended TDR Measurements
Measuring Transmission Line Percent Reflection

Figure 7-16

Wide Trace
Discontinuity

Since the generated TDR step has a risetime of 35 ps, it is impossible to directly
measure the percent reflection to a 500 ps step. One way to measure the percent
reflection of a 500 ps step is by using the waveform math bandwidth limit
function. The bandwidth limit function effectively applies a low pass filter to
the selected waveform.
The following shows how to use the waveform math bandwidth limit function
to measure percent reflection.

1
2
3
4
5
6
7
8
9
10
11
12

Change the Scale to 1 ns/div.
Press the SETUP Math key.
Press the Function softkey to select f2.
Press the Define function . . . softkey.
Press the Operand 1 softkey and select channel 1.
Press the Enter softkey.
Press the Operator softkey and select bw limit.
Press the Enter softkey.
Change the Risetime to 500 ps.
Press the Done softkey.
Press the Display softkey to display the f2 function.
Press the SETUP Channel 1/3 key.

7-27

Single-ended TDR Measurements
Measuring Transmission Line Percent Reflection

Press the Display softkey to turn off channel 1 display.

Figure 7-17

The green waveform is function 2 which is the 500 ps filtered waveform of
channel 1. Because 500 ps is much greater than 35 ps, the overall system
risetime is approximately 500 ps.
We will now calculate the peak percent reflection using the measured voltage
and the following formula:
v – 200 mV
p max = 100 × ---------------------------200 mV

This method does not require establishing normalization or the reference plane
which is useful for quick measurements or when establishing normalization and
the reference plane is difficult to do.

1
2
3
4
5
6
7

Press the SETUP Marker key.
Press the Mode softkey and select TDR/TDT.
Press the Enter softkey.
Press the + Source softkey and select function 2.
Press the Enter softkey.
Press the Reference softkey to select ref plane.
Press the Marker units . . . softkey.

7-28

Single-ended TDR Measurements
Measuring Transmission Line Percent Reflection

8
9
10
11

Press the Vertical units softkey to select Volt.
Press the Done softkey.
Change the + Position until the + marker is over the negative peak of the
wide trace discontinuity.
Read the Y voltage value at the bottom of the display.

Figure 7-18

Wide Trace
Voltage

The measured voltage (v) is 155.20 mV. Substituting into the equation:
155.20 – 200
p max = 100 × ------------------------------- = – 22.40 %
200

We will now have the oscilloscope make this measurement for us. This requires
establishing normalization and the reference plane as follows.

1
2
3
4
5
6
7

Press the + Source softkey and select response 1.
Press the Enter softkey.
Press the SETUP Math key.
Press the Display softkey to turn the function 2 display off.
Press the TDR/TDT Setup key.
Remove the cable end connected to the demo board.
Press the Normalize response . . . softkey.

7-29

Single-ended TDR Measurements
Measuring Transmission Line Percent Reflection

8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25

Press the Establish normalization & ref plane softkey.
Connect an SMA short to the end of the cable.
Press the Continue softkey.
Remove the short from the end of the cable and connect an SMA 50 ohm
load to the end of the cable.
Press the Continue softkey.
Reconnect the demo board.
Set the Risetime to 500 ps.
Press the TDR normalize softkey to turn the normalized trace on.
Press the SETUP Channel 1/3 key.
Press the Alternate scale . . . softkey.
Press the Units softkey and select % reflect.
Press the Enter softkey.
Press the Done softkey.
Press the SETUP Marker key.
Press the Reference softkey to select ref plane.
Press the Marker units . . . softkey.
Press the Vertical units softkey to select % reflect.
Press the Done softkey.

7-30

Single-ended TDR Measurements
Measuring Transmission Line Percent Reflection

Figure 7-19

Wide Trace Percent
Reflection

The automated percent reflection at the + marker is seen at the bottom of the
display. The measured value of -22.870 % agrees approximately with the
previously calculated value of -22.40 %.

7-31

Single-ended TDR Measurements
Measuring Excess L/C

Measuring Excess L/C
This section shows how to measure excess inductance and capacitance. To
perform the tasks in this section, you need the following:
• 1 good quality SMA cable one meter in length, such as the Agilent 8120-4948
cable.
• 1 each demo board (54754-66503) supplied with the TDR plug-in.
• 1 SMA short.
• 1 SMA 50 ohm load.
• 1 Agilent 54754A or Agilent 54753A TDR Module.
The following procedure shows how to perform an excess L/C measurement.

1
2
3
4
5
6
7
8
9

Connect a 1 m SMA cable to channel 1 of the TDR plug-in module.
Press the STORAGE Setup menu key above the display.
Press the Default setup softkey.
Press TDR/TDT Setup on the TDR plug-in module.
Select 1 only (on for the Agilent 54753A) in the Stimulus menu.
Press the Enter softkey.
Press the Preset TDR/TDT softkey.
Press the Time base key.
Change the Position to bring the reflected step on screen. The display
should be similar to Figure 7-20.

7-32

Single-ended TDR Measurements
Measuring Excess L/C

Figure 7-20

1
2
3
4
5
6
7
8

Press TDR/TDT Setup on the TDR plug-in module.
Press the Normalize response . . . softkey.
Press the Establish normalization & ref plane softkey.
Connect an SMA short to the end of the cable.
Press the Continue softkey.
Remove the short from the end of the cable and connect an SMA 50 ohm
load to the end of the cable.
Press the Continue softkey.
Remove the 50 ohm load from the end of the cable.

7-33

Single-ended TDR Measurements
Measuring Excess L/C

Connect the cable to single transmission line connector closest to the
narrow trace (Figure 7-21).

Press the Clear display key. Whenever an external connection is changed,
Clear display should be pressed to reset averaging.
Press the SETUP Marker key.
Press the Mode softkey and select TDR/TDT.
Press the Enter softkey.
Press the Reference softkey to select ref plane.
Change the + Position until the + marker is on the right side of the positive

Figure 7-21

2
3
4
5
6
7

bump.
Change the X Position until the X marker is on the left side of the positive
bump.

7-34

Single-ended TDR Measurements
Measuring Excess L/C

Figure 7-22

+ Marker

X Marker

Excess L/C

At the bottom of the display is shown the excess L/C which is 7.111091 nH for
the narrow trace discontinuity. We will now measure the wide trace
discontinuity’s excess L/C.

7-35

Single-ended TDR Measurements
Measuring Excess L/C

1
2

Change the + Position until + marker is on the right side of the negative
bump.
Change the X Position until the X marker is on the left side of the negative
bump.

Figure 7-23

+ Marker

X Marker

Excess L/C

The excess L/C of the negative bump is 2.353307 pF. The negative bump is not
as well defined (or as square) as the positive bump. This is due to the filtering
effect that the positive bump (inductive section) has on the negative bump
(capacitive section). For example, the reflections off the capacitive section
have to pass back through the inductive section before it can be viewed. To get
a more accurate measure of the capacitive section, connect the cable to the
single transmission line closer to the wide trace.

7-36

Single-ended TDR Measurements
Measuring the Distance to a Discontinuity

Measuring the Distance to a Discontinuity
This section shows how to measure the distance to a capacitive or inductive
discontinuity. To perform the tasks in this section, you need the following:
• 1 good quality SMA cable one meter in length, such as the Agilent 8120-4948
cable.
• 1 each demo board (54754-66503) supplied with the TDR plug-in.
• 1 SMA short.
• 1 SMA 50 ohm load.
• 1 Agilent 54754A or Agilent 54753A TDR Module.
The following procedure shows how to perform a distance measurement.

1
2
3
4
5
6
7
8

7-37

Single-ended TDR Measurements
Measuring the Distance to a Discontinuity

Change the Position until the display is similar to Figure 7-24.

1
2
3
4
5

Press TDR/TDT Setup on the TDR plug-in module.
Press the Normalize response . . . softkey.
Press the Establish normalization & ref plane softkey.
Connect an SMA short to the end of the cable.
Remove the short from the end of the cable and connect an SMA 50 ohm
load to the end of the cable.
Press the Continue softkey.
Press the Done softkey.
Remove the 50 ohm load from the end of the cable.
Connect the channel 1 cable to the single transmission line connector
closest to the narrow trace on the demo board (Figure 7-25).

Figure 7-24

6
7
8
9

7-38

Single-ended TDR Measurements
Measuring the Distance to a Discontinuity

Figure 7-25

1
2
3
4
5
6

7
8

Press the Clear display key. Whenever an external connection is changed,
Clear display should be pressed to reset averaging.
Press the SETUP Marker key.
Press the Mode softkey and select TDR/TDT.
Press the Reference softkey to select ref plane.
Press the Marker units . . . softkey.
Press the Horiz units softkey to select meter.
Before the distance from the reference plane (the end of the cable) to the
narrow trace can be computed, either the dielectric constant or the velocity of
the transmission line must be known. For the demo board the dielectric
constant is approximately 3.945.
Change the Dielectric c to 3.945.
Press the Done softkey.

7-39

Single-ended TDR Measurements
Measuring the Distance to a Discontinuity

Change the + Position until the + marker is on the left side of the positive
bump.

Figure 7-26

+ Marker

Distance

The distance from the reference plane to the narrow trace is shown at the
bottom of the display (Figure 7-26).

7-40

Differential TDR Measurements

What you’ll find in this chapter
This chapter describes how to make differential TDR measurements and
describes the reason for the processes required to make these measurements.

CAUTION

8-2

Differential TDR Measurements
Differential TDR Features

Differential TDR Features
The Agilent 54754A TDR plug-in module is capable of performing differential
TDR measurements. These measurements include characterizing differential
microstrip lines, differential PC board traces, and differential cables. Because
differential TDR measurements are complex, the differential TDR plug-in
module has several features which make measurements easier.
The Preset TDR/TDT Feature
The Preset TDR/TDT feature prepares the oscilloscope for making TDR and
TDT measurements by automatically setting several menu fields. The Preset
TDR/TDT feature appears in the TDR/TDT Setup menu once a stimulus has
been selected.
Differential and Common Mode Stimulus Feature
The feature allows you to deliver to a TDR system either differential or common
mode stimulus. Differential stimulus launches a positive going step on channel
1 and an effective negative going step on channel 2. Common mode stimulus
launches a positive going step on both channels.
Responses to Differential and Common Mode Stimulus
Response controls are provided which show the differential or common mode
response of a TDR system under test stimulated by differential or common mode
stimulus. It is also possible to view the individual channels responses.
TDR Channel Deskewing Feature
Before accurate differential or common mode TDR measurements can be made,
it is necessary to deskew the two TDR step generators. Under the channel menu
for each TDR channel there is a Calibrate . . . softkey. When this softkey is
selected, it allows you to enter a channel Skew and a TDR skew parameter.
Channel Skew is a feature that allows the acquired signal to be moved in time
with respect to other acquired channels. Since sampling oscilloscopes do not
have negative time, this control only allows waveforms to be moved to the left
or more positive in time. This control is used in deskewing differential TDR and
in normal signal acquisition applications.
TDR skew is a feature that allows positioning of the actual TDR step. It has an
approximate range of ±400ps. The units of the control are in % of the maximum
range of ±100%. This control moves the position of the TDR step whereas the

8-3

Differential TDR Measurements
Differential TDR Features

channel Skew control moves the position of the acquired waveform. Both
controls are needed to properly deskew the TDR step generators for differential
TDR.
Differential TDR measurements require two cables to be connected from the
TDR channels to the device under test. These cables should be good quality
cables of equal length (within 1 ns delay of each other). The procedure for
deskewing the differential TDR to the ends of the cables is as follows:
• Insure that the open reflection of each TDR channel overlay each other with
no cables attached. If they are not, either do a plug-in calibration or adjust
the TDR Skew so they overlay each other.
• Attach the cables. Notice that unless the cables are matched in electrical
length, the open reflections now do not overlay each other. Using the ∆Time
auto measurement or manual markers, measure the delta time (skew)
between the TDR channels.
• Now go to the channel Skew control for the channel that is more to the right
on the screen. Adjust this control until the skew between the TDR channels
is reduced to half of what it was initially.
• Now use one of the TDR Skew controls from either TDR channel to reduce
the remaining skew to approximately 0.
The TDR step generators are now deskewed to the ends of the cables. The open
reflections off the end of the cables should overlay each other. Note that the
incident steps of the two channels probably do not overlay each other. They
won't unless the cables are the same electrical length. This is not a problem
because only the reflected signals need to be deskewed. The system in now
ready to make differential TDR measurements. If differential TDT
measurements are to be made, an additional deskew of the destination channels
needs to be done for TDT.
• Connect an additional two cables to the two TDT destination channels.
• Using some SMA or 3.5 mm F-F adaptors, connect the TDR channels to the
TDT channels. The transmitted steps should now be seen on the TDT
destination channels.
• Unless the electrical lengths of the two transmission paths happen to be
identical, the transmitted steps will not overlay each other.
• Now go to the channel Skew control for the channel that is more to the right
on the screen. Adjust this control until the skew between the TDT channels
is reduced to approximately 0.
The TDT paths are now deskewed. The device under test can now be connected
to the 4 cables, and the transmitted response observed.

8-4

Differential TDR Measurements
Measuring Differential and Common Mode Impedance

If markers are to be used on the differential responses, then a Establish ref plane
should be done after the system is deskewed. This will allow the channel scales
to be set to Ohm or % reflect, and allow the Reference under the Marker Mode
TDR/TDT menu to be set to ref plane. The marker will be based on the
average of the two channels’ reference planes.
Alternate Channel Scales
As in single ended TDR, the TDR channels can be set to have a vertical scale of
% reflection or ohms. If both TDR channels of a differential TDR measurement
are set to either % reflection or ohm, then the combined response, for example
Response 1 or Response 2, will also be in those units.

Measuring Differential and Common Mode Impedance
This section will show how to deskew the TDR step generators, establish the
reference planes, and measure the differential and common mode impedances.
To perform the tasks in this section, you need the following:
• 2 good quality SMA cables one meter in length, such as the Agilent 8120-4948
cable.
• 1 each demo board (54754-66503) supplied with the TDR plug-in.
• 1 SMA short.
• 1 SMA 50 ohm load.
• 1 Agilent 54754A TDR Module.
Deskewing Differential TDR Step Generators
This first thing that must be done before you can measure differential
impedance is to deskew the TDR step generators. There are two ways to deskew
the TDR step generators. One way is to calibrate the TDR plug-in module. The
other way is to use the channel TDR skew control. This section describes how
to use the TDR skew control to deskew the TDR step generators.
Refer to the Agilent 54750A, Agilent 83480A User’s Guide for information on
calibrating plug-in modules.
The following steps describe the deskewing process.

1
2
3

Press the STORAGE Setup menu key above the display.
Press the Default setup softkey.
Press TDR/TDT Setup on the TDR plug-in module.

8-5

Differential TDR Measurements
Measuring Differential and Common Mode Impedance

4
5
6
7
8

Press the Stimulus softkey and select differential.
Press the Enter softkey.
Press the Preset TDR/TDT softkey.
Press the SETUP Acquisition menu key below the display.
Change the # of averages from 16 to 4.
These steps set the oscilloscope to a known condition and activate the TDR
steps on channel 1 and channel 2. You should see a display similar to Figure 8-1.
Channel skew is set to 0.0 s for both channels. It is important to set these controls
to 0.0 s before adjusting the TDR skew otherwise the TDR step generator skew will
be incorrectly set.

Figure 8-1

Incident
Steps

Reflected
Steps

1
2
3
4
5

Press the SETUP Time base key.
Change the Scale to 100 ps/div.
Change the Position until the incident edge is off screen to the left.
Press the blue key followed by the milli key to turn on the ∆Time
measurement.
Press the Stop src softkey and select channel 2.

8-6

Differential TDR Measurements
Measuring Differential and Common Mode Impedance

Press the Enter softkey (Figure 8-2).

Press the SETUP Channel key for the channel whose reflected step is the
right-most step on the display.
Press the Calibrate . . . softkey.
Change the TDR Skew until the remaining ∆Time is approximately 0. You
will need to move the control slowly as it takes a short amount of time
for the waveform to settle.

Figure 8-2

2
3

Deskewing Differential Cables
The next thing that must be done before you can measure differential
impedance is to deskew the TDR step generators to the end of the cables. The
following steps describe the deskewing process.

1
2
3

Connect an SMA cable to the TDR plug-in channel 1.
Connect an SMA cable to the TDR plug-in channel 2.
Press the SETUP Time base key.

8-7

Differential TDR Measurements
Measuring Differential and Common Mode Impedance

Change the Position until the reflected edge is approximately centered
in the display (Figure 8-3).

Figure 8-3

Measured Skew

A ∆time (skew) value of 7 ps (one fifth the TDR step risetime of 35 ps) or less
is small enough that the skew will not introduce errors into TDR measurements.
However, for the purposes of demonstration, the following process shows how
to deskew the cables.

1
2
3
4

Press the SETUP Channel key for the channel whose reflected step is the
right-most step on the display.
Press the Calibrate.... softkey.
Change the Skew until the ∆Time is ½ its initial value.
Change the TDR Skew until the remaining ∆Time is approximately 0. You
will need to move the control slowly as it takes a short amount of time
for the waveform to settle (Figure 8-4).

8-8

Differential TDR Measurements
Measuring Differential and Common Mode Impedance

Figure 8-4

Establishing the Reference Plane
Since we want to measure the impedance of a differential line, we must establish
the reference plane so the scope can measure the height of the TDR step height.

1
2
3
4
5
6
7
8
9
10
11
12

Press the TDR/TDT Setup key.
Press the Establish ref plane softkey.
Connect an SMA short to the end of the cable on channel 1.
Press the Continue softkey.
Remove the SMA short from the end of the cable and connect an SMA
50 ohm load to the end of the cable.
Press the Continue softkey.
Remove the SMA 50 load from the end of the cable.
Connect an SMA short to the end of the cable on channel 2.
Press the Continue softkey.
Remove the SMA short from the end of the cable and connect an SMA
50 ohm load to the end of the cable.
Press the Continue softkey.
Remove the SMA 50 load from the end of the cable.

8-9

Differential TDR Measurements
Measuring Differential and Common Mode Impedance

Measuring Differential Impedance
We are now ready to measure the differential impedance of the differential line.

1
2

Connect the channel 1 cable to the differential line closest to the edge
of the board.
Connect the channel 2 cable to the other differential line (Figure 8-5).

Figure 8-5

1
2
3
4
5

Make sure that the switch on the demo board is in the off position
(toward the outside edge of the board away from the differential line).
Press the blue key followed by the Clr key.
Press the SETUP Time base key.
Change the Scale to 500 ps/div.
Change the Position until the reflected steps are in the middle of the
display (Figure 8-6).

8-10

Differential TDR Measurements
Measuring Differential and Common Mode Impedance

Figure 8-6

Reflected step at the
end of differential line
Start of
differential line
Waveform
separation

The portion of the waveforms starting at the left-hand of the display is where
the cables are connected to the differential line. The positive and negative going
steps are the reflected steps from the end of the differential line. The waveform
separation seen is due to the difference in impedance along the differential line.
The differential mode impedance for two 50 ohm uncoupled lines is 100 ohms.
Before performing the next step, be sure to wear a grounding strap connected to
the mainframe ground.
To verify which part of the waveform represents a certain part of the differential
line, touch the differential line with your finger. A bump in the waveform will
appear which represents the location of your finger along the differential line.
To measure the impedance of the differential line, use the following procedure.

1
2
3
4
5
6

Press SETUP Channel 1/3 key.
Press the Alternate scale . . . softkey.
Press the Units softkey and select Ohm.
Press the Enter softkey.
Press the Done softkey.
Press SETUP Channel 2/4 key.

8-11

Differential TDR Measurements
Measuring Differential and Common Mode Impedance

7
8
9
10

Press the Alternate scale . . . softkey.
Press the Units softkey and select Ohm.
Press the Enter softkey.
Press the Done softkey (Figure 8-7).

Figure 8-7

Both channel waveforms have positive going steps because ohms is always
positive in a passive system.
We will now measure the differential line impedance.

1
2
3
4
5

Press the TDR/TDT Setup key.
Press the TDR response 1 off . . . softkey.
Press the Response softkey and select differential.
Press the Enter softkey.
Press the Done softkey (Figure 8-8).

8-12

Differential TDR Measurements
Measuring Differential and Common Mode Impedance

Figure 8-8

Switch connection
Response 1
Waveform

Differential
Impedance

1
2
3
4
5
6

Press the SETUP Marker key.
Press the Mode softkey and select TDR/TDT.
Press the Enter softkey.
Press the + Source softkey and select response 1.
Press the Enter softkey.
Change the + Position until the + marker is on screen.
As the + marker moves along the waveform, the differential impedance at the
current + marker is shown at the bottom of the display. If you move the + marker
to the portion of the waveform representing the cable response, the impedance
is the sum of the two cable impedances (approximately 100 ohms depending
on the quality of the cable). The negative bump where the + marker is located
in Figure 8-8 is the parasitic capacitance due to the leg of the switch connected
to the trace.

8-13

Differential TDR Measurements
Measuring Differential and Common Mode Impedance

Measuring Common Mode Impedance
We are now ready to measure the common mode impedance of the differential
line. This section assumes that the differential mode impedance section has
been completed and that the oscilloscope settings have not been changed.

1
2
3
4
5
6
7

Press the TDR/TDT Setup key.
Press the Stimulus softkey and select common mode.
Press the Enter softkey.
Press the TDR response 1 differential . . . softkey.
Press the Response softkey and select common mode.
Press the Enter softkey.
Press the Done softkey (Figure 8-9).

Figure 8-9

The common mode impedance of two 50 ohm uncoupled lines is 25 ohms.

8-14

Differential TDR Measurements
Making Differential TDT Measurements

Making Differential TDT Measurements
This section will show how to deskew the TDR step generators, establish the
reference planes, and make differential and common mode TDT measurements.
To perform the tasks in this section, you need the following:
• 4 good quality SMA cables one meter in length, such as the Agilent 8120-4948
cable.
• 2 female-to-female SMA adapters.
• 1 each demo board (54754-66503) supplied with the TDR plug-in.
• 1 SMA short.
• 1 SMA 50 ohm load.
• 1 Agilent 54754A TDR Module.
• 1 two channel electrical plug-in module, for example, an Agilent 54751A or
Agilent 83483A 20 GHz Module.
Deskewing Differential TDR Step Generators
If you have just completed the Measuring Differential and Common Mode
Impedance section, set the stimulus to differential and go to the Deskewing the
TDT Channels section.

1
2
3
4
5
6
7
8

The first thing that must be done before you can make differential TDT
measurements is to deskew the TDR step generators to the end of the cables.
The following steps describe the deskewing process.
Press the STORAGE Setup menu key above the display.
Press the Default setup softkey.
Press TDR/TDT Setup on the TDR plug-in module.
Press the Stimulus softkey and select differential.
Press the Enter softkey.
Press the Preset TDR/TDT softkey.
Press the SETUP Acquisition menu key below the display.
Change the # of averages from 16 to 4.

8-15

Differential TDR Measurements
Making Differential TDT Measurements

These steps set the oscilloscope to a known condition and activates the TDR
steps on channel 1 and channel 2. You should see a display similar to Figure 8-10.
Channel skew is set to 0.0 s for both channels. It is important to set these controls
to 0.0 s before adjusting the TDR skew otherwise the TDR step generator skew will
be incorrectly set.

Figure 8-10

1
2
3
4
5
6

8-16

Differential TDR Measurements
Making Differential TDT Measurements

Figure 8-11

1
2
3

Press the SETUP Channel key for the channel whose reflected step is the
right-most step on the display.
Press the Calibrate.... softkey.
Change the TDR Skew until the remaining ∆Time is approximately 0. You
will need to move the control slowly as it takes a short amount of time
for the waveform to settle.
Deskewing Differential Cables
The next thing that must be done before you can measure differential
impedance is to deskew the TDR step generators to the end of the cables. The
following steps describe the deskewing process.

1
2
3

Connect an SMA cable to the TDR plug-in channel 1.
Connect an SMA cable to the TDR plug-in channel 2.
Press the SETUP Time base key.

8-17

Differential TDR Measurements
Making Differential TDT Measurements

Change the Position until the reflected edge is approximately centered
in the display (Figure 8-12).

Figure 8-12

Measured Skew

1
2
3
4

8-18

Differential TDR Measurements
Making Differential TDT Measurements

Figure 8-13

Establishing the Reference Plane
Since we want to measure the impedance of a differential line, we must establish
the reference plane so the scope can measure the height of TDR step height.

1
2
3
4
5
6
7
8
9
10
11
12

8-19

Differential TDR Measurements
Making Differential TDT Measurements

Deskewing the TDT Channels
In addition to the TDR step generators requiring deskewing the two TDT
destination channels require deskewing. Use the following procedure to deskew
the TDT destination channels.

1
2
3
4
5
6
7
8
9
10
11
12
13
14

Connect an SMA cable to channel 3 of the electrical plug-in module.
Connect another SMA cable to channel 4 of the electrical plug-in
module.
Press the TDR/TDT softkey to select TDT.
Press the Preset TDR/TDT softkey.
Connect one end of a female-to-female SMA adapter to the end of the
channel 1 cable and the other end of the adapter to the channel 3 cable.
Connect one end of a female-to-female SMA adapter to the end of the
channel 2 cable and the other end of the adapter to the channel 4 cable.
Press the Time base key.
Change the Position until positive going TDT step is in the middle of the
display.
Press the blue key followed by the milli key.
Press the Start src softkey and select channel 3.
Press the Enter softkey.
Press the Stop src softkey and select channel 4.
Press the Enter softkey.
Press the Enter softkey (Figure 8-14).

8-20

Differential TDR Measurements
Making Differential TDT Measurements

Figure 8-14

2
3
4

Press the electrical plug-in module’s SETUP Channel key whose waveform
is closest to the right side of the display. (For the example shown in
Figure 8-14, channel 4 is chosen.)
Press the Calibrate . . . softkey.
Change the Skew until the ∆Time is reduced to approximately 0 ps.
Press the Done softkey.
This completes the deskewing process for the TDT channels.
TDT Response Analysis
Analyzing TDT responses can help determine problems in differential lines. The
following procedure will show how to use TDT find a problem on the demo
board.

1
2
3
4

Remove the female-to-female adapters from both cable pairs.
Connect the channel 1 cable to the differential line closest to the single
transmission line on the demo board.
Connect the channel 3 cable to the other end of the differential line
closest to the single transmission line on the demo board.
Connect the channel 2 cable to the differential line closest to the edge
of the demo board.

8-21

Differential TDR Measurements
Making Differential TDT Measurements

Connect the channel 4 cable to the other end of the differential line
closest to the edge of the demo board(Figure 8-15).

1
2
3
4
5
6

Press the SETUP Channel 1/3 key.
Press the Display softkey to turn off the display of channel 1.
Press the SETUP Channel 2/4 key.
Press the Display softkey to turn off the display of channel 2.
Press the SETUP Time base key.
Change the Position until the positive and negative going steps are on the
third graticule from the left side of the display.

Figure 8-15

The transmitted steps’ edges, seen on channels 3 and 4, nearly overlay one
another (Figure 8-16).

8-22

Differential TDR Measurements
Making Differential TDT Measurements

Figure 8-16

1
2
3
4
5
6
7
8
9
10

Press the blue key followed by the Clr key.
Press the SETUP Channel 3 key of the electrical plug-in module.
Press the Display softkey to turn off the channel 3 display.
Press the SETUP Channel 4 key of the electrical plug-in module.
Press the Display softkey to turn off the channel 4 display.
Press the TDR/TDT Setup key.
Press the TDT response 3 off.... softkey.
Press the Response softkey and select common mode.
Press the Enter softkey.
Press the Done softkey.
You should see a response similar to that in Figure 8-17. The response is nearly
flat, as should be expected from a balanced differential line.

8-23

Differential TDR Measurements
Making Differential TDT Measurements

Figure 8-17

1
2
3
4
5

Press the SETUP Channel 3 key of the electrical plug-in module.
Press the Display softkey to turn on the channel 3 display.
Press the SETUP Channel 4 key of the electrical plug-in module.
Press the Display softkey to turn on the channel 4 display.
Change the demo board switch position to on.
This places a 10 pF capacitive load on one of the differential lines. Note that the
response waveform is no longer flat and the transmitted steps on channels 3
and 4 no longer overlay each other (Figure 8-18). This is due to the fact that
the differential lines are unbalanced.

8-24

Differential TDR Measurements
Making Differential TDT Measurements

Figure 8-18

This situation might occur on a clock distribution line of a digital PC board where
an input pin of a gate connected one side of line has an excess capacitance of
10 pF. The gate receiving the differential signal might produce a glitch or might
not switch properly.
Even though the extra capacitance was connected to only one of the differential
lines, the waveforms for both differential lines were affected. This shows that
the differential lines are coupled. If the differential lines were separated by a
greater distance, the capacitive load would only affect one side and not both
sides of the differential line.
While both differential lines show the effects of the capacitive load, the
differential line connected to the capacitive line shows the most change.
Knowing this allows you to find the differential line which has the problem.

8-25

8-26

TDR Fundamentals

Introduction

The most common method for evaluating a transmission line and its load
has traditionally involved applying a sine wave to a system and measuring
waves resulting from discontinuities on the line. From these
measurements, the standing wave ratio (SWR) is calculated and used as
a figure of merit for the transmission system. When the system includes
several discontinuities, however, the SWR measurement fails to isolate
them. Consider a case where the load is well matched to the transmission
line (i.e., ZL = Zo) but several connector joining segments of the line act
as minor discontinuities. This is a realistic situation since BNC
connectors, for example, will typically look like small inductors in series
with the line. The SWR measurement does not single out the component
or components causing the discontinuity; it only indicates their
aggregate effect. Any attempt to improve the system, becomes a trial
and error method of component substitution. In addition SWR
techniques fail to demonstrate whether one discontinuity is generating
a reflection of the proper phase and magnitude to cancel (at a particular
frequency) the reflection from a second discontinuity. When the
broadband quality of a transmission system is to be determined, SWR
measurements must be made at many frequencies, and this method soon
becomes very time consuming and tedious.
TDR avoids all of these disadvantages of the SWR method. TDR employs
a step generator and an oscilloscope in a system best described as
"closed- loop radar." A voltage step is propagated down the transmission
line under investigation, and the incident and reflected voltage waves
are monitored by the oscilloscope at a particular point on the line.
This echo technique (see Figure 9-1) reveals at a glance the characteristic
impedance of the line, and it shows both the position and the nature
(resistive, inductive, or capacitive) of each discontinuity along the line.
TDR also demonstrates whether losses in a transmission system are
series losses or shunt losses. All of this information is immediately
available from the oscilloscope's display. TDR also gives more meaningful
information concerning the broadband response of a transmission
system than any other measuring technique.
Since the basic principles of time domain reflectometry are easily
grasped, even those with limited experience in high frequency
measurements can quickly master this technique. This chapter attempts

9-2

TDR Fundamentals

a concise presentation of the fundamentals of TDR and then relates these
fundamentals to the parameters that can be measured in actual test
situations. Before discussing these principles further we will briefly
review transmission line theory.
Figure 9-1

Voltage vs Time at a Particular Point on a Mismatched Transmission Line Driven with a Step of
Height Ei

9-3

TDR Fundamentals
Propagation on a Transmission Line

Propagation on a Transmission Line
The classical transmission line is assumed to consist of a continuous structure
of resistors (R), inductors (L) and capacitors (C), as shown in Figure 9-2. By
studying this equivalent circuit, several characteristics of the transmission line
can be determined.
Figure 9-2

The Classical Model for a Transmission Line.

If the line is infinitely long and R, L, G, and C are defined per unit length, then
Z in = Z o =

R + jωL
-------------------G + jωL

where Zo is the characteristic impedance of the line. A voltage introduced at
the generator will require a finite time to travel down the line to a point x. The
phase of the voltage moving down the line will lag behind the voltage introduced
at the generator by an amount β per unit length. Furthermore, the voltage will
be attenuated by an amount α per unit length by the series resistance and shunt
conductance of the line. The phase shift and attenuation are defined by the
propagation constant γ,where
γ = α + jβ =

where

9-4

( R + jωL ) ( G + jωC )

α = attenuation in nepers per unit length
β = phase shift in radians per unit length

TDR Fundamentals
Propagation on a Transmission Line

The velocity at which the voltage travels down the line can be defined in terms
of β where:
ω
v p = ---- unit length per second
β

The velocity of propagation approaches the speed of light, vc, for transmission
lines with air dielectric. For the general case where εr is the dielectric constant.
vc
v p = -------εr

The propagation constant γ can be used to define the voltage and the current
at any distance x down an infinitely long line by the relations
Ex = E in e
I x = I in e

– ϒx

Since the voltage and the current are related at any point by the characteristic
impedance of the line
– ϒx

Ein e
Ein
= ------= Z in
Z o = -----------------– ϒx
I in
I in e

When the transmission line is finite in length and is terminated in a load whose
impedance matches the characteristic impedance of the line, the voltage and
current relationships are satisfied by the preceding equations.
If the load is different from Zo, these equations are not satisfied unless a second
wave is considered to originate at the load and to propagate back up the line
toward the source. This reflected wave is energy that is not delivered to the
load. Therefore, the quality of the transmission system is indicated by the ratio
of this reflected wave to the incident wave originating at the source. This ratio
is called the voltage reflection coefficient, ρ, and is related to the transmission
line impedance by the equation:
ZL – Zo
E
ρ = -----r = ----------------Ei
ZL + Z o

The magnitude of the steady-state sinusoidal voltage along a line terminated in
a load other than Zo varies periodically as a function of distance between a
maximum and minimum value. This variation called a standing wave, is caused

9-5

TDR Fundamentals
Step Reflection Testing

by the phase relationship between incident and reflected waves. The ratio of
the maximum and minimum values of this voltage is called the voltage standing
wave ratio, σ, and is related to the reflection coefficient by the equation
+ ρσ = 1-------------1– ρ

As has been said, either of the above coefficients can be measured with
presently available test equipment. But the value of the SWR measurement is
limited. Again, if a system consists of a connector, a short transmission line and
a load, the measured standing wave ratio indicates only the overall quality of
the system. It does not tell which of the system components is causing the
reflection. It does not tell if the reflection from one component is of such a
phase as to cancel the reflection from another. The engineer must make detailed
measurements at many frequencies before he can know what must be done to
improve the broadband transmission quality of the system.

Step Reflection Testing
A time domain reflectometer setup is shown in Figure 9-3.
Figure 9-3

A Time Domain Reflectometer

The step generator produces a positive-going incident wave that is applied to
the transmission system under test. The step travels down the transmission line
at the velocity of propagation of the line. If the load impedance is equal to the

9-6

TDR Fundamentals
Step Reflection Testing

characteristic impedance of the line no wave is reflected and all that will be seen
on the oscilloscope is the incident voltage step recorded as the wave passes the
point on the line monitored by the oscilloscope. Refer to Figure 9-4.
Figure 9-4

Oscilloscope Display When Er = 0

If a mismatch exists at the load, part of the incident wave is reflected. The
reflected voltage wave will appear on the oscilloscope display algebraically
added to the incident wave. Refer to Figure 9-5.
Figure 9-5

Oscilloscope Display When Er ≠ 0

Locating Mismatches
The reflected wave is readily identified since it is separated in time from the
incident wave. This time is also valuable in determining the length of the
transmission system from the monitoring point to the mismatch. Letting D
denote this length:
vp T
T
D = vp • --- = -------2
2

wherevp = velocity of propagation
T =
transit time from monitoring point to the mismatch and
back again, as measured on the oscilloscope (Figure 9-5)

9-7

TDR Fundamentals
Step Reflection Testing

The velocity of propagation can be determined from an experiment on a known
length of the same type of cable for example, the time required for the incident
wave to travel down and the reflected wave to travel back from an open circuit
termination at the end of a 120 cm piece of RG-9A/U is 11.4 ns resulting in a vp
= 2.1 x 10 cm/sec. Knowing vp and reading T from the oscilloscope determines
D. The mismatch is then located down the line.
Analyzing Reflections
The shape of the reflected wave is also valuable since it reveals both the nature
and magnitude of the mismatch. Figure 9-6 shows four typical oscilloscope
displays and the load impedance responsible for each.

9-8

TDR Fundamentals
Step Reflection Testing

Figure 9-6

TDR Displays for Typical Loads

These displays are easily interpreted by recalling this equation:
E
ZL – Zo
ρ = -----r = ----------------Ei
ZL + Z o

Knowledge of Ei and Er, as measured on the oscilloscope, allows ZL to be
determined in terms of Zo, or vice versa. In Figure 9-6, for example, we may
verify that the reflections are actually from the terminations specified.

9-9

TDR Fundamentals
Step Reflection Testing

Assuming Zo is real (approximately true for high quality commercial cable), it
is seen that resistive mismatches reflect a voltage of the same shape as the
driving voltage, with the magnitude and polarity of Er determined by the relative
values of Zo and RL.
Also of interest are the reflections produced by complex load impedances. Four
basic examples of these reflections are shown in Figure 9-7.
These waveforms could be verified by writing the expression for ρ(s) in terms
of the specific ZL for example:
R
Z L = R + sL , -------------------- , etc.
1 + RCs

multiplying ρ(s) by Ei ⁄ s the transform of a step function of Ei, and then
transforming this product back into the time domain to find an expression for
er(t). This procedure is useful, but a simpler analysis is possible without
resorting to Laplace transforms. The more direct analysis involves evaluating
the reflected voltage at time t = 0 and at time t = ∞ and assuming any transition
between these two values to be exponential. (For simplicity, time is chosen to
be zero when the reflected wave arrives back at the monitoring point.) In the
case of the series R-L combination, for example, at t = 0 the reflected voltage
is +Ei. This is because the inductor will not accept a sudden change in current;
it initially looks like an infinite impedance, and ρ = +1 at t = 0. Then current in
L builds up exponentially and its impedance drops toward zero. At t = ∞,
therefore er(t) is determined only by the value of R.
R–Z
ρ = ---------------o when ι = ∞
R + Zo

The exponential transition of er(t) has a time constant determined by the
effective resistance seen by the inductor. Since the output impedance of the
transmission line is Zo, the inductor sees Zo in series with R, and
L
ϒ = --------------R + Zo

9-10

TDR Fundamentals
Step Reflection Testing

Figure 9-7

WHERE

Oscilloscope Displays for Complex ZL

9-11

TDR Fundamentals
Step Reflection Testing

A similar analysis is possible for the case of the parallel R-C termination. At
time zero, the load appears as a short circuit since the capacitor will not accept
a sudden change in voltage. Therefore ρ = - 1 when t = 0. After some time,
however, voltage builds up on C and its impedance rises. At t = ∞, the capacitor
is effectively an open circuit:
ZL = R
R–Z
∴ ρ = ---------------o
R + Zo

The resistance seen by the capacitor is Zo in parallel with R, and therefore the
time constant of the exponential transition of er(t) is:
Zo R
---------------C
Zo + R

The two remaining cases can be treated in exactly the same way. The results of
this analysis are summarized in Figure 9-7.
Measuring the Time Constant of the Reflected Wave from Complex
Loads
When one encounters a transmission line terminated in a complex impedance,
determining the element values comprising ZL involves measuring two things:
1. Either er(t) at t = 0 or at t = ∞
and
2. The time constant of the exponential transition from er(0) to er(∞).
Number 1 is a straight forward procedure from the information given in
Figure 9-7. Number 2 is most conveniently done by measuring the time to
complete one half of the exponential transition from er(0) to er(∞). The time
for this to occur corresponds to 0.69 t, where t denotes the time constant of the
exponential. Adjusting the vertical sensitivity of the oscilloscope in the TDR
system so that the exponential portion of the reflected wave fills the full vertical
dimension of the graticule makes this measurement very easy (Figure 9-8).

9-12

TDR Fundamentals
Step Reflection Testing

Figure 9-8

Determining the Time Constant of a Reflected Wave Returning from a Complex ZL

Discontinuities on the Line
So far, mention has been made only about the effect of a mismatched load at
the end of a transmission line. Often, however, one is not only concerned with
what is happening at the load, but also at intermediate points along the line.
Consider the transmission system in Figure 9-9.
Figure 9-9

Transmission System

The junction of the two lines (both of characteristic impedance Zo) employs a
connector of some sort. Let us assume that the connector adds a small inductor
in series with the line. Analyzing this discontinuity on the line is not much
different from analyzing a mismatched termination. In effect, one treats
everything to the right of M in the figure as an equivalent impedance in series
with the small inductor and then calls this series combination the effective load
impedance for the system at the point M. Since the input impedance to the right

9-13

TDR Fundamentals
Step Reflection Testing

of M is Zo, an equivalent representation is shown in Figure 9-10. The pattern on
the oscilloscope is merely a special case of Figure 9-7A and is shown on Figure
9-11.
Figure 9-10

Equivalent System

Figure 9-11

Evaluating Cable Loss

Time domain reflectometry is also useful for comparing losses in transmission
lines. Cables where series losses predominate, reflect a voltage wave with an
exponentially rising characteristic, while cables where shunt losses
predominate, reflect a voltage wave with an exponentially-decaying
characteristic. This can be understood by looking at the input impedance of
the lossy line.
Assuming that the lossy line is infinitely long, the input impedance is given by:
Z in = Z o =

R + jωL
--------------------G + jωC

Treating first the case where series losses predominate, G is so small compared
to ωC that it can be neglected:
1
---

Z in =

9-14

R
+ jωL =
------------------jωC

L-  1 + --------R 2
--
C
jωL

TDR Fundamentals
Step Reflection Testing

Recalling the approximation (1 + x)a ≅ (1 + ax) for x<1, Zin can be approximated
by:
L
R
Z in ≅ ----  1 + ------------- when R< ωL
C
2jωL

Since the leading edge of the incident step is made up almost entirely of high
frequency components, R is certainly less than ωL for t = 0+. Therefore, the
above approximation for the lossy line which looks like a simple series R-C
network, is valid for a short time after t = 0. It turns out that this model is all
that is necessary to determine the transmission line's loss.
In terms of an equivalent circuit valid at t = 0+, the transmission line with series
losses is shown in Figure 9-12.
Figure 9-12

A Simple Model Valid at t = 0+ for a line with series losses

The response to a step of height E appears as shown in Figure 9-13, where
Zs = source impedance, and assumed resistive.

9-15

TDR Fundamentals
Step Reflection Testing

Figure 9-13

Transmission Line with Series Losses

In the case where Zs = R´, and τ = 2ZsC´ and the initial slope of ein(t) is given by:
E
E
m i = --------------- = ------R
8L
4R′C′

The series resistance of the lossy line (R) is a function of the skin depth of the
conductor and therefore is not constant with frequency. As a result, it is difficult
to relate the initial slope with an actual value of R. However, the magnitude of
the slope is useful in comparing cables of different loss.
A similar analysis is possible for a cable where shunt losses predominate. Here
the input admittance of the lossy cable is given by:
1
Y in = ------- =
Z in

G + jωC
--------------------- =
R + jωL

G + jωC
--------------------jωL

Since R is assumed small, re-writing this expression for Yin:
1
---

Y in =

C
G 2
---- 1 + ----------

L
jωC

Again approximating the polynomial under the square root sign:
C
G
Y in ≅ ----  1 + ------------- when G < ωC
L
2jωC

Going to an equivalent circuit () valid at t = 0+, ein(t) will look like Figure 9-15.

9-16

TDR Fundamentals
Step Reflection Testing

Figure 9-14

A Simple Model Valid at 0+ for a Line with Shunt Losses

Figure 9-15

Assuming

1
G′ = -------- , ""ι = 2G′L′ and the initial slope of ein(t) is given by:
Zs ″
EE - = – -----G
m i = – -------------4G′L′
8C

Again G depends on frequency, but relative loss can be estimated from the value
of mi.
A qualitative interpretation of why ein(t) behaves as it does is quite simple in
both these cases. For series losses, the line looks more and more like an open
circuit as time goes on because the voltage wave traveling down the line
accumulates more and more series resistance to force current through. In the
case of shunt losses, the input eventually looks like a short circuit because the
current traveling down the line sees more and more accumulated shunt
conductance to develop voltage across.

9-17

TDR Fundamentals
Step Reflection Testing

Multiple Discontinuities
One of the advantages of TDR is its ability to handle cases involving more than
one discontinuity. An example of this is Figure 9-16.
Figure 9-16

The oscilloscope's display for this situation would be similar to the diagram in
Figure 9-17 (drawn for the case where ZL > Zo > Z’o):
Figure 9-17

It is seen that the two mismatches produce reflections that can be analyzed
separately. The mismatch at the junction of the two transmission lines
generates a reflected wave, Er1, where
′

 Z o – Z o
- E i
Er1 = ρ 1 E i =  -----------------Z ′ + Z 
o

Similarly, the mismatch at the load also creates a reflection due to its reflection
coefficient
′

ZL – Z o
ρ2 = ------------------′
Z L + Zo

9-18

TDR Fundamentals
Step Reflection Testing

Two things must be considered before the apparent reflection from ZL, as shown
on the oscilloscope, is used to determine ρ2. First, the voltage step incident on
ZL is (1 + ρ1) Ei, not merely Ei: Second, the reflection from the load is
[ ρ 2 ( 1 + ρ 1 )E i ] = E rL

but this is not equal to Er2 since a re-reflection occurs at the mismatched
junction of the two transmission lines. The wave that returns to the monitoring
point is
′

′

Er2 = ( 1 + ρ 1 )E rL = ( 1 + ρ 1 ) [ ρ 2 ( 1 + ρ 1 )Ei ]

Since ρ1′ = -ρ1, Er2 may be re-written as:
2

E r2 = [ ρ2 ( 1 – ρ 1 ) ]Ei

The part of ErL reflected from the junction of Zo′ and Zo, such as ρ1′ErL, is again
reflected off the load and heads back to the monitoring point only to be partially
reflected at the junction of Zo′‚ and Zo. This continues indefinitely, but after
some time the magnitude of the reflections approaches zero.
Practical Handling of Multiple Discontinuities.
It is now seen that although TDR is useful when observing multiple
discontinuities, one must be aware of the slight complication they introduce
when analyzing the display. It is fortunate that most practical measuring
situations involve only small mismatches (e.g., Zo ≅ Zo′) and the effect of
multiple reflections is almost nil. Even in this situation, however, it is advisable
to analyze and clean up a system from the generator end. The reflection from
the first of any number of discontinuities is unaffected by the presence of others.
Therefore if it is remedied first and one then moves on to the second
discontinuity, the complications introduced by re-reflections will not exist.

9-19

TDR Fundamentals
Step Reflection Testing

Matching Source Impedance to Transmission Line Impedance
Until now nothing has been said concerning reflections that may have occurred
at the generator end of the transmission line. In general, the source impedance
of the step generator may not be equal to the characteristic impedance of the
transmission line it drives. When this is the case, voltage waves returning from
a mismatch or discontinuity in the system under test will be re- reflected at the
generator end and will complicate the analysis of the display. Referring to
Figure 9-18 and Figure 9-19, it is almost essential that the source impedance of
the step generator match the cable it drives. When this is the case, all rereflections returning from the system under test pass the oscilloscope's
monitoring point only once and are then absorbed in the source impedance of
the step generator.
Figure 9-18 is the oscilloscope display of a TDR system investigating a
transmission line terminated into an open circuit. The source impedance of the
step generator matches the characteristic impedance of the line under test
(Zs = Zo = 50 Ω).
Figure 9-18

A 50 Ω TDR System Testing a 50 Ω Line Terminated With an Open Circuit.

In Figure 9-19 this was not the case. Here the source impedance of the step
generator is 50 Ω and the line impedance is 75 Ω. The jump from a 50 Ω to a 75
Ω cable is evident and follows TDR rules. But the step from the 75 Ω cable to
the open circuit does not. Instead of jumping to a +1 reflection coefficient for
an open circuit, the trace actually exceeds that value.
The 50 Ω to 75 Ω mis-match caused the reflected wave returning from the open
circuit to be re-reflected at the source, thus launching a second incident wave
down the line. This second wave travels back to the monitoring point. The

9-20

TDR Fundamentals
Step Reflection Testing

second reflected wave, in turn, launches a third incident wave, down the line.
This process continues indefinitely, but unless the reflection coefficient at each
end is equal to ±1, the reflections decrease in magnitude and only the first few
are noticeable.
Figure 9-19

A 50 Ω TDR System Testing a 75 Ω Line Terminated With an Open Circuit Yields a Display That is
More Difficult to Interpret

Balun For measurements of transmission lines in the 200 Ω to 300 Ω
region, a balun is the best solution. A good balun will permit a 200 Ω line
to be tested without the danger of re-reflections from the 50 Ω source. A
broadband balun should be used so that the incident step is not appreciably
affected by sag or loss of risetime.

9-21

TDR Fundamentals
Step Reflection Testing

Matching L-Pad To completely eliminate the effect of multiple
reflections in a non 50 Ω system, use a simple matching L-pad. Refer to
Figure 9-20 and Figure 9-21.
Figure 9-20

L-Pad Matching 50 Ω Source to 75 Ω System Impedance

for Zo > 50 Ω:
Resistance in series with Zo, R 1 =
Shunt resistance, R2 =

Z o ( Z o – 50 )

50Z o
-----------R1

for Zo < 50 Ω:
Resistance in series with source, R2 =
Shunt resistance, R2 =

50 ( 50 – Z o )

50Z o
-----------R1

The incident step and the reflections will be attenuated considerably. Refer to
Figure 9-21. The sacrifice made to achieve the reflectionless connection is
sensitivity, and a loss of calibration. It is a good rule of thumb to use the L-pad
technique when major discontinuities are to be encountered and a tapered
section when small discontinuities are present (such as in cable testing).
The ±100% reflection points may be determined with the voltage markers and
by using a short and open at the transmission line's end.

9-22

TDR Fundamentals
Instrument Configuration

Figure 9-21

Α 50 Ω TDR System with a Matching L-Pad to the 75 Ω cable. The Amplitude Corresponding to Rho
= ±1 is Reduced Using the Matching L-Pad

Instrument Configuration
In the proceeding sections little consideration was given to the effects of the
configuration of the oscilloscope and step generator on the measurement. Now
lets examine this important part of the TDR measurement.
There are several different architectures for accomplishing a TDR
measurement. They are:
• Terminated step generator and through-line sampler.
• Terminated sampler and through-line step generator.
• Terminated sampler, terminated step generator, and power splitter.
Traditionally, TDR systems have used the terminated step generator and
through-line sampler architecture shown in Figure 9-22.

9-23

TDR Fundamentals
Instrument Configuration

Figure 9-22

THRU-LINE
SAMPLER

The step generator was implemented using a tunnel diode. Because a tunnel
diode is a low impedance device, it lends itself to a terminated configuration.
The major drawback of this architecture is that small reflections from the
terminated step generator are measured directly by the through-line sampler.
In the Agilent 54750 Series TDR systems, the terminated sampler and throughline step generator architecture in Figure 9-23 is used. In this case step
generation is accomplished using a switched current source driven from a step
recovery diode. This configuration is advantageous because small reflections
from the terminated sampler propagate back to the through-line step generator
where only a small portion of the already small reflection is sent back to the
terminated sampler and measured. None of the reflections from the step
generator or the sampler are measured directly, thus improving the system
performance.

9-24

TDR Fundamentals
Instrument Configuration

Figure 9-23

The terminated sampler, terminated step generator, and power splitter
architecture in Figure 9-24 is usable but is typically not used because both the
incident and reflected step are attenuated when they pass through the power
splitter. This decreases the system signal-to-noise ratio.
Figure 9-24

Risetime and Distance Resolution
The examples shown so far have assumed that the TDR step has zero risetime.
Practical TDR systems have a finite risetime for both the step generator and the
sampler. The effect of the finite risetime of the TDR system is to low-pass filter
the ideal (zero risetime) response of a given discontinuity with a filter that has
a risetime equal to the combined risetime of the step generator, sampler, and
test setup which is approximated by:
2

t r system ≅ ( tr step gen ) + ( tr sampler ) + ( test setup )

9-25

TDR Fundamentals
Instrument Configuration

The distance or time resolution of a Time Domain Network Analysis (TDNA)
system is related to the system risetime. The distance to a discontinuity is given
by:
d = υt

so that
c t
d = -------- ---oεr 2

where c is the speed of light, to is the Delta time between the incident step and
the reflected signal, and εr is the relative dielectric constant of the dielectric of
the transmission line. Therefore the distance that separates two discontinuities
is given by:
c t 2 – t1
∆d = -------- ------------εr 2

where t1 is the two way travel time to one discontinuity and t2 is the two way
travel time to second discontinuity. These two discontinuities become
indistinguishable when separated by a time (t2 - t1) of less than half the system
risetime. Therefore the minimum distinguishable distance between two
discontinuities is given by:
c t
d min = -------- ---r
εr 4

This means that with the Agilent 54750 Series TDR system risetime of 45 ps,
two discontinuities merge together and become indistinguishable at 3.5 mm for
an air dielectric. For practical systems the Agilent 54750 Series TDR systems
define the distance resolution to be twice this number, or 7 mm in air.
An example of how risetime affects distance resolution is an airline with two
washers (capacitive discontinuities) placed 2 mm apart on the center
conductor. The risetime needed to distinguish these as separate discontinuities
is given by:
c t
d min = -------- ---r
εr 4

or
4d min ε r
t r = ---------------------- = 26.7 ps
c

9-26

TDR Fundamentals
Instrument Configuration

The results of a TDR measurement, using normalization to decrease the system
risetime, on this airline at three different risetimes (40, 26, and 10 ps) is shown
in Figure 9-25. At 40 ps it is not possible to distinguish each discontinuity. At
26 ps the separate discontinuities begin to show. Finally, at 10 ps risetime both
discontinuities are clearly discernible.
Figure 9-25

Two Discontinuities 2 mm Apart can be Distinguished with a System Risetime of 10 ps

Small L's and C's
Figure 9-26 is an example of risetime effects for a series L discontinuity in a
50 Ω line. If the combined step generator and sampler risetime, tr system, is much
less than the risetime of the low-pass filter, tr lpf, created by the discontinuity
(where tr lpf ~ 2.2 × T where T = L ÷ 100 Ω), the result approaches the ideal as
shown in Figure 9-26 Plot A. If tr system ~ tr lpf, the result is as shown in
Figure 9-26 Plot C. If tr system > tr lpf, the result is as shown in Figure 9-26 Plot D.
Plot A: tr lpf = 100 × tr system = approaches ideal
Plot B: tr lpf = 10 × tr system
Plot C: tr lpf = tr system
Plot D: tr lpf = 1/10 × tr system
tr system = Combined risetime of the step generator and sampler.
tr lpf = Risetime of the low pass filter created by the discontinuity.

9-27

TDR Fundamentals
Instrument Configuration

Figure 9-26

System Risetime Affects the TDR Results

In analyzing TDR results so far, we have assumed that the time constant and
therefore risetime (tr lpf) created by a discontinuity were known and therefore
the value of the inductor L or capacitor C was also known. In most TDNA
measurements, only the combined step generator and sampler risetime (tr
system) and the reflected waveform are known. From this information, you may
want to derive the value of the discontinuity L or C. Again three cases exist in
this analysis. If tr system << tr lpf, then, as stated earlier, the TDR response
approaches the ideal result and a value for the L or C can be calculated (as in
Figure 9-7) from the measured time constant of the exponential decay or rise
to the final value. If tr system is of the same order of magnitude as tr lpf, then
calculating the L or C discontinuity becomes much more difficult due to the
interaction of the time constants.
One way to find the value of the L or C in this case is to use a SPICE simulation
program to model the response and vary the L or C value until the maximum
reflection on the SPICE simulation program and the TDR waveform match.
Accuracy depends on using realistic waveforms in the SPICE simulation. When
tr system >> tr lpf, such as small reflections, it is possible to relate the reflected
signal to the value of the L or C by assuming the L or C is driven by a current
or voltage source. This is equivalent to saying that for the frequencies contained
in the step, the impedance of a discontinuity does not significantly alter the
impedance of the circuit loading it. Using this approximation, we can relate the
maximum slope of the step to the maximum reflection from the discontinuity.
If the TDNA step is Gaussian (or can be normalized to an approximately
Gaussian step), then it can be shown that the maximum slope of the step is 27%

9-28

TDR Fundamentals
Instrument Configuration

higher than the slope of a line through the 10% and 90% risetime points. For
reflections less than 10%, the error resulting from this method is less than 3%,
not including measurement error of the TDR system.
For a series inductive discontinuity, the relationship between the reflected
signal and the inductor, L, is found as follows:

Since ωL << 100 Ω for frequencies of interest.
iL ~ vstep / 100 Ω where iL is the current through L and vstep is the open
circuit step amplitude

The voltage across the inductor therefore is:
di
L dv step
v L = L -------L = --------- -------------dt
100 dt

where vL is the voltage across the inductor, and
L dv step
v Lmax =  --------- ------------- 100 dt  max

As discussed above the max slope is
dv step
( 0.8 ) ( v step ) ( 1.27 )
1.016v step
 -------------= ----------------------- dt  max = -------------------------------------------trL
t rL

If the incident voltage at the inductor is viL and the reflected voltage at
the inductor is vrL then
v iL = 0.5v step or v step = 2v iL

and
v rL = 0.5v L or v L = 2v rL

Combining steps 2, 3 4 above produces
L v step
v Lmax = ---------  ---------100  dt  max
L 1.016v step
v Lmax = --------- -----------------------100
t rL
L 2v iL2v rLmax = --------- --------100 trL
100 ( v rL ) ( t rL )
L = --------------------------------1.016v iL

9-29

TDR Fundamentals
Instrument Configuration

Since
v rL
ρ = -----v iL

then
L = 98.4ρt r system

For a series L discontinuity ±3% when ρ ≤ to 10%
Using a similar derivation for a shunt C interline discontinuity, the
relationship between shunt C and the reflection is:
C = 0.0303ρtrL

For a shunt C discontinuity ±3% when ρ ≤ 10%
Cable Loss
As a step travels down a non-ideal transmission line, the higher frequencies are
attenuated by skin effect losses and dielectric losses. This distorts the step, and
is called cable loss. The effect of cable loss is shown in Figure 9-27 Plot A, which
shows the reflection of a short at the end of a 1 meter cable. Since cable loss
degrades the risetime of the TDR step, it can limit the distance resolution and
the accuracy of reflection measurements made at the end of a cable.
If fast risetime TDR measurements are needed, short interconnecting cables
should be used to reduce the effects of cable loss. The same reflection off a
short is shown in Figure 9-27 Plot B except now it is at the end of a very short
cable (approximately 5 cm).
Another way to reduce cable loss effects is to use normalization, if the TDR
system has this capability. Normalization to an ideal (approximating a Gaussian)
step removes the effects of cable loss to the point in the cable where a calibration
is done which establishes the reference plane from which TDR measurements
can be made without suffering effects from the cable. Calibration typically
involves connecting a 50 Ω termination and a short termination at the reference
plane. Figure 9-27 Plot C shows the results of normalizing the reflection of a
short at the end of a 1 meter cable. Normalization can also be used to remove
cable loss effects from transmission measurements.

9-30

TDR Fundamentals
Instrument Configuration

Figure 9-27

Short Cables (B) and Normalization (C) can Reduce the Effects of Cable Loss Seen in (A)

Multiple Discontinuities
Multiple discontinuities are another source of error in TDR measurements. A
discontinuity that occurs before the discontinuity of interest will cause a
degradation of risetime and accuracy of reflection measurements similar to
cable losses. Typically in a TDR system, if high accuracy and resolution are
needed to examine a particular discontinuity on a transmission line, the
reflections due to discontinuities that are before the one of interest must be
small. One example involves a transmission line with two discontinuities on it.
The first one has a maximum reflection coefficient of ρ1 and the second of ρ2.
The percent error in ρ2 due to ρ1 is:
ρ1

% error in ρ2

0.01

<0.25%

0.05

~2%

0.10

~6%

These results are computed values and are useful for estimating errors in
measurements. As with cable loss, you can remove the effects of multiple
discontinuities using normalization up to the point in the transmission line
where a calibration is done.

9-31

TDR Fundamentals
Instrument Configuration

Using TDR to Test Interconnects
One of the largest applications of TDR measurements is optimizing and testing
transmission line systems. An example of this involves the interface from a PC
board 50 Ω line to a thickfilm hybrid 50 Ω line. If the connection was made with
a 3 mm wire bond, then this would introduce a series inductive discontinuity
into the line. Where Lwb is the inductance of the wire bond. Refer to
Figure 9-28. A wire bond in free space would have an inductance of about
1.26 nH/mm but since it is located near the ground planes of the transmission
lines the inductance is somewhat lower. A measured inductance for typical wire
bonds on hybrids is about 1 nH/mm. If we assume this number, then the
inductance of the 3 mm wire bond is 3 nH. This then says that the low-pass filter
created by Lwb in the 50 Ω line has a risetime given by:
t r = 2.2T

where T = time constant = Lwb/100 Ω = 2.2 × 3 nH/100 Ω = 66 ps

9-32

TDR Fundamentals
Instrument Configuration

Therefore the risetime of the signal that is to pass through this discontinuity
should be greater than 66 ps if it is not to be significantly degraded. If the signal
to be transmitted through the discontinuity was a 350 ps risetime logic signal,
then the risetime degradation would be small. Even though the risetime
degradation is small there will be a significant reflection off the wire bond.
Assuming the reflection is less than 10%, then an equation predicts a maximum
reflection of:
L = 98.4ρt r system

or
3 nH
L
ρ = ----------------------------- = ------------------------------------ = 8.7
( 98.4 ) ( 350 ps )
98.4t r system

if the edge was an ideal Gaussian step.
Figure 9-28
Lwb

If the step is not ideal, this gives an approximate answer. This reflection may or
may not be a problem. If the circuit driving the 50 Ω line is source-terminated
in 50 Ω then this will not be a problem, but if it is driven from a current source
such as an open collector of a transistor, then it could. If it is desired to minimize
reflections of this discontinuity, then there are methods to do this. Refer to
Figure 9-28. If a TDR system is used to measure the transmission line, a response
would be seen as shown in Figure 9-30 which is an inductive response with a
max reflection of about 8.7% as predicted before. If the capacitance along the
wire bond could be increased, this would reduce the maximum reflection since
the wire bond section is moving towards a 50 Ω line. While it may not be possible
to do this, it is possible to increase the capacitance at the two ends of the wire
bond by widening the 50 Ω lines there. The circuit would now resemble the
circuit shown in Figure 9-29. When the value of C1 and C2 are chosen properly,
the TDR response of the system would now be as shown in
Figure 9-31. The value of C1 and C2 which minimizes the maximum reflection
is 0.6 pF which can be calculated from the equation.
L
Z o = 50Ω = ---C

9-33

TDR Fundamentals
Instrument Configuration

where
C = C1 + C2

therefore
L
C1 = C2 = -------- = 0.6 pF
2
Zo

The resultant circuit is actually a third order Butterworth filter. Refer to
Figure 9-31. The bandwidth of the resultant Butterworth filter has the same
bandwidth as the initial single pole filter. Since the risetime of the step to be
transmitted is much greater than the risetime of either the single pole or
Butterworth filter there will be little effect on the transmitted step.

9-34

TDR Fundamentals
Instrument Configuration

Figure 9-29
Lwb

Figure 9-30

Inductance of the Wirebond Causes a Reflection

9-35

Figure 9-31

Extra Capacitance can Compensate for the Wirebond's Inductance, Reducing the Reflection

9-36

Improving Time Domain Network
Measurements

Time Domain Network Analysis and
Normalization

Normalization, an error-correction process, helps ensure that time
domain network analysis measurements are as accurate as possible. The
Agilent 54750A Series digitizing oscilloscopes with TDR capability
include normalization as a standard feature. With normalization software
built into the oscilloscope, external controllers and variable edge speed
step generators or risetime converters are not needed. Normalization
not only enhances measurement accuracy, it simplifies the measurement
process.
Time domain network analysis (TDNA), includes both time domain
reflectometry (TDR) and time domain transmission (TDT)
measurements. TDNA measurement accuracies can be improved using
normalization techniques. This chapter discusses normalization and
assumes the reader is familiar with basic TDNA measurements.
Time domain reflectometry (TDR) sends a very fast edge down a
transmission line to a test device and then measures the reflections from
that device. The measured reflections can help to design signal path
interconnects and transmission lines in IC packages, PC board traces,
and coaxial connectors.
Time domain transmission (TDT) measurements are made by passing
an edge through the test device. Parameters typically measured are gain
and propagation delay. Transmission measurements also characterize
crosstalk between traces.
Imperfect connectors, cabling, and even the response of the oscilloscope
itself can introduce errors into TDNA measurements. Understanding
the effects of these errors, and more importantly, how to remove them,
will result in more accurate and useful measurements.
Normalization can be used in TDNA to remove the oscilloscope response,
step aberrations, and cable losses and reflections so that the only
response measured is that of the device under test (DUT). In addition,
normalization can be used to predict how the DUT would respond to an
ideal step of any arbitrary risetime.

10--2

Improving Time Domain Network Measurements
Sources of Measurement Error

Sources of Measurement Error
There are three primary sources of error in TDNA measurements: the cables
and connectors, the oscilloscope, and the step generator.
Cables and Connectors Cause Losses and Reflections
Cables and connectors between the step source, the DUT, and the oscilloscope
can significantly affect measurement results. Impedance mismatches and
imperfect connectors add reflections to the actual signal being measured.
These can distort the signal and make it difficult to determine which reflections
are from the DUT and which are from other sources.
In addition, cables are imperfect conductors that become more imperfect as
frequency increases. Cable losses, which increase at higher frequencies,
increase the risetime of edges and cause the edges to droop as they approach
their final value.
Figure 10-1 illustrates how cables and connectors affect TDNA measurements.
The upper waveform is the reflection of a step from a short circuit. Connections
cause the reflections at the peak of the step and along the baseline. Cable loss
yields the rounded transition of the step to its baseline level. Normalization can
correct the measured data, resulting in the lower waveform.

10--3

Improving Time Domain Network Measurements
Sources of Measurement Error

Figure 10-1

The top waveform shows distortions caused by cables and connectors. The bottom waveform
shows how normalization corrects for these distortions

The Oscilloscope as an Error Source
Oscilloscopes introduce errors into measurements in several ways. The finite
bandwidth of the oscilloscope translates to limited risetime. Edges with
risetimes that approach the minimum risetime of the oscilloscope are measured
slower than they actually are. When measuring how a device responds to a very
fast edge, the oscilloscope's limited risetime may distort or hide some of the
device response.
The oscilloscope can also introduce small errors that are due to the trigger
coupling into the channels and channel crosstalk. These errors appear as
ringing and other non-flatness in the display of the measurement channel
baseline and are superimposed on the measured waveform. They are generally
small and are only significant when measuring small signals.
The Step Generator as an Error Source
The shape of the step stimulus is also important for accurate TDNA
measurements. The DUT responds not only to the step, but also to the
aberrations on the step such as overshoot and non-flatness. If the overshoot is
substantial, the DUT's response can be more difficult to interpret.

10--4

Improving Time Domain Network Measurements
Sources of Measurement Error

The risetime of the step is also extremely important. In most cases the step
generator used for TDNA will have a fixed risetime. A hardware filter known
as a risetime convertor can be used in some systems to change the risetime.
To determine how the DUT will actually respond in it’s intended application,
you should test it at edge speeds similar to those it will actually encounter.
Consider the example of a BNC connector (Figure 10-2). Only about 3% of a
350 ps risetime edge (top waveform) is reflected by a BNC connector whereas
6% of a 100 ps risetime edge (middle waveform) is reflected and about 8% of a
50 ps risetime edge (bottom waveform) is reflected.
Figure 10-2

Variable edge speed helps determine the amount of reflection in actual applications. The top
waveform (tested to 350 ps) shows less reflection than the middle waveform (tested to 100 ps) or
the bottom waveform (tested to 50 ps)

In the case of this measurement, the results obtained using a 50 ps risetime step
stimulus do not apply for a connector that sees edges that are always slower
than 350 ps. The connector might be acceptable for 350 ps edges but not for
50 ps edges. Measurements made at inappropriate risetimes can yield invalid
conclusions.

10--5

Improving Time Domain Network Measurements
Removing Measurement Errors

Edge speed is also critical when using TDR to locate the source of a discontinuity
along a transmission line. Just as the limited risetime of the oscilloscope can
limit the accuracy of this kind of measurement, the risetime of the step source
can also limit accuracy.
The edge speed also affects the spatial resolution of a TDR measurement or its
ability to resolve discontinuities along a transmission line. This can be important
when trying to extract models for an interconnect.
The risetime of the measurement system is limited by the combined risetimes
of the oscilloscope and the step generator. It can be approximated by equation
1.
System risetime =

( Step risetime ) + ( Scope risetime ) + ( Test setup risetime )

(1)

In a system with zero minimum risetime, the response of a discontinuity would
not be attenuated at all. A real system has a limited risetime, which acts as a
lowpass filter. If the step stimulus used is too slow, the true nature of the
discontinuity may be disguised or may not even be visible. The cause may be
more difficult to physically locate. Notice in Figure 10-2 that as the risetime of
the step stimulus is decreased, the true nature of the reflection from the DUT
becomes more apparent.

Removing Measurement Errors
Waveform Subtraction has Limitations
In the past, waveform subtraction was used to reduce the effects of some of the
errors discussed above. It was convenient because many digitizing oscilloscopes
provided this feature without the aid of an external controller. A known good
reference device was measured and the reference waveform stored in memory.
The reference waveform could then be subtracted from the waveform measured
from the DUT. The result showed how the DUT response differed from the
reference response. This technique removed error terms common to both the
reference and DUT waveforms, such as trigger coupling, channel crosstalk, and
reflections from cables and connectors.
Waveform subtraction has, however, several shortcomings. First, it requires
that a known good reference DUT exists and is available to measure. In some
cases a good DUT may not be readily available or may not exist at all. Second,
the waveform which results from the subtraction process is a description of how

10--6

Improving Time Domain Network Measurements
Removing Measurement Errors

the DUT response differs from the reference response. Hence, there is no way
to view the actual DUT response without the errors introduced by the test
system.
Finally, the most significant shortcoming is that measurements are limited to
the risetime of the test system. Determining the DUT response at multiple
risetimes is cumbersome. Either multiple step generators or multiple risetime
convertors are necessary and a separate reference waveform is required for
each risetime.
Normalization Improves on Error Correction
A digital error-correction method known as normalization can significantly
reduce or remove all of the above types of errors from TDNA measurements.
Taking full advantage of its powerful internal microprocessor, the
Agilent 54750A Series digitizing oscilloscopes with TDR capability include
normalization as a standard feature.
Normalization can predict how the DUT will respond to an ideal step of the userspecified risetime. Only one step generator and one normalization process are
required. No risetime convertors are necessary, and the normalization
standards are not related to the DUT.
Unlike a risetime converter, normalization can also increase the bandwidth (i.e.,
decrease the risetime) of the system by some amount depending on the noise
floor. This means that when more bandwidth is critical, such as when trying to
locate a discontinuity along a transmission line, the waveform data acquired by
the oscilloscope can be "squeezed" for every bit of useful information it contains.
Examples of What Normalization Can Do
The following two examples illustrate what normalization can accomplish:
Example 1 Correcting for the TDR measurement errors introduced by
connecting hardware.
Consider trying to model a device at the end of some imperfect test fixture as in
Figure 10-3.

10--7

Improving Time Domain Network Measurements
Removing Measurement Errors

Figure 10-3

Test system with the device at the end of an imperfect test fixture

This example uses two identical printed circuit boards (PCBs) to model this
measurement. The PCBs have a 50 Ω trace on them with two discontinuities.
The first PCB represents the test fixture, and the second PCB represents the
DUT. The goal is to accurately measure the reflections caused by the DUT
(second PCB). Figure 10-4 is the unnormalized response of the system.
Figure 10-4

In an unnormalized measurement, the reflections from the DUT are masked by the imperfect test
fixture

The TDR response shows the reflections of the second PCB to be different from
the first PCB. TDR accurately measures the first discontinuity. But TDR
measures each succeeding discontinuity with less accuracy, as the transmitted
step degrades and multiple reflections occur. Thus the two identical boards
show different responses.

10--8

Improving Time Domain Network Measurements
Removing Measurement Errors

By defining a reference plane to be at the end of the test fixture (first PCB) and
then normalizing, the errors can be corrected.
Figure 10-5

Normalization uses a short, then a 50 Ω termination to define a reference plane and to generate a
digital filter

Figure 10-6

The normalized measurement corrects for the errors introduced by test fixture.

normalization first defines a reference plane and generates a digital filter. The
normalizing measurement then corrects for the errors introduced by the test
fixture. Notice how the normalized response of the second PCB (DUT) now
matches the response measured earlier of the nearly identical first PCB.

10--9

Improving Time Domain Network Measurements
Removing Measurement Errors

To further verify the accuracy of the normalization, the response of the second
PCB is measured without the first PCB.
Figure 10-7

The unnormalized response of the DUT, measured without the test fixture

Example 2 Resolving two discontinuities separated by 2 mm.
Normalization can improve the TDR's ability to resolve adjacent discontinuities.
Figure 10-8 shows the TDR measurement results of two capacitive
discontinuities 2 mm apart in an air dielectric. Note that at a system risetime
slower than 45 ps, the two discontinuities appear to be one. By normalizing the
response to a system risetime of 10 ps, both discontinuities can be seen.

10--10

Improving Time Domain Network Measurements
Removing Measurement Errors

Figure 10-8

Normalization improves the ability to distinguish two discontinuities by decreasing the system
risetime.
a. System risetime = 45 ps.
b. System risetime = 100 ps.
c. System risetime = 50 ps.
d. System risetime = 20 ps.
e. System risetime = 10 ps

Normalizing the Test System
The normalization process characterizes the test system and is made with all
cables and connections in place but without the DUT.
Removing Systematic Errors
The first part of TDNA normalization removes systematic errors due to trigger
coupling, channel crosstalk, and reflections from cables and connectors.
For TDR, this is done by replacing the DUT with a termination having an
impedance equal to the characteristic impedance of the transmission line. If
the termination is properly matched, all of the energy that reaches it will be
absorbed. The only reflections measured result from discontinuities along the
transmission line.
For TDT, this normalization step is done with nothing connected to the
oscilloscope input.

10--11

Improving Time Domain Network Measurements
Removing Measurement Errors

In both cases, the measured waveforms are stored and subtracted directly from
the measured DUT response before the response is filtered. Ideally, these
normalization waveforms are flat lines. Any non-flatness or ringing is
superimposed on the measured DUT response and represents a potential
measurement error source. These errors are not related to the magnitude of
the response of the DUT. Therefore, it is valid to subtract them directly. Notice
in Figure 10-9 that the errors present in the TDR normalization waveform
(bottom) are also visible in the measured DUT waveform (top), particularly at
the left side of the figure.
Figure 10-9

Errors present in the TDR normalization waveform (bottom) are visible in the measured waveform
(top)

10--12

Improving Time Domain Network Measurements
Removing Measurement Errors

Generating the Digital Filter
The second part of the normalization process generates the digital filter. Unlike
the errors removed by subtracting the first normalization signal, the errors
removed by the filter are proportional to the amplitude of the DUT response.
For the second part of the TDR normalization process, the DUT is replaced by
a short circuit. The frequency response of the test system is derived from the
measured short cal signal. Note that a short circuit should be used rather than
an open circuit. When a step hits an open circuit at the end of a real-world
transmission line, some of the energy is lost due to radiation rather than being
reflected. Of course there is no such thing as a perfect short either, but the
energy lost due to resistance in the short has a much smaller effect.
It is important that a good quality short be used, because the normalization
process assumes a perfect short circuit termination. Any non-ideal components
in the measured short cal signal are attributed to the test system. If any of the
non-ideal components are, in reality due to the short itself, the filter will attempt
to correct for error terms which do not exist in the test system. By attempting
to correct for errors which do not exist, the filter can actually add error terms
into the normalized measurement results.
In the second part of the TDT normalization process, the transmission throughpath is connected without the DUT. The frequency response of the test system
is then measured with the aid of the step stimulus. With this information, a
digital filter can be computed that will compensate for errors due to anomalies
in the frequency response of the test system.
Correcting for Secondary Reflections
Secondary reflections caused by the impedance mismatch between the test port
and the transmission media can also be corrected. In step TDNA, airlines can
separate the primary reflection from the secondary reflection. Time windowing
can then be used to remove the secondary reflection. In CW TDNA, a third
normalization is used.
The impedance mismatch between test port and transmission media reflects a
portion of the primary reflection back towards the DUT. A secondary reflection
from the DUT may then be measured. Secondary reflections are usually very
small.
Figure 10-10 shows the relative size of primary and secondary reflections. The
lower waveform is a copy of the upper waveform with the voltage scale greatly
expanded about the baseline to show more clearly the shape of the secondary
reflection. The DUT is a short circuit connected to the oscilloscope through a

10--13

Improving Time Domain Network Measurements
Removing Measurement Errors

BNC connector. A secondary reflection from the DUT is visible at the right end
of the baseline. Notice that the secondary reflection is indeed quite small. It has
a peak voltage value of about 1.5 mV at 40 ps risetime, which is about 0.75% of
the 200 mV incident step.
Figure 10-10

The lower waveform is a copy of the upper waveform with the voltage scale greatly expanded about
the baseline to show more clearly the shape of the secondary reflection

In step TDNA, a section of airline may be placed between the test port and the
DUT to provide time separation between the primary reflection and secondary
reflections. Figure 10-11 illustrates the use of this technique. A secondary
reflection is visible very close to the primary reflection in the top waveform. It
is difficult to tell them apart. A short section of airline was placed between the
DUT and the test port, resulting in the lower waveform. Note that the primary
and secondary reflections are clearly separated. When the primary and
secondary reflections are close together, the shapes of both may be distorted.
If they are adequately separated in time, as is the case in the lower waveform,
they no longer have a significant effect on each other.

10--14

Improving Time Domain Network Measurements
Removing Measurement Errors

Figure 10-11

By adding a section of airline between the test port and the DUT, you can more clearly distinguish
primary and secondary reflections

After an adequate separation has been achieved, a time window can be selected
which does not include the undesirable secondary reflections. Figure 10-12
illustrates the removal of secondary reflections from the measurement data
using time windowing. The top waveform in Figure 10-12 contains a secondary
reflection visible at the right end of the baseline. Note that moving the time
window to the left (less delay after the trigger) removes the secondary reflection

10--15

Improving Time Domain Network Measurements
Removing Measurement Errors

from the measurement without losing any of the primary reflection data. In CW
TDNA, time windowing is cumbersome, thus a third normalization
measurement is used.
Figure 10-12

Decreasing delay in the bottom waveform removes the secondary reflection shown at the right end
of the baseline in the top waveform.

The Digital Filter Corrects the Measured Response
The digital filter describes how the frequency response of the test system varies
from the ideal. If the signal was passed through the filter, the result would be
the ideal response. The filter removes errors by attenuating or amplifying and
phase-shifting components of the frequency response as necessary.
Consider, for example, overshoot on the step stimulus. The frequency response
of a DUT will include unwanted response to the overshoot. During
normalization, the filter will phase-shift the frequencies responsible for the
overshoot and thus attenuate the DUT response to the overshoot. The filter
works similarly to correct for cable losses due to attenuation of high frequencies.
It compensates for cable losses by boosting high frequency components in the
DUT response back up to their proper levels.
The digital filter defines an ideal impulse response. A good basis for a
normalization filter is a four-term, frequency-domain sum of cosines window,
W(f) (see equation 2) with the appropriate coefficients.

10--16

Improving Time Domain Network Measurements
Removing Measurement Errors

W(f) =

- , for  ------ < f < ---
∑ ak cos  ---------2
L 
2
2πfk

–L

(2)

k=0

where :

a0 + a1 + a2 + a3 = 1
L = the full width of the window in hertz
f = frequency in hertz

A window of this form may be selected that rolls off quickly and has an almost
Gaussian impulse response. The impulse response of the window defines the
ideal response. The Gaussian response is considered ideal because it has a
minimum settling time after a transition from one voltage level to another.
Minimizing the settling time minimizes the interference between closely-spaced
discontinuities, thus making them easier to see and analyze. The filter's
bandwidth, and therefore risetime, is determined by the choice of L, the width
of the sum of the cosines window. The actual normalization filter, F(f), is
computed by dividing the sum of cosines window by the frequency response of
the test system, S(f) (see equation 3). The frequency response is the Fourier
transform of the impulse response.
W( f )
F ( f ) = ----------S(f)

(3)

By varying the bandwidth of the filter, normalization can predict how the DUT
would respond to ideal steps of various risetimes. The bandwidth of the test
system is the frequency at which the frequency response is attenuated by 3 dB.
The response beyond the cutoff frequency is not zero; it is only attenuated
(Figure 10-13). By carefully changing the - 3 dB point in the frequency response,
the bandwidth can be increased or decreased.

10--17

Improving Time Domain Network Measurements
Removing Measurement Errors

Figure 10-13

Basic system frequency response

In the Agilent 54750A Series digitizing oscilloscopes with TDR capability, the
user-specified risetime determines the bandwidth of the filter. Decreasing the
bandwidth is accomplished by attenuating the frequencies that are beyond the
bandwidth of interest (Figure 10-14). Increasing the bandwidth requires more
consideration.
Figure 10-14

Normalized system frequency response (system bandwidth reduced)

10--18

Improving Time Domain Network Measurements
Removing Measurement Errors

To increase the bandwidth, the response beyond the initial -3 dB frequency
needs to be amplified. While this is a valid step, it is important to realize that
the system noise at these frequencies and at nearby higher frequencies is also
amplified (see Figure 10-15).
Figure 10-15

Normalized system frequency response (system bandwidth increased)

The limit to which the risetime of real systems may be extended is determined
by the noise floor. In real systems, there is a point beyond which the amplitude
of the frequency response data is below the noise floor. Any further increase
in bandwidth only adds noise.
Because waveform averaging reduces the initial level of the noise floor,
WAVEFORM AVERAGING SHOULD BE USED WHEN NORMALIZING.
An equation can be used to describe the filtering process. The test system
frequency response, S(f), can be thought of as the ideal frequency response
defined by the sum of cosines window, W(f) multiplied by an error frequency
response, E(f) (see equation 4). Further, the measured response of the DUT,
M(f), can be thought of as the DUT frequency response, D(f), multiplied by the
test system frequency response, S(f). Filtering is accomplished by multiplying
the measured frequency response of the DUT by the filter, F(f). N(f) is the
normalized (filtered) frequency response of the DUT. Equation 5 describes the
filtering process using the above definitions.

S ( f ) = W ( f )E ( f )

(4)

10--19

Improving Time Domain Network Measurements
Removing Measurement Errors

M(f) = D(f)

(5)

S ( f )N ( f ) = M ( f )F ( f )
N ( f ) = D ( f )S ( f )F ( f )
W(f )
N ( f ) = D ( f )W ( f )E ( f ) ---------------------W ( f )E ( f )
N ( f ) = D ( f )W ( f )

The normalized response is the DUT frequency response multiplied by the
frequency response of an ideal impulse. Note that the error response has been
removed, and that N(f) is an impulse response.
When N(f) is converted to the time domain, the result is ni(t), a normalized
impulse response.
Because a step stimulus is used, a normalized step response, ns(t), is desired.
An ideal step can be defined in the time domain by convolving w(t), the ideal
impulse response, with u(t), the unit step function. Given this modification,
equation 6 further describes the effect of the filtering process.
n i ( t ) = d ( t )w ( t )

(6)

n s ( t ) = n i ( t )u ( t )
n s ( t ) = d ( t ) [ w ( t )u ( t ) ]

The normalized response, ns(t), is the impulse response of the DUT convolved
with the ideal step defined by the convolution of w(t) with u(t). The result of
normalization is, therefore, the response of the DUT to an ideal step of risetime
determined by w(t). By varying the width, L, of W(f), normalization can predict
the response of the DUT at multiple risetimes based on a single-step response
measurement.

10--20

Improving Time Domain Network Measurements
Removing Measurement Errors

Putting It All Together
The actual normalization of a DUT response is accomplished in two steps. A
stored waveform, derived in the normalization and which represents the
systematic errors, is subtracted from the measured DUT waveform. This result
is then convolved with the digital filter to yield the response of the DUT,
normalized to an ideal step input with the user-specified risetime.
Figure 10-16 illustrates the power of normalization. It shows discontinuities in
a transmission path measured using TDR. The bottom waveform was measured
in a test system with an approximate risetime of 35 ps. The top waveform is the
bottom waveform normalized to 20 ps risetime. Note that in the bottom
waveform there appears to be only one inductive discontinuity. Using
normalization, it becomes obvious that there are actually two inductive
discontinuities. Because it is difficult to build a 20 ps risetime step stimulus with
a clean response and a test system with adequate bandwidth to measure it, this
measurement probably could not have been made without normalization.
Figure 10-16

The top waveform is the same signal as the bottom waveform, except that it has been normalized.
Normalization reveals that there are actually two inductive discontinuities, rather than one as
shown in the bottom waveform

10--21

10--22

Transmission Line Theory Applied to
Digital Systems

Introduction

Understanding the operation of transmission lines used in conjunction
with high speed MECL circuits is necessary in order to be able to
completely characterize system operation. This Chapter describes
transmission lines with respect to both line reflections and propagation
delay times. Also discussed will be the use of the Time Domain
Reflectometer (TDR) for measuring transmission line characteristics.

Transmission Line Design
A transmission line, as used with high speed MECL, is a signal path that exhibits
a characteristic impedance. Coaxial cables and twisted pairs have a defined
characteristic impedance and are commonly referred to as transmission lines.
Equally important, printed circuit fabrication of microstrip and stripline results
in closely-controlled transmission-line impedance.
Transmission lines may be approximated by the lumped constant
representation shown in Figure 11-1. The effect of the line resistance Ro, of the
line on characteristic impedance, Zo, is negligible, but it will cause some loss in
voltage at the receiving end of long lines. The inductance and capacitance of
the line in the presence of a ground plane are a function of the dielectric
medium, the thickness and width of the line, and the spacing from the ground
plane. The inductance and capacitance of the line can be measured using an LC
meter.
Figure 11-1

Equivalent Circuit of a Transmission Line

11--2

Transmission Line Theory Applied to Digital Systems
Transmission Line Design

Microstrip and strip lines may be treated as operating in the transverse electromagnetic (TEM) mode. Although microstrip propagation is not purely TEM
because of non-uniform dielectrics, for all practical purposes it can be treated
as TEM. The characteristic impedance of the line is:
Zo =

Lo ⁄ Co

and the propagation delay is:
t pd =

Lo Co = Zo Co

For a homogeneous medium the propagation delay is also equal to:
tpd =

µε =

µ o µ r ε o εr

Where µ is the permeability and ε is the permittivity of the medium. In
transmission lines, the relative permeability (µr) is unity,
µo = 4π × 10-7 henries/metre, and εo = 8.85 x 10-12 farads/metre.
Therefore, tpd = 1.017 ns/ft, εr is the relative dielectric constant. For microstrip
lines on glass epoxy boards εr = 3.0, and for strip lines εr = 5.0.
From transmission line theory for a lossless line, it can be shown that a signal
sent down a line of constant characteristic impedance will travel along the line
without distortion. However, when the signal reaches the end of the line, a
reflection will occur if the line is not properly terminated. Proper termination
requires the terminator value to be equal to the characteristic impedance of the
line.
Figure 11-2 shows a MECL gate driving a transmission line terminated in a load
resistor, RL. A negative-going transition on the input to the gate will result in a
positive-going transition at the NOR output. The MECL gate is essentially a VHF
linear differential amplifier with a bandwidth of 0.37 ÷ tr (MHz), where tr is the
risetime of the gate in nanoseconds. The effect of the capacitance of the
transmission line will not decrease the bandwidth or affect the risetime at the
MECL gate output. However, the signal at the end of a long transmission line
may be attenuated due to bandwidth limitations in the particular type of
transmission line used. For the purposes of this discussion, a long line is defined
as a line having a propagation delay larger than the risetime of the driving circuit
divided by two: TD > tr ÷ 2.

11--3

Transmission Line Theory Applied to Digital Systems
Transmission Line Design

Figure 11-2

MECL Gate Driving a Transmission Line

The circuit of Figure 11-2 can be redrawn as shown in Figure 11-3 to include
the equivalent circuit of the MECL gate. The resistor, Ro, is the output source
impedance (for MECL 10K/10KH it is 7 Ω, and MECL III it is 5 Ω). According
to theory, the risetime of the driving voltage source is not affected by the
capacitance of the transmission line. Except for skin effect and dielectric losses,
the signal will remain undistorted until it reaches the load.
Figure 11-3
R

Equivalent MECL Gate Output, Driving a Transmission Line

The equation representing the voltage waveform going down the line as a
function of distance and time can be written as:
v 1 (x,t) = v A ( t ) • u ( t – xt pd ) , for t < T D

11--4

(1)

Transmission Line Theory Applied to Digital Systems
Transmission Line Design

where:
Zo 
v A ( t ) = E s ( t )  ----------------Z + R 
o

vA(t) = voltage at point A,
x = the distance to an arbitrary point on the line,
l = the total line length,
tpd = the propagation delay of the line in ns/unit distance,
TD = l tpd,
u(t) = a unit step function occurring at t = 0, and
ES(t) = the source voltage at the sending end of the line.
When the incident voltage v1 reaches the end of the long line, a reflected voltage
v’1 will occur if RL ≠ Zo. The reflection coefficient at the load, ρL, can be obtained
by applying Ohm's Law.
The voltage at the load is v1 + v’1 which must be equal to (i1 + i’1) RL. But i1
= v1/Zo, and i’1 = -v’1/Zo (the minus sign is due to v’1, travelling toward the
source). Therefore,
v
v′
v 1 + v′ 1 =  -----1 – ------1- RL
Z

o Zo

(2)

By definition
v′
reflected voltage
ρ S = ---------------------------------------- = ------1incident voltage
v1

Solving for v’1 + v1 in equation 2, and substituting in the relation for ρL results in:
RL – Zo
ρ L = ----------------R L + Zo

(3)

Similarly, the reflection coefficient at the source is:

11--5

Transmission Line Theory Applied to Digital Systems
Transmission Line Design

R o – Zo
ρ S = ----------------Ro + Z o

(4)

By summing the incident voltage v1 (equation 1), together with similar voltage
contributions from the various orders of reflection (due to ρL and ρS), a general
equation for total line voltage can be written, and used to develop practical
design information:
v ( x, t ) = v A ( t ) [ u ( t – t pd x ) + ρ L u ( t – tdp ( 2l – x ) ) + ρ L ρ S u ( t – tpd ( 2l + x ) )
+ ρ L 2ρ S u ( t – t pd ( 4l – x ) ) + ρ L 2ρ S 2u ( t – t pd ( 4l + x ) ) + … ] + V dc

(5)

Note that as time progresses, the unit step function (u) brings successively
higher order reflection coefficient terms into v(x,t). Successive terms may be
positive or negative, depending on the resulting sign and so damped ringing can
occur. Equation 5 expresses the voltage at any point, x, on the line for any time,
t. The equation can be used graphically with a lattice diagram to find v(x,t).

Example 1
Figure 11-4 will be used to illustrate the lattice diagram method for finding
v(x,t) and the use of equation 5. The source impedance of the MECL III gate is
5 Ω, resulting in a reflection coefficient at the source of - 0.82 for a line
impedance of 50 Ω.

11--6

Transmission Line Theory Applied to Digital Systems
Transmission Line Design

Figure 11-4

Latice Diagram for a Typical Reflection Example

The load resistor is arbitrarily chosen to be 30 percent greater (65 Ω) than the
characteristic impedance (50 Ω) so that reflections will occur. The resulting
reflection coefficient at the load is ρL = + 0.13. Two vertical lines are drawn to
represent the input of the line, point A, and the output of the line, point B. A
line is drawn from point A to point B before t = 0 to represent the steady state
conditions. Note that for VCC = 2 V and VEE = - 3.2 V, the nominal logic levels
are approximately logic 0 = 0.3 V, and logic 1 = 1.14 V. (These power supply
conditions are used to permit convenient measurements when output resistors
are returned directly to ground). For steady state conditions, the line looks like
a short line with a resistance equal to Rdc. It can be assumed that Rdc is
negligible for this example.

11--7

Transmission Line Theory Applied to Digital Systems
Transmission Line Design

The voltage and current at points A and B are the same initially, as shown in
Figure 11-4. At t = 0, the voltage at the source switches from a logic 0 to a logic
1 level. The voltage term, vA(t), in equation 1 is:
Zo 
- = v 1 = 0.81 volts,
v A ( t ) = ( V′ OH – V′ OL )  ----------------Z + R 
o

where: (v’OH - v’OL) = ES(t) = internal voltage swings in the circuit = ∆VINT

Therefore, at time t = 0 a voltage waveform, V = 0.81 V, and a current, I = 16.2
mA, travel down the line as shown in Figure 11-4 by the line from t = 0 to t =
TD (TD is the time it takes for the wavefront to travel down the length of the
line). A line is drawn from t = TD to t = 2TD. Voltage and current values are as
indicated. Note that the reflected current is negative, indicating the current is
flowing back toward the source; the reflection coefficient for the current is a
minus one times the reflection coefficient for the voltage.
To find the voltage at point B for t = TD all the voltages entering and leaving
this point are summed. The same is done to determine the load current. The
process continues until the voltage at the load approaches the new steady state
condition in the example. This condition occurs when t = 3TD. (The steady
state logic 1 voltage is actually 1.13 V).
This example indicates that for a case in which the load resistor is 30% higher
than the characteristic impedance, 85 mV of overshoot and 10 mV of undershoot
would occur. Generally, as far as noise immunity is concerned, only the
undershoot need be considered. The typical noise immunity (or noise margin)
for a MECL circuit is greater than 200 mV. Since the undershoot in this example
was 10 mV, the typical noise immunity would exceed 190 mV. In actual system
design, typically more than 100 mV of undershoot can be tolerated. Regarding
overshoot, 300 mV can be tolerated, except in some early ac coupled flip-flops
(MECL I and II). This restriction insures that saturation of the input transistor
does not occur (if it did, the gate would slow down). If a 100 Ω load resistor
were used in Figure 11-4, the resulting overshoot would be about 220 mV and
the undershoot, about 80 mV. If the load resistor is twice the characteristic
impedance, the noise margin is typically 120 mV which is more than acceptable
for MECL circuits.

11--8

Transmission Line Theory Applied to Digital Systems
Transmission Line Design

A slightly different situation can exist when the output of the MECL gate
switches from a logic 1 to a logic 0. The output of the MECL gate will turn off if
the termination resistor, RL, is somewhat larger than the characteristic
impedance of the line. For the conditions in Figure 11-4, the output transistor
of the MECL gate will turn off at t = 0 for the negative going transition, when
RL > 70 Ω.
An equation for the value for RL at which the gate will turn off can be derived
as follows. The maximum voltage change at point A in Figure 11-4, (due to
turning off the output transistor) is the product of the dc current in the line and
the characteristic impedance of the line:
V′ OH
-(Z )
∆V A = I LINE ( Z o ) = ----------------R o + Zo o

The voltage at point A is also dependent on the internal resistance of the driving
gate Ro and the internal logic swing.
Zo
- ( ∆VINT )
∆VA = ----------------R o + Zo

Equating the two and solving for RL:
V′ OH ( R o + Z o )
- – Ro
R L = ----------------------------------∆VINT

(6)

Thus for the conditions given in Figure 11-4, the output transistor will turn off at
1.22 ( 5 + 50 )
t = 0 when R L = ------------------------------- – 5 = 70Ω is exceeded.
0.9

The case for which the MECL output turns off is not in itself a serious problem,
although it makes a thorough analysis more difficult. Two reflection coefficients
must be used at the sending end and a piecewise approach used in determining
the voltage reflections.

11--9

Transmission Line Theory Applied to Digital Systems
Transmission Line Design

Example 2
The condition for a negative-going transition will now be analyzed. Refer to
Figure 11-5. The steady state high logic level current is:
V′ OH
- = 11.6 mA
I dc = ----------------Ro + Z o

For the conditions shown in Figure 11-5, the use of equation 6 shows that the
load resistor is indeed larger than required to turn off the output transistor
during a negative transition.

11--10

Transmission Line Theory Applied to Digital Systems
Transmission Line Design

Figure 11-5

Lattice Diagram for Negative-Going Voltage Transition

To determine the voltage V1 at t = 0, the following equation results from the
application of Ohm's Law to the circuit:
V A + 3.2 + V 1
V1 =  I dc + -------------------------------- Zo


R

(7)

For the example shown, let RE = ∞, then:

11--11

Transmission Line Theory Applied to Digital Systems
Transmission Line Design

V 1 = ( – I dc )Z o

(8)

Solving equation 8, V1 = 0.58 V. The implication of this result is that stubbing
off the line with gate loads in a distributed fashion is not recommended, due to
the reduced initial voltage swing. However, it would be acceptable to lump the
loads at the end of the line.
Since the value of the load resistor is greater than the characteristic impedance,
the voltage swing at the load resistor is greater than v1 by the amount of ρLV1,
(in this example, 193 mV). When t = TD + T1, the voltage at B is equal to 0.387
V; so 82 mV of undershoot occurs. Undershoot on the falling edge is defined as
the amount of voltage step above the nominal logic 0 level of 0.305 V. Overshoot
in the low logic state is defined as the amount of voltage change below the logic
0 level.
Figure 11-6

Voltage Waveforms for Points A and B in Example 2

In Figure 11-6, the voltage waveforms at points A and B of this example are
shown as a function of time. To be more realistic, the waveform in the figure is
shown to be a negative-going ramp rather than an abrupt step function. The

11--12

Transmission Line Theory Applied to Digital Systems
Transmission Line Design

term, T1, is the amount of time it takes for the waveform at A to switch to the
level at which the output transistor turns off. The fall time of the signal would
have been longer by an amount equal to:
( 1.16 – 0.305 )
T′ 1 = ----------------------------------T 1
( 1.16 – 0.58 )

if the termination resistor had been 70 Ω or less.
The reflected voltage waveform leaving point B at t = TD arrives at point A at t
= 2TD. The source impedance is very high initially (ρS = + 1.0), with the output
transistor being in the off condition until the voltage at A falls to 0.32 V. Then,
the source impedance changes to 5 Ω (ρS = - 0.82).
The following formula may be used to determine the point at which the
transistor turns on:
∆Vsource = V 1 + ρ S V 1 = 2V1

(9)

where V1 is now the incident voltage approaching the source and ∆Vsource is
the change in voltage at the source necessary to turn the transistor on.
In this example the actual voltage change for this conduction to occur is
∆Vsource = 0.32 - 0.58 = - 0.26 V. Therefore, the voltage waveform approaching
the source (193 mV) can be broken into two signals V11 = -0.13, and
V12 = - 0.063 V. The reflected voltage due to V11 is V’11 = -0.13 V, and for V12,
the reflected voltage is V’12 = (-0.82) (-0.063) = 0.052 V. The two reflected
voltages of opposite polarity at point A going toward point B are the reason for
the increased overshoot of short duration at point B, when t = 3TD + (0.13 ÷
0.193) T1. Refer to Figure 11-6.
The steady state voltage reflection that occurs after t = 2TD + T1 is the sum of
-0.13 V and +0.052 V, equal to -78 mV as shown in Figure 11-5. The steady state
voltage reflection can be calculated using the relation:
(10)
Zo
Zo
 1 + -------
 1 + -------


R o2
R o2
V′ = ρ S2 ∆V source  ------------------ + ρ S1 V 1 – ∆V source  ------------------
 2 
 2 





11--13

Transmission Line Theory Applied to Digital Systems
Transmission Line Design

Equation 10 may be illustrated by solving for the steady state reflection voltage
at t = 2TD + T1:
50
50
 1 + ------
 1 + ------
∞
∞


V′ = ( 1.0 ) ( 0.32 – 0.58 ) --------------- + ( – 0.82 ) – 0.193 – ( 0.32 – 0.58 ) --------------- = 78 mV
 2 
 2 





From the analysis of Figure 11-5, it is concluded that the MECL gate can safely
drive the transmission line (Zo = 50 Ω) with a 100 Ω load resistor and with the
gate loads lumped at the end of the line, since less than 100 mV of undershoot
occurs. The remaining noise margin will be typically greater than 100 mV.

11--14

Transmission Line Theory Applied to Digital Systems
Signal Propagation Delay for Microstrip and Strip Lines with Distributed or Lumped
Loads

Signal Propagation Delay for Microstrip and Strip Lines
with Distributed or Lumped Loads
The propagation delay, tpd, has been shown to be 1.77 ns/ft for microstrip lines
and 2.26 ns/ft for strip lines, when a glass epoxy dielectric is the surrounding
medium. The propagation delay time of the line will increase with gate loading
and the altered delay can be derived as follows. The unloaded propagation delay
for a transmission line is:
t pd =

Lo Co

If a lumped load, Cd, is placed along the line, then the propagation delay will be
modified to t’pd:
(11)
t′ pd =

Lo ( Co + Cd ) =

Lo Co

C
C
1 + -----d- = t pd 1 + -----dC0
Co

where Lo and Co are the intrinsic line inductance and capacitance per unit
length.
Therefore, the signal propagation down the line will increase by the factor of:
C
1 + -----dCo

A MECL gate input should be considered to have 5 pF of capacitance for ac
loading considerations (includes stray capacitance). If 4 gate loads are placed
on a 1 foot signal line, then the distributed capacitance, Cd, is equal to 20 pF/ft
or 1.67 pF/in. As an example, a propagation delay increase is to be found for a
50 Ω microstrip line on a glass epoxy board. Given a line width of 25 mils, the
dielectric material would have a thickness of 15 mils to yield Zo = 50 Ω and a
capacitance of 35 pF/ft. Therefore, the modified propagation delay would be:
t′ pd = 1.77 ns/ft

20
1 + ------ = 2.21 ns/ft
35

11--15

Transmission Line Theory Applied to Digital Systems
SignalPropagationDelayforMicrostripandStripLineswithDistributedorLumpedLoads

For a 50 Ω strip line on a glass epoxy board with a 15 mil spacing between the
strip line and ground plane, a 12 mil width would be required, and the strip line
would exhibit a capacitance of 41 pF/ft. The modified propagation delay for
such a strip line would be:
t′ pd = 2.26 ns/ft

1 + 20
------ = 2.75 ns/ft
41

Notice that the propagation delay for the strip line and the microstrip line
change by approximately the same factor when the separation between the line
and ground plane, and the characteristic impedance are the same. However,
the line width of the strip line is less (by a factor of 2) than the microstrip line
for the same characteristic impedance.
It should be noted that to obtain the minimum change and lowest propagation
delay as a function of gate loading, the lowest characteristic impedance line
should be used. This will result in the largest intrinsic line capacitance. With
MECL 10K/10KH the lowest impedance that can be used is about 35 Ω
(VTT = - 2.0 V and RTT = 35 Ω).
According to theory, when an open line (stub) is driven by a pulse, the resultant
undershoot and ringing are held to about 15 percent of the logic swing if the
two way delay of the line is less than the risetime of the pulse. The maximum
line length, lmax may be calculated using the equality:
tr
- (inches)
l max = ----------2t′ pd

where tr is the risetime of the pulse in nanoseconds, and t’pd is the modified
propagation delay in nanoseconds/inch from equation 11.
A quadratic equation for maximum line length for G-10 fiber glass epoxy
microstrip conductors may be written in terms of CD, Co, and tr as

CD
2
max + ------- l max – 11.1t r = 0 (for microstrip lines)
Co

where CD is the total gate capacitance.

11--16

(12)

Transmission Line Theory Applied to Digital Systems
Signal Propagation Delay for Microstrip and Strip Lines with Distributed or Lumped
Loads

An equation for maximum open line length for a strip line (using G-10 fiber glass
epoxy material) can be written in a similar fashion as follows:
(13)
C
l 2 max + ------D- l max – 7.1t 2 r = 0 (for strip lines)
Co

Using the lattice diagram, it has been found that the rule of thumb used to derive
equations 12 and 13 should be modified for an open line because the incident
voltage doubles at the end of the line. This results in a faster risetime at the
receiving end of an unloaded line than at the driving end. An approximate value
of maximum open line length can be generated from equations 12 and 13 if the
risetime that is substituted into the equations is multiplied by an adjustment
factor, 0.75. This maintains an approximate overshoot and undershoot of less
than 35% and 12% respectively.
To demonstrate how equations 12 and 13 may be used, the maximum open line
length will be computed for a 50 Ω line with a fanout of one MECL 10K gate.
Using the equation tpd = Zo Co, the line capacitance, Co, is found to be
C = 2.96 pF/in for microstrip, and Co = 3.76 pF/in for strip line. For a fanout of
one, CD is equal to 5 pF when the device is in a socket. The risetime for MECL
10K is 3.5 ns which means that a value of tr = 0.75 X 3.5 = 2.6 ns should be used
in the equations. Solving equations 12 and 13, lmax for a 50 Ω microstrip line
and lmax = 6.2 inches for a 50 Ω strip line.
Equations 12 and 13 can be very useful in finding the approximate maximum
line length under various conditions. Suggested maximum open line lengths for
MECL 10K/10KH and MECL III are tabulated in tables Table 11-1, Table 11-2,
and Table 11-3 for various fanouts and line impedances. For these tables, line
lengths are chosen to limit overshoot to 3.5% of logic swing and undershoot to
12%. Note that the tables give the maximum line lengths for fanouts of 1, 2, 4,
and 8 for various types of lines with a wide range of characteristic impedances.

11--17

Transmission Line Theory Applied to Digital Systems
SignalPropagationDelayforMicrostripandStripLineswithDistributedorLumpedLoads

Table 11-1
Maximum Open Line Length for MECL 10,100 (Gate Rise Time = 3.5 ns)
Zo (OHMS)

MICROSTRIP
(Propagation Delay
0.148 ns/in.)

STRIPLINE
(Propagation Delay
0.188 ns/in.)

BACKPLANE
(Propagation Delay
0.140 ns/in.)

FANOUT = 1
(2.9pF)

FANOUT = 2
(5.8pF)

FANOUT = 4
(11.6pF)

FANOUT = 8
(23.2pF)

lMAX (IN)

8.3

7.5

6.7

5.7

7.0

6.2

5.0

4.0

6.9

5.9

4.6

3.6

6.6

5.7

4.2

3.3

6.5

5.4

3.9

3.0

100

6.3

5.1

3.6

2.6

6.5

5.9

5.2

4.5

5.6

4.9

3.9

3.2

5.3

4.7

3.6

2.8

5.2

4.4

3.3

2.6

5.1

4.3

3.1

2.4

100

4.9

4.0

2.8

2.1

100

6.6

5.4

3.8

2.8

140

5.9

4.3

2.8

1.9

180

5.2

3.6

2.1

1.3

11--18

Transmission Line Theory Applied to Digital Systems
Signal Propagation Delay for Microstrip and Strip Lines with Distributed or Lumped
Loads

Table 11-2
Maximum Open Line Length for MECL 10,200, MECL 10H100, 10H210, 10H211 (Gate Rise Time = 2 ns)
Zo (OHMS)

MICROSTRIP
(Propagation
Delay 0.148 ns/in.)

STRIPLINE
(Propagation
Delay 0.188 ns/in.)

BACKPLANE
(Propagation
Delay 0.140 ns/in.)

FANOUT = 1
(3.3pF)

FANOUT = 2
(6.6pF)

FANOUT = 4
(13.2pF)

FANOUT = 8
(26.4pF)

lMAX (IN)

3.5

2.8

1.9

1.2

3.2

2.3

1.5

0.8

3.0

2.2

1.3

0.7

2.9

2.0

1.2

0.6

2.8

1.9

1.0

0.5

100

2.6

1.8

0.9

0.4

2.8

2.2

1.5

1.0

2.5

1.9

1.2

0.6

2.4

1.7

1.1

0.6

2.3

1.6

0.9

0.5

2.2

1.5

0.8

0.4

100

2.0

1.4

0.7

0.3

100

2.8

1.8

0.9

0.4

140

2.4

1.4

0.5

0.3

180

2.0

1.0

0.3

0.1

11--19

Transmission Line Theory Applied to Digital Systems
SignalPropagationDelayforMicrostripandStripLineswithDistributedorLumpedLoads

Table 11-3
Maximum Open Line Length for MECL III, MECL 10H209 (Gate Rise Time 1.1 ns)
Zo (OHMS)

MICROSTRIP
(Propagation
Delay 0.148 ns/in.)

STRIPLINE
(Propagation
Delay 0.188 ns/in.)

BACKPLANE
(Propagation
Delay 0.140 ns/in.)

FANOUT = 1
(3.3pF)

FANOUT = 2
(6.6pF)

FANOUT = 4
(13.2pF)

FANOUT = 8
(26.4pF)

lMAX (IN)

1.6

1.1

0.7

0.6

1.4

0.8

0.5

0.4

1.3

0.8

0.4

0.3

1.2

0.7

0.4

0.2

1.1

0.6

0.3

0.2

100

1.0

0.5

0.2

0.1

1.2

0.8

0.6

0.5

1.1

0.7

0.4

0.3

1.0

0.6

0.3

0.2

0.9

0.6

0.3

0.2

0.9

0.5

0.2

0.1

100

0.8

0.4

0.2

0.1

100

1.1

0.6

0.2

0.1

140

0.8

0.3

0.0

180

0.6

0.2

0.0

The maximum line lengths are also given for various characteristic impedances
in the backplane. The characteristic impedance of the backplane should be
between 100 Ω and 180 Ω if a ground screen is used. For MECL 10K from
Table 11-1, 5.9 inches of open backplane wiring can be driven for a fanout of one.
It should be remembered that these line lengths are based on 100 mV maximum
undershoot, and are not absolute maximum lengths with which MECL circuits
will operate. It is possible to use longer unterminated lines than shown — the
tradeoff being an associated loss of noise immunity due to increased ringing.

11--20

Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements

From these calculations, it can be concluded that lower impedance lines result
in longer line lengths before termination is required. The lower impedance lines
are preferred over higher impedance lines because longer open lines are
possible, and the propagation delay down the line is reduced. In addition, more
stubbed-off gate loads can be driven with a terminated line due to its higher
capacitance per unit length.

Microstrip Transmission Line Techniques Evaluated
Using TDR Measurements
The time domain reflectometer (TDR) employs a step generator and an
oscilloscope in a system which might be described as "closed-loop radar. Refer
to Figure 11-7. In operation, a voltage step is propagated down the transmission
line under investigation. Both the incident and reflected voltage waves are
monitored on the oscilloscope at a particular point on the line.
Figure 11-7

Time Domain Reflectometer

For the examples the incident voltage setup, E1, is a positive edge with an
amplitude of 1 V and a risetime of 30 ps. It is generated from a source impedance
of 50 Ω. Also, the output edge has very little overshoot (less than ±5%).
This TDR technique reveals the characteristic impedance of the line under test.
It shows both the position and the nature (resistive, inductive, or capacitive) of
each discontinuity along the line, and signifies whether losses in a transmission
system are series losses or shunt losses. All of this information is immediately
available from the oscilloscope's display. An example of a microstrip line
evaluated with TDR techniques is shown below:

11--21

Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements

TDR Example 1
Given the following:
Board material:
Dielectric thickness:
Copper thickness:
Dielectric constant:

Norplex Type G-10
h = 0.062 inch;
t = 0.0014 inch;
εr = 5.3.

The formula for the characteristic impedance is:
87
5.98h
Z o = ------------------------- ln  -------------------

0.8w
+ t
εr + 1.41

(14)

For a line width, w = 0.1 inch, the characteristic impedance of the line is
calculated to be 51 Ω. A board was fabricated as shown in Figure 11-8 to the
dimensions specified above. Figure 11-8 and Figure 11-9 show the incident and
reflected waveforms observed with the TDR. The vertical scale is calibrated
both in terms of the voltage and the reflection coefficient, ρ. Equation 3 can be
rearranged to determine the characteristic impedance of the line:
1 + 0.01
Z line =  ------------------- • Z reference
1 – 0.01

where:
and

(15)

Zline = characteristic impedance of the line under test
Zreference = impedance of the known line.

The 50 Ω reference point is shown in Figure 11-9. The mean level of the reflected
waveform due to the line has a ρ = + 0.01. Substituting values into equation 15
permits calculation of the line impedance:
1 + 0.01
Z line =  ------------------- • 50 ohms = 51 ohms
 1 – 0.01

which agrees closely with the calculated value.

11--22

Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements

Figure 11-8

TDR Determination of Line Characteristic Impedance

Figure 11-9

TDR Determination of Line Characteristic Impedance (Continued)

11--23

Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements

Figure 11-10

TDR Determination of Line Characteristic Impedance (Continued)

The reflected voltage due to the connector is ±40 mV. The line reflects a voltage
of ±25 mV due to variations in the characteristic impedance of the line. The
reflection of 88 mV shown for the termination resistor (ρ = 0.088) is due to the
inductance of the resistor. It can be calculated that the inductance of the resistor
is less than 0.9 nH.
In these experiments, the input waveform comes from a generator which has a
risetime of 28 ps. There is some attenuation of the signal noticeable as it reaches
the termination resistor (tr = 80 ps at the load). When driving the line with a
MECL III gate with a risetime of 1 ns, the reflection due to the inductance of
the resistor would be much less (about 10 mV).

TDR Example 2
An equation can be derived to determine the maximum reflection voltage due
to the inductance of the resistor leads. The circuit shown in will be used in the
derivation.
The reflection coefficient at the load is:
RL – Zo
s + ----------------( R L + sL ) – Z o
ZL – Zo
L
ρ L ( s ) = ------------------ = ------------------------------------ = --------------------------( R L + sL ) + Z o
ZL + Zo
R L + Zo
s + -----------------L

11--24

(16)

Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements

Figure 11-11

Circuit for Determining the Maximum Reflected Voltage Due to the Inductance of the Resistor Leads

where s is the LaPlace operator for jω. The driving voltage will be represented
as:
e i ( t ) = mtu ( t ) – m ( t – T 1 )u ( t – T 1 )

(17)

where u(t) is a step function occurring at t = 0. Taking the LaPlace transform
of equation 17 gives:
(18)

–T 1 s
m
)
Ei ( s ) = ---- ( 1 – e
2
s

The reflected voltage at the load is then the product of the driving voltage and
the reflection coefficient (both in the transformed plane):
RL – Zo
s + ----------------( – T 1 )s
L
)
Ei ( s ) = Ei ( s )ρ L ( s ) = -------------------------------------- • m ( 1 – e
R L + Z o
2
s  s + -----------------L 

(19)

Taking the inverse LaPlace transform yields:
( R L + Z o )t

R L – Z o  2Z o L  – -----------------------2Z o L
L
mu ( t ) –
E ref1 ( t ) = ------------------------t –  -------------------------- e
- +  -----------------2 R + Z 
 ( R + Z )2
L
o
( RL + Z o )
L
o

(20)

( RL + Z o ) ( t – T1 )

 2Z o L  – ----------------------------------------R L – Z o
2Z o L
L
- e
( t – T 1 ) –  ------------------------mu ( t – T 1 )
------------------------- +  -----------------2
2 R + Z 

L
o
( R L + Zo )
( RL + Z o )

11--25

Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements

The maximum reflection voltage occurs at t = T1. Then, for R = Zo:

T ref1 ( t = T 1 ) = Eref1max

(21)

2Z

– --------o- T 1
L 
mL 
= --------- 1 – e

2Z o 



This equation relates the maximum reflected voltage, which can be measured
by TDR, and the inductance, which can then be calculated for the circuit of
Figure 11-11.

TDR Example 3
This example indicates how to measure the effect of resistor leads using the
TDR. Figure 11-12A shows the construction of a microstrip board used for
determining the effects of a resistor with 1" lead lengths. The reflected voltage
determined from the TDR measurement is 480 mV (see Figure 11-12B). The
risetime at the input to the line is 28 ps but it is lengthened to about 80 ps as
the wavefront reaches the termination resistor.
The time, T1, associated with the slope of the input voltage rise at the
terminating resistor can be approximated as:
tr
- = 100ps
T 1 ≈ --------0.80

(22)

The inductance can be computed by using equation 21, giving L = 6 nH.
Additional information can be obtained from the decay of the reflection shown
in Figure 11-12B. The decay lasts about 0.3 ns implying a time constant of about
0.3 ns/5 = 60 ps (using 5 time constants as decay time). The calculated time
constant for an inductance of 6 nH is:
L
--------- = 60ps
2Z o

The two results agree closely.
When driving the line with a MECL III gate risetime = 1 ns the reflection would
be only 50 mV. Most carbon resistor types will have less than 10 nH of
inductance. This inductance gives a reflection of less than 75 mV when the line
is driven by a MECL III gate. Note that the reflection is positive, indicating that
the noise immunity of a MECL gate connected at the load would be unchanged.

11--26

Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements

Figure 11-12

Effects Due to Termination Resistor Leads

11--27

Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements

TDR Example 4
Experiments have also been performed to determine the effects of a ground
plane on the characteristic impedance of microstrip lines. Figure 11-13
illustrates what happens when the ground plane width under the transmission
line abruptly drops to the width of an active line. The TDR waveform shows that
a 12% reflection occurs due to this discontinuity in the ground plane.
Using equation 15 the impedance of the 2½ inch-long strip can be calculated as:
1 + 0.12
Z line = ------------------- • 50 = 68ohms
1 – 0.12

Figure 11-13

Effects of Ground Plane Discontinuities.

11--28

Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements

Figure 11-14 shows a curve that approximates the change in the characteristic
impedance of the line for various ratios of ground plane width to active line
width. Note that when the ground width is greater than 3 times the line width,
the characteristic impedance is constant according to equation 14.
Figure 11-14

Variation of Microstrip Impedance as a Function of Ground Width - Line Width

11--29

Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements

A related experiment was performed to find the reflection due to a ground plane
near the active line, but not directly under it. The test configuration and test
results are shown in Figure 11-15.
Figure 11-15

Effects of Ground Plane Discontinuity

As indicated by the TDR measurement, the reflection is about 36%. Again using
equation 15, the impedance of the 2½ inch strip can be calculated:
1 + 0.36
Z line = ------------------- • 50 = 106ohms
1 – 0.36

The reason for the reflection is the change in the characteristic impedance along
with the line resulting from the ground plane not being under part of the active
line. In such a region, capacitance of the line to ground decreases while the
inductance of the line increases, the net result being a higher characteristic
impedance.

11--30

Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements

It must be remembered that the TDR input waveform has a risetime of 28 ps.
Consequently, in a real logic circuit situation where perhaps, a MECL III gate
with a 1 ns risetime is driving the line the reflection would actually be less than
27%, not 36% as in this example.1 This can be determined by scaling the value
of ρ found with the TDR waveshape in Figure 11-15B, with a 1 ns risetime. When
the length of the ground plane discontinuity is less than the distance travelled
by the signal during its risetime, then the reflection coefficient can also be
calculated as:
2lt pd
2lt pd
- • ρ , for  ----------- < 1
ρ′ = ---------- t

tr
r

where:

(23)

tpd = the propagation delay time of the line in ns / in.
tr = the risetime of the signal in ns
l = the length of the discontinuity in inches
ρ = the reflection coefficient for 2lpd / tr ≥ 1
(in this case the value found with the TDR waveshape
with tr = 28 ns).

For a discontinuity in the ground plane of 2.5 inches length, a propagation delay
of the line of 0.15 ns / in, and a MECL III gate with 1 ns risetime, the percent
reflected voltage can be calculated. From Figure 11-15B, ρ is found to be 0.36.
Using equation 23
2 ( 0.36 )2.5 ( 0.15 )
ρ′ = ------------------------------------------ = 0.27
1

Therefore, the reflection would be 27%. For a MECL 10K series gate with a
risetime of 3.5 ns, the reflection would only be 7.7%, and a MECL 10KH gate
with a rise time of 1.8 ns, the reflection would be 15%.

TDR Example 5
Another measurement was performed to observe the reflections due to the use
of a hybrid divider. The construction of the microstrip board used is shown in
the Figure 11-16. Note that the 50 Ω line branches out into two 100 Ω lines. A
reflection of 4% is observed at point 2 where the junction occurs. Notice that
the resistor exhibits a reflection of - 8%, due to capacitance of the resistor.

The Agilent 54753A and Agilent 54754A TDR plug-ins’ normalization allows
the user to change the risetime of the measurement system to simulate actual
circuit risetimes.

11--31

Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements

Previously it was found that the 50 Ω resistor was inductive. The lower values
of resistors (< 75 Ω) exhibit inductance, while the higher values behave
capacitively. No mismatch appears due to the crosstalk between the two
100 Ω branches, because of their wide separation.
Figure 11-16

Hybrid Divider

11--32

Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements

Figure 11-17B shows the reflection due to the construction of Figure 11-17A
where the two 100 Ω lines have been brought close together. The reflection at
point 2 is now equal to 8% because the two lines are cross coupled.
Figure 11-17

Hybrid Divider with Crosstalk Problem

Even mode or odd mode characteristic impedance (Zoe or Zoo) can be
considered to exist in a circuit with crosstalk. One, Zoe, is due to the strips being
at the same potential and carrying equal currents in the same direction. The

11--33

Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements

other, Zoo, is due to the strips being at equal but opposite potentials and carrying
equal currents in opposite directions. The backward crosstalk voltage, VB, on a
passive line is:
Z oe – Z oo
- E
V B =  ---------------------Z + Z  1
oe
oo

(24)

where E1 is the signal propagating down the active line. The backward crosstalk
voltage shown in Figure 11-17B at point 2 is equal to 8% of the incident voltage
E1. Since both lines are active, the crosstalk due to one active line is 4% of E1
for a spacing of 80 mils.
Crosstalk is not ordinarily a problem when using MECL III on microstrip or strip
line circuit boards, when line spacings are greater than 30 mils. The mutual
inductance and capacitance between two lines are used to determine the
crosstalk coefficient. Forward crosstalk is normally much smaller than the
backward crosstalk on microstrip lines except for very long lines ( > 5 feet).
Forward crosstalk does not exist at all on strip lines, since they are made with
a homogeneous medium, so that the inductively and capacitively induced
currents cancel.
The backward crosstalk coefficients for various types of microstrip lines on glass
epoxy boards are shown in Figure 11-18.

11--34

Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements

Figure 11-18

Backward Crosstalk Coefficient for Microstrip Lines on Glass Epoxy Boards (G-10 Material)

The backward crosstalk coefficient is equal to:

where:

1 LM
- + C M Z o
= ----------  -----
4tpd  Z o

(25)

LM = the inductive coupling
CM = the capacitive coupling
tpd = the propagation delay of the line per unit length

11--35

Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements

TDR Example 6
The graph data in Figure 11-18 will be used to determine the percent of crosstalk
coupling for the circuit of Figure 11-17. From the dimensions of the lines given
in Figure 11-17 (a), KB is found to be 0.055 from the graph. This means that if
one line (the active line) were driven with a signal, the other line (passive)
would have a coupled signal of 5.5% of the amplitude on the active line, in a
direction opposite to that of the driving signal. Since both 100 Ω lines are active
simultaneously, the reflection observed on the TDR is twice as much, or 11%.
From Figure 11-17, the actual crosstalk can be seen to be about 8%.
In very high speed systems, the exact shape of a line can be important, if
reflections are to be kept to a minimum. The arrangement shown in Figure 1119A has been used to investigate the behavior of two different line shapes. For
one line, corners are sharp. This permits the width of the line to be larger at
corners than elsewhere. Figure 11-19B shows that a -7.5% reflection occurs at
point 6 due to the lowered characteristic impedance at the corner. For the other
line, the corners are rounded to produce a constant line width. Figure 11-19C
shows that a constant line impedance exists for the second line. Note that an
inductive reflection, as discussed before, occurs at the end of the line due to
the inductance of the resistor. In conclusion, it is desirable to have smooth,
rounded line edges and constant line widths when designing transmission lines
for high speed systems. Resistor leads should be kept short to minimize
termination inductance.

11--36

Transmission Line Theory Applied to Digital Systems
Microstrip Transmission Line Techniques Evaluated Using TDR Measurements

Figure 11-19

Reflections Caused by Signal-Line Shape Variations

11--37

References
1 Kaupp, H. R., "Characteristics of Microstrip Transmission Lines", IEEE
Transactions on Electronic Computers, Vol. EC-16 No. 2, April 1967, pp.
185-193.
2 Cohn, S. B., "Characteristic Impedance of the Shielded Strip
Transmission Line", Transactions IRE, Vol. MTT-2, July 1954, pp. 52-57
3 Springfield, W. K., "Designing Transmission Lines into Multilayer Circuit
Boards". Electronics, November 1, 1965 pp. 90-96.
4 Skilling, H. H., "Electric Transmission Lines", New York McGraw-Hill,
1951.
5 DeFalco, J. A., "Reflections and Crosstalk in Logic Circuit
Interconnections", IEEE Spectrum. July 1970, pp. 44-50.
6 Millman, J., and Taub, H., "Pulse, Digital and Switching Waveforms", New
York, McGraw-Hill, 1965, pp. 83-106
7 "Time Domain Reflectometry", Hewlett-Packard2 Application Note 62,
1964.
8 Botos, Bob, "Nanosecond Pulse Handling Techniques in I/C
Interconnections", Motorola Application Note AN-270.
9 Schwarzmann, A., "Microstrip Plus Equations Adds Up to Fast Designs",
Electronics, October 2, 1967, pp. 109-112
10 Catt, I., "Crosstalk (Noise) in Digital Systems". IEEE Transactions on
Electronic Computers, Vol. EC-16, No. 6 December 1967, pp. 743-763.
11 Feller, A., H. R. Kaupp. J. J. Digiacoma. "Crosstalk and Reflections in
High-Speed Digital Systems". Proceedings, Fall Joint Computer
Conference, 1965, pp. 511-525.
12 Cohn, S. B., "Shielded Coupled-Strip Transmission Line", IRE
Transactions-Microwave Theory and Techniques, October 1955. pp. 2938.
13 Gabbard, O. G., "High-Speed Digital Logic for Satellite Communications",
Electro-Technology, April 1969, pp. 59-65.
14 Henschen, I. E. and E. M. Reyner II, "Adapting PC Connectors for
Impedance Matching", Proceedings, NEPCON, 1970.

Hewlett-Packard and HP are U.S. registered trademarks of Hewlett-Packard
Company.

11--38

Specifications and Characteristics

What you’ll find in this chapter
This chapter lists the system specifications and characteristics of the
Agilent 54753A, 54754A TDR plug-in module when it is combined with either the
Agilent 83480A or Agilent 54750A mainframes. The specifications and
characteristics for the mainframe are in the Agilent 83480A, 54750A User’s Guide.
All specifications, unless otherwise noted, require a 60 minute warm-up period.
Definitions of terms
The distinction between specifications, characteristics, typical performance, and
nominal values is described as follows:
• Specifications describe warranted performance over the temperature
range +15° C to +35° C (unless otherwise noted). All specifications apply
after the instrument’s temperature has been stabilized after 60 minute
continuous operation. Unless otherwise noted, corrected limits are given
when specifications are subject to minimization with error-correction
routines.
• Characteristics provide useful, but nonwarranted information about the
functions and performance of the instrument. Characteristics are printed in
italics.
• Typical Performance, where listed, is not warranted, but indicates
performance which most units will exhibit. Typical performance is printed
in italics.
• Nominal Value indicates the expected, but not warranted, value of the
parameter.

12-2

Specifications and Characteristics
Specifications

Specifications
The following are specifications used to test the Agilent 54753A, 54754A plugin modules. Specifications are valid after a 60 minute warm-up period. See the
Agilent 54701A Active Probe Service Guide for complete probe
specifications.

12-3

Specifications and Characteristics
Vertical Specifications

Vertical Specifications
Channels (Vertical)1

dc to 12.4 or 18.0 GHz, user selectable
dc to 12.4 or 20.0 GHz, user selectable2

Bandwidth (−3 dB)

dc to 12.4 or 18.0 GHz3, user selectable
dc to 12.4 or 20.0 GHz, user selectable2

dc Accuracy—single marker4 5
12.4 GHz bandwidth

±0.4% of full scale
±2 mV ±0.6% of reading-channel offset6
± (1%/°C) (∆Tcal) (reading) 7

18 GHz bandwidth

±0.4% of full scale
±2 mV ±1.2% of reading-channel offset6
± (1%/°C) (∆Tcal) (reading) 7

20 GHz bandwidth2

±0.4% of full scale
±2 mV ±1.2% of reading-channel offset6
± (1%/°C) (∆Tcal) (reading) 7

dc Difference—two marker accuracy on same
channel 5
12.4 GHz bandwidth

±0.8% of full scale
±0.6% of delta reading6
± (1%/°C) (∆Tcal) (delta reading) 7

18 GHz bandwidth

±0.8% of full scale
±1.2% of delta reading6
± (1%/°C) (∆Tcal) (delta reading) 7

20 GHz bandwidth2

±0.8% of full scale
±1.2% of delta reading6
± (1%/°C) (∆Tcal) (delta reading) 7

Transition Time (10%–90%) characteristic,
calculated from T=0.35/BW, electrical
12.4 GHz bandwidth

28.2 ps

18 GHz bandwidth

19.4 ps

20 GHz bandwidth 2

17.5 ps

RMS Noise
Typical
12.4 GHz

0.25 mV

18 GHz

0.5 mV

20 GHz 2

0.5 mV

12-4

Specifications and Characteristics
Environmental Specifications

Maximum
12.4 GHz

0.5 mV

18 GHz

1.0 mV

20 GHz2

1.0 mV

Scale Factor

full scale is eight divisions

Minimum

1 mV/div

Maximum

100 mV/div

Display Resolution

256 points

dc Offset Range8

±500 mV

Nominal Input Impedance

50 [ohm ]

Connectors

3.5mm (m), channel and trigger

Input Reflection/Return Loss

≤5% for 30 ps rise time

Number of Channels

Dynamic Range/Maximum Specified Input Power

±400 mV relative to channel offset

Maximum Safe Input

±2V + peak ac (+16 dBm)

When operated within ±5°C (±9°F) of the temperature of the last plug-in calibration.

For the Agilent 54753A channel 2 only

The input sampler is biased differently for increased bandwidth in the 18 GHz bandwidth mode.

When driven from a 0 ohm source.

It is recommended that a user vertical calibration be performed after every 10 hours of continuous use or if
the temperature has changed by greater in 2°C from the previous vertical calibration.

When operated within ±2°C (±3.6°F) of the temperature of the last plug-in calibration. When operated within
±5°C (±9°F) of the temperature of the last plug-in calibration, the final term in the dc accuracy specification
is 2.5 times higher.

Where ∆Tcal represents the temperature change in Celsius from the last user vertical calibration.

An effective offset of ±900 mV can be achieved using the ±500 mV of channel offset and adding ±400 mV of
offset using the waveform math offset scaling function.

Environmental Specifications
Temperature
Operating

15 °C to +35 °C

Non-operating

−40 °C to +70 °C

12-5

Specifications and Characteristics
Environmental Specifications

Humidity
Operating

up to 90% relative humidity at 35 °C

Non-operating

up to 90% relative humidity at 35 °C

12-6

Specifications and Characteristics
Power Requirements

Power Requirements
Supplied by mainframe.

Weight
Net

approximately 1.1 kg (2.4 lb.)

Shipping

approximately 2.0 kg (4.4 lb.)

Characteristics
The following characteristics are typical for the Agilent 54753A and 54754A
TDR plug-in modules. See the Agilent 54701A Active Probe Service Guide for
complete probe characteristics.

Trigger Input Characteristics
Nominal Impedance

50 [ohm ]

Input Connector

3.5 mm (m)

Trigger Level Range

±1 V

Maximum Safe Input Voltage

±2 Vdc + ac peak (+16 dBm)

Percent Reflection

≤10% for 200 ps rise time

Refer to the Agilent 83480A, 54750A User’s Guide for Trigger specifications.

12-7

Product Regulations
Safety

IEC 1010
UL 3111
CSA Standard C22.2 No. 1010.1-92

EMC

This product meets the requirement of the European Communities (EC) EMC
Directive 89/336/EEC.
Emissions
EN55011/CISPR 11 (ISM Group 1, Class A equipment)
Immunity
EN50082-1
Code1
IEC 801-2 (ESD) 4kV CD, 8kV AD

IEC 801-3 (Rad.) 3V/m

IEC 801-4 (EFT) 1kV

Notes2

Performance Codes:
1 PASS - Normal operation, no effect.
2 PASS - Temporary degradation, self recoverable.
3 PASS -Temporary degradation, operator intervention required.
4 FAIL - Not recoverable, component damage.

Notes:
(None)

12-8

In Case of Difficulty

What you’ll find in this chapter
This chapter provides a list of suggestions for you to follow if the plug-in module
fails to operate. A list of messages that may be display is also included.
For complete service information, refer to the optional Agilent 54753A, 54754A
Service Guide.

CAUTION

Electrostatic discharge (ESD) on or near input connectors can damage circuits
inside the instrument. Repair of damage due to misuse is not covered under
warranty. Before connecting any cable to the electrical input, momentarily
short the center and outer conductors of the cable together. Personnel should
be properly grounded, and should touch the frame of the instrument before
touching any connector.

13-2

In Case of Difficulty
If You Have Problems

If You Have Problems
Review the procedure being performed when the problem occurred. Before
calling Agilent Technologies or returning the unit for service, a few minutes
spent performing some simple checks may save waiting for your instrument to
be repaired.

If the Mainframe Does Not Operate
Please make the following checks:

1
2
3
4
5
6

Is the line fuse good?
Does the line socket have power?
Is the unit plugged in to the proper ac power source?
Is the mainframe turned on?
Is the rear-panel line switch set to on?
Will the mainframe power up without the plug-in module installed?
If the mainframe still does not power up, refer to the optional Agilent 83480A,
54750A Service Guide or return the mainframe to a qualified service
department.

13-3

In Case of Difficulty
If the Plug-in Does Not Operate

If the Plug-in Does Not Operate
Make the following checks:

Is the plug-in module firmly seated in the mainframe slot?
• Are the knurled screws at the bottom of the plug-in module finger-tight?
• Is a trigger signal connected to a trigger input?
• If other equipment, cables, and connectors are being used with the plug-in
module are they connected properly and operating correctly?
• Review the procedure for the test being performed when the problem
appeared. Are all the settings correct? Can the problem be reproduced?

Perform the following procedures:
• Make sure the instrument is ready to acquire data by pressing Run.
• Find any signals on the channel inputs by pressing Autoscale.
• See if any signals are present at the channel inputs by pressing:
Trigger
Sweep
freerun
After viewing the signal, press triggered.
• Make sure Channel Display is on by pressing:
Channel
Display on off on
• Make sure the channel offset is adjusted so the waveform is not clipped off
the display.
• If you are using the plug-in module only as a trigger source, make sure at
least one other channel is turned on.
• If all of the channels are turned off, the mainframe will not trigger.
• Make sure the mainframe identifies the plug-in module by pressing:
Utility
System config...
The calibration status of the plug-in modules is listed near the bottom of the
display, in the box labeled “Plug-ins”. If the model number of the plug-in
module is listed next to the appropriate slot number, then the mainframe has
identified the plug-in.

13-4

In Case of Difficulty
Error Messages

If “~known” is displayed instead of the model number of the plug-in module,
remove and reinsert the plug-in module in the same slot. If “~known” is still
displayed, then the memory contents of the plug-in module are corrupt. Refer
to the optional Agilent 54753A, 54754A Service Guide or contact a qualified
service department.
If all of the above steps check out okay, and the plug-in module still does not
operate properly, then the problem is beyond the scope of this book. Refer to
the optional Agilent 54753A, 54754A Service Guide or return the plug-in
module to a qualified service department.

Error Messages
The following error messages are for the plug-in module. Typically, the error
messages indicate there is a problem with either the plug-in or the mainframe.
This section explains what the messages mean and offers a few suggestions that
might help resolve the error condition. If the suggestions do not eliminate the
error message, then additional troubleshooting is required that is beyond the
scope of this book. Refer to the optional Agilent 54753A, 54754A Service
Guide and Agilent 83480A, 54750A Service Guide for additional
troubleshooting information.
Additional error messages are listed in the Agilent 83480A, 54750A User’s
Guide for the mainframe.
Memory error occurred in plug-in : Try reinstalling plugin
The mainframe could not correctly read the contents of the memory in the plugin.

1
2
3
4
5

Remove and reinstall the plug-in module.
Each time a plug-in is installed, the mainframe rereads the plug-in
module’s memory.
Verify the plug-in module is firmly seated in the mainframe slot.
Verify the knurled screws at the bottom of the plug-in module are fingertight.
Install the plug-in in a different slot in the mainframe.
Busy timeout occurred with plug-in
plug-in

: Try reinstalling

13-5

In Case of Difficulty
Error Messages

The mainframe is having trouble communicating with the plug-in module. Make
sure there is a good connection between the mainframe and the plug-in module.

1
2
3
4

Remove and reinstall the plug-in module.
Verify the plug-in module is firmly seated in the mainframe slot.
Verify the knurled screws at the bottom of the plug-in module are fingertight.
Install the plug-in in a different slot in the mainframe.
Communication failure exists at slot : Service is required
An illegal hardware state is detected at the mainframe to plug-in module
interface of the specified slot.
If the slot is empty, there is a mainframe hardware problem. Refer to the
Agilent 83480A, 54750A Service Guide.
If a plug-in is installed in the slot, there is a plug-in module hardware problem.
Refer to the optional Agilent 54753A, 54754A Service Guide.
ID error occurred in plug-in : Service is required
The information read from the plug-in module’s memory does not match the
hardware in the plug-in module. This can be caused by a communication
problem between the mainframe and the plug-in module. Make sure there is a
good connection between the mainframe and the plug-in.

1
2
3

Remove and reinstall the plug-in module.
Verify the plug-in module is firmly seated in the mainframe slot.
Verify the knurled screws at the bottom of the plug-in module are fingertight.

13-6

Index

Numerics
2 only stimulus
menus 5-13

A
accuracy performance 1-5
active probe 3-5
Alternate scale softkey 3-6
Atten units softkey 3-6, 3-12, 3-13
attenuation
probe 3-12
range 3-6
Attenuation softkey 3-6, 3-13
automatic measurement 3-4
auxiliary power connector 1-4

B
bandwidth limit 7-5
Bandwidth softkey 3-6

CAL signal 3-11, 3-12
Cal status softkey 3-10
Calibrate plug-in softkey 3-12
Calibrate probe softkey 3-11, 3-12
Calibrate softkey 3-8, 3-12
calibration
probe 3-12
status 13-4
validity 3-11
vertical 1-5, 3-12
voltage probe 3-12
capacitance 7-6
channel
display 3-4
input 1-4
scale 3-7
setup 3-2
Channel 1 Calibration Status message 3-11
Channel key 3-2
characteristics 12-7
common mode stimulus
measuring impedance 8-5
menus 5-23
connector

accuracy 2-6
alignment 2-4
connecting devices 2-16
dimensions 2-10
disconnecting devices 2-6, 2-7
inspection 2-4
mechanical inspection 2-8
mechanical mismatch 2-4
visual inspection 2-8
Current Date message 3-10
Current Frame ∆Temp message 3-10

D
decibel calculation 3-6
delay 6-7
description of the plug-in module ii
deskewing 8-5
cables 8-7, 8-17
TDT channels 8-20
destination
TDT 5-18
differential
stimulus 5-7
stimulus menus 5-23
differential stimulus
measuring impedance 8-5
differential TDT
measurements 8-15
digital offset 3-5
Display softkey 3-4
distance
measuring 7-37

E
environmental specifications 12-5
error messages 13-5, 13-6
Establish normalization & ref plane softkey 4-7, 5-10, 5-15,
5-20, 5-32
Establish ref plane softkey 5-26
Examples
measuring common mode impedance 8-5
measuring differential impedance 8-5
measuring distance 7-37
measuring excess L/C 7-32
measuring impedance 7-19
measuring percent reflection 7-25

Index-1

Index

normalization 7-8
single-ended TDR 7-3
excess L/C 7-6
measuring 7-32
Ext gain softkey 3-7, 3-13
Ext offset softkey 3-7, 3-13
extender cables 1-6
external
gain 3-12
offset 3-12
external stimulus
menus 5-30
external trigger
input 1-4
level 1-6

F
front panel overview 1-4

G
gain
calculation 6-7

Horizontal softkey 4-7, 5-10, 5-14, 5-20, 5-24, 5-32
horizontal waveform 3-8

reference plane 4-7
marker
reference plane 5-15
Marker menu 6-9
Marker units softkey 6-9
marker value 3-6
markers
reference plane 5-10, 5-21
math function 3-4
measuring
capacitance 7-6
inductance 7-6
menu overview 1-3

normalization 5-8, 5-10, 5-13, 5-15, 5-19, 5-20, 5-31, 5-32,
7-3, 7-4, 7-8
TDR 5-9, 5-14, 5-15
TDR step risetime
changing 5-8
TDT 5-9, 5-14, 5-15
Normalize 1 response 5-19
Normalize 2 response softkey 5-19
normalize response
risetime 5-13
Normalize response softkey 4-5, 5-8, 5-13, 5-31
Normalize scaling 5-9
Normalize scaling softkey 4-6, 5-14, 5-20, 5-32
normalizing 4-7

I
impedance
calculation 7-21
common mode 8-5
differential 8-5
measuring 7-19
inductance 7-6
installing the plug-in module 1-6

offset 3-4, 3-5, 3-7, 5-8
probe 3-12
Offset softkey 3-5
ohms 7-7, 8-5

K
key conventions 1-3

M
mainframe troubleshooting 13-3
Marker

Index-2

percent reflection 7-7, 7-25, 8-5
calculation 7-28
plug-in
characteristics 12-7
Plug-in message 3-11
plug-in module
features 1-4
front panel 1-4

Index

installation 1-6
power requirements 12-7
purpose 1-4
specifications 12-3, 12-7
troubleshooting 13-4
weight 12-7
power level 3-5
power requirements 12-7
Preset TDR/TDT 7-3, 8-3
Preset TDR/TDT softkey 4-8, 5-12, 5-17, 5-22, 5-28, 5-34
probe
attenuation 3-6, 3-11, 3-12
calibration 3-12
characteristics 3-7
offset 3-12
power 3-5
power connector 1-4
propagation
constant 6-9
propagation delay
automatic measurement 6-7
calculation 6-7
pulse parameter measurements 3-4

reference plane 4-7, 5-10, 5-15, 5-20, 5-31, 5-32, 6-9, 7-3,
7-4, 7-8, 8-9
common mode 5-26
differential 5-26
markers 5-10, 5-15, 5-21
setting markers 4-7
Reference softkey 6-9
reflection 6-4, 6-6
calculation 6-4, 6-6
maximum 6-6
minimum 6-4
repetition rate 5-27
response
common mode 8-3
differential 8-3
menus 6-10
TDT 5-25
Response scaling softkey 5-23, 5-25
RF connectors 1-6
risetime 4-5
Risetime softkey 5-8, 5-13, 5-19, 5-31

S
safety information 1-6
scale
percent reflection 7-7
Scale softkey 3-4
scaling
normalization 5-14, 5-32
changing 5-20
horizontal 5-14, 5-20, 5-32
vertical 5-14, 5-20, 5-32
normalize 5-9
changing 4-6
horizontal 4-7, 5-10
vertical 4-6, 5-10
response 5-23, 5-25
horizontal 5-24
vertical 5-23
shifted function keys 1-3
skew 8-4
Skew softkey 3-8
softkey
menu 1-4
overview 1-3
specifications 12-3, 12-7
stimulus
1 and 2 5-7
1 and 2 menus 5-18
1 only 5-7
1 only menus 5-8
2 only 5-7, 5-13
common mode 5-7, 5-23, 8-3
differential 5-7, 5-23, 8-3
external 5-7, 5-30
Stimulus softkey 4-4, 5-7
SWR 9-2

T
TDR
normalization 7-3
reference plane 7-3
TDR 1 dest softkey 4-5, 5-30
TDR destination
changing 4-5, 5-30
TDR Maximum Reflection softkey 6-6
TDR Minimum Reflection softkey 6-4
TDR normalization 4-5, 4-6, 5-15

Index-3

Index

TDR normalize softkey 5-9, 5-14, 5-20, 5-32
TDR rate
automatic 5-11, 5-16, 5-21, 5-27
manual 5-11, 5-16, 5-21, 5-27
repetition
changing 4-8
repetition rate 5-34
changing 5-27
TDR rate softkey 5-11, 5-16, 5-21, 5-27, 5-34
automatic 4-8
manual 4-8
TDR response 5-23
TDR response 1 softkey 5-23
TDR skew 8-3
TDR Skew softkey 3-9
TDR step
repetition rate 5-11, 5-16, 5-21
risetime 4-5, 5-19, 5-31
TDR/TDT
automatic setup 4-8, 5-12, 5-17, 5-22, 5-28, 5-34
TDR/TDT Measure
menus 6-4
TDR/TDT Setup menu 4-4
Agilent 54754A 5-7
TDR/TDT softkey 4-6, 5-9, 5-14, 5-19, 5-23, 5-31
TDT
amperes 7-7
deskewing 8-20
destination
changing 5-18
differential measurements 8-15
gain 7-7
normalization 7-4
reference plane 7-4
response 8-21
volts 7-7
watts 7-7
TDT 1 dest softkey 4-4, 5-8, 5-30
TDT 2 dest softkey 5-13, 5-18
TDT destination
changing 5-8, 5-13, 5-30
TDT Gain
automatic measurement 6-7
TDT Gain softkey 6-7
TDT normalization 4-5, 4-6, 5-15
TDT normalize softkey 5-9, 5-14, 5-20, 5-32
TDT Propagation Delay softkey 6-7

Index-4

TDT response
scaling 5-25
TDT response 1 softkey 5-25
temperature change 3-10, 3-11
trigger
external 1-6
input characteristics 12-7
level 3-7
source 3-4
troubleshooting 13-3, 13-6

Units softkey 3-4, 3-7, 3-12, 3-13
Utility key 3-12

V
vertical
calibration 1-5, 3-12
measurement 3-7
scale 3-4
specifications 12-4
waveform 3-5
Vertical softkey 4-6, 5-10, 5-14, 5-20, 5-23, 5-32
voltage
measurement 3-6
probe 3-11
probe calibration 3-12

W
wattage measurement 3-6
waveform
display 3-4
horizontal 3-8
weight of plug-in module 12-7

© Copyright
Agilent Technologies 2000
All Rights Reserved.
Reproduction, adaptation, or
translation without prior
written permission is
prohibited, except as allowed
under the copyright laws.
Document Warranty
The information contained in
this document is subject to
change without notice.
Agilent Technologies makes
no warranty of any kind with
regard to this material,
including, but not limited to,
the implied warranties of
merchantability or fitness for
a particular purpose.
Agilent Technologies shall
not be liable for errors
contained herein or for
damages in connection with
the furnishing, performance,
or use of this material.

Safety
This apparatus has been
designed and tested in
accordance with IEC
Publication 61010-1, Safety
Requirements for Measuring
Apparatus, and has been
supplied in a safe condition.
This is a Safety Class I
instrument (provided with
terminal for protective
earthing). Before applying
power, verify that the correct
safety precautions are taken
(see the following warnings).
In addition, note the external
markings on the instrument
that are described under
"Safety Symbols."
Warning
• Before turning on the
instrument, you must
connect the protective earth
terminal of the instrument to
the protective conductor of
the (mains) power cord. The
mains plug shall only be
inserted in a socket outlet
provided with a protective
earth contact. You must not
negate the protective action
by using an extension cord
(power cable) without a
protective conductor
(grounding). Grounding one
conductor of a twoconductor outlet is not
sufficient protection.
• Only fuses with the
required rated current,
voltage, and specified type
(normal blow, time delay,
etc.) should be used. Do not
use repaired fuses or shortcircuited fuseholders. To do
so could cause a shock of fire
hazard.

Agilent Technologies
Lightwave Division
1400 Fountaingrove Parkway
Santa Rosa, CA 95403-1799, USA
(707) 577-1400

• Service instructions are for
trained service personnel. To
avoid dangerous electric
shock, do not perform any
service unless qualified to do
so. Do not attempt internal
service or adjustment unless
another person, capable of
rendering first aid and
resuscitation, is present.
• If you energize this
instrument by an auto
transformer (for voltage
reduction), make sure the
common terminal is
connected to the earth
terminal of the power source.
• Whenever it is likely that
the ground protection is
impaired, you must make the
instrument inoperative and
secure it against any
unintended operation.
• Do not operate the
instrument in the presence
of flammable gasses or
fumes. Operation of any
electrical instrument in such
an environment constitutes a
definite safety hazard.
• Do not install substitute
parts or perform any
unauthorized modification to
the instrument.
• Capacitors inside the
instrument may retain a
charge even if the
instrument is disconnected
from its source of supply.
• Use caution when exposing
or handling the CRT.
Handling or replacing the
CRT shall be done only by
qualified maintenance
personnel.

Safety Symbols

!
Instruction manual symbol:
the product is marked with
this symbol when it is
necessary for you to refer to
the instruction manual in
order to protect against
damage to the product.

Hazardous voltage symbol.

Earth terminal symbol: Used
to indicate a circuit common
connected to grounded
chassis.
WARNING

The Warning sign denotes a
hazard. It calls attention to a
procedure, practice, or the
like, which, if not correctly
performed or adhered to,
could result in personal
injury. Do not proceed
beyond a Warning sign until
the indicated conditions are
fully understood and met.
CAUTION

The Caution sign denotes a
hazard. It calls attention to
an operating procedure,
practice, or the like, which, if
not correctly performed or
adhered to, could result in
damage to or destruction of
part or all of the product. Do
not proceed beyond a
Caution symbol until the
indicated conditions are fully
understood or met.

Product Warranty
This Agilent Technologies
product has a warranty
against defects in material
and workmanship for a
period of one year from date
of shipment. During the
warranty period, Agilent
Technologies will, at its
option, either repair or
replace products that prove
to be defective. For warranty
service or repair, this
product must be returned to
a service facility designated
by Agilent Technologies. For
products returned to Agilent
Technologies for warranty
service, the Buyer shall
prepay shipping charges to
Agilent Technologies and
Agilent Technologies shall
pay shipping charges to
return the product to the
Buyer. However, the Buyer
shall pay all shipping
charges, duties, and taxes for
products returned to Agilent
Technologies from another
country. Agilent
Technologies warrants that
its software and firmware
designated by Agilent
Technologies for use with an
instrument will execute its
programming instructions
when properly installed on
that instrument. Agilent
Technologies does not
warrant that the operation of
the instrument software, or
firmware will be
uninterrupted or error free.
Limitation of Warranty
The foregoing warranty shall
not apply to defects resulting
from improper or inadequate
maintenance by the Buyer,
Buyer- supplied software or
interfacing, unauthorized
modification or misuse,
operation outside of the
environmental specifications
for the product, or improper
site preparation or
maintenance.

No other warranty is
expressed or implied. Agilent
Technologies specifically
disclaims the implied
warranties of
merchantability or fitness for
a particular purpose.
Exclusive Remedies
The remedies provided
herein are the buyer's sole
and exclusive remedies.
Agilent Technologies shall
not be liable for any direct,
indirect, special, incidental,
or consequential damages,
whether based on contract,
tort, or any other legal
theory.
Assistance
Product maintenance
agreements and other
customer assistance
agreements are available for
Agilent Technologies
products. For any assistance,
contact your nearest Agilent
Technologies Sales Office.
Certification
Agilent Technologies
certifies that this product
met its published
specifications at the time of
shipment from the factory.
Agilent Technologies further
certifies that its calibration
measurements are traceable
to the United States National
Institute of Standards and
Technology, to the extent
allowed by the Institute's
calibration facility, and to the
calibration facilities of other
International Standards
Organization members.

About this edition
This is the second edition of
the Agilent 54753A and
Agilent 54754A TDR Plug-in
Modules User's Guide.

The following list of pages
gives the date of the current
edition and of any changed
pages to that edition.
All pages original edition

Publication number
54753-97015
Printed in USA.
New editions are complete
revisions of the manual.
Update packages, which are
issued between editions,
contain additional and
replacement pages to be
merged into the manual by
you. The dates on the title
page change only when a
new edition is published. A
software or firmware code
may be printed before the
date. This code indicates the
version level of the software
or firmware of this product
at the time the manual or
update was issued. Many
product updates do not
require manual changes; and,
conversely, manual
corrections may be done
without accompanying
product changes. Therefore,
do not expect a one-to-one
correspondence between
product updates and manual
updates.

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