Fluke 190 202 Application Note

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A rst look at DSOs

This introduction to digital storage oscilloscopes

(DSOs) takes you on a quick but comprehensive

tour of DSO functions and measurements.

An oscilloscope measures and

displays voltage signals on a

time-versus-voltage graph. In

most applications the graph

shows how the signal changes

over time: the vertical (Y) axis

represents voltage, and the hori-

zontal (X) axis represents time.

This simple graph can tell you

many things about a signal:

• View the signal for anomalies.

• Calculate the frequency of an

oscillating signal.

• Tell if a malfunctioning compo-

nent is distorting the signal.

• Tell how much of the signal is

noise and whether the noise is

changing with time.

Today’s handheld digital stor-

age oscilloscopes offer two

critical advantages over benchtop

models: they are battery-oper-

ated, and they use isolated,

electrically floating inputs.

These designs make safety-

certified measurements possible

in 1000 V CAT III and 600 V

CAT IV environments—a critical

need for safely troubleshooting

electrical devices in high-energy

applications.

Scopes and DMMs

The difference between an

oscilloscope and a DMM (Digital

Multimeter) can be summarily

stated as “pictures vs. numbers.”

A DMM is a tool for making

precise measurements of discrete

signals, enabling readings of up

to eight digits of resolution for

the voltage, current or frequency

of a signal. On the other hand, it

cannot depict waveforms visually

to reveal signal strength, wave-

shape, or the instantaneous value

of the signal. Nor is it equipped to

reveal a transient or a harmonic

signal that could compromise the

operation of a system.

A scope adds a wealth of infor-

mation to the numeric readings of

a DMM. While displaying instan-

taneous numerical values of a

wave, it also reveals the shape of

the wave, including its ampli-

tude (voltage) and frequency.

With such visual information, a

transient signal that may pose

major consequences to a system

can be displayed, measured and

isolated.

Reach for a scope if you want

to make both quantitative and

qualitative measurements. Use

a DMM to make high-precision

checks of voltage, current,

resistance and other electrical

parameters.

Sampling

Sampling is the process of

converting a portion of an input

signal into a number of discrete

electrical values for the pur-

pose of storage, processing and

display. The magnitude of each

sampled point is equal to the

amplitude of the input signal at

the time the signal is sampled.

The input waveform appears as

a series of dots on the display. If

the dots are widely spaced and

difficult to interpret as a wave-

form, they can be connected

using a process called interpola-

tion, which connects the dots

with lines, or vectors.

Volts

vertical

Y-axis

Time

horizontal X-axis

Figure 1. Time-versus-voltage graph.

The 4-channel Fluke 190 Series II

ScopeMeter has a 200 MHz bandwidth

and 2.5 GS/s real-time sampling rate.

The Fluke 123 ScopeMeter with 20 MHz

dual-input measurement shows both

meter reading and waveform.

Application Note

From the Fluke Digital Library @ www.fluke.com/library

2 Fluke Corporation A first look at DSOs

Triggering

Trigger controls allow you to

stabilize and display a repetitive

waveform.

Edge triggering is the most

common form of triggering. In

this mode, the trigger level and

slope controls provide the basic

trigger point definition. The slope

control determines whether the

trigger point is on the rising or

the falling edge of a signal, and

the level control determines

where on the edge the trigger

point occurs.

For even greater control and

visibility into signal phenom-

ena, you can use the ability of

some DSOs to capture events

leading up to the trigger point

(“pre-trigger”) or after the trig-

ger point (“post-trigger”) on the

input waveform. As an example,

by using pre-triggering or post-

triggering, you may catch a spike

that occurs in between two

occurrences of a signal.

Pulse width triggering trig-

gers on specific pulses within a

series or it can identify one-time

or sporadic problems in a pulsed

signal. In this mode, you can

monitor a signal indefinitely and

trigger on the first occurrence of

a pulse whose duration, or pulse

width, is either outside of or

within set limits. The goal is to

isolate and display a pulse that

meets the predetermined time

criteria.

Single-shot triggering is

useful for capturing a one-time

event—such as electrical arcing

or a relay closure. A DSO with a

single-shot mode waits until it

receives a trigger and then sets

itself in a hold mode to store the

signal at the moment the one-

time event occurs.

Video triggering is a power-

ful feature of advanced DSOs.

Video signals can be extremely

complex, providing no unique

edge that is repetitive and that

can be isolated to stabilize the

signal. With an extensive range

of signaling protocols in use in

today’s electronic video equip-

ment and systems, an effective

DSO is one that recognizes the

predominant video protocols and

provides appropriate triggering

functionality.

Setup

The task of capturing and ana-

lyzing an unknown waveform on

an oscilloscope can be routine, or

it can seem like taking a shot in

the dark. However, in most cases,

taking a methodical approach to

setting the oscilloscope will cap-

ture a stable waveform or help

you determine how the scope

controls need to be set so that

you can capture the waveform.

1) Start with Auto

Connect the ground reference

lead and then connect the probe

tip to the circuit test point. Most

oscilloscopes have the ability to

either perform a one-time auto

setup or continuously analyze the

unknown input signal. Press the

AUTO button or verify that the

scope is already in Auto mode.

Pressing the AUTO button will

typically set up the oscilloscope

to automatically adjust three key

parameters:

Vertical sensitivity. Adjusts

the vertical sensitivity so that the

vertical amplitude spans approxi-

mately three to six divisions.

Horizontal timing. Adjusts

the horizontal time per division

so that there are three to four

periods of the waveform across

the width of the display.

Figure 2. Sampling and interpolation, sampling is depicted by the dots while interpolation

is shown as the black line.

t

Figure 3. Unknown trace adjusted for 3-6

vertical divisions.

Figure 4. Unknown trace adjusted for 3-4

periods horizontally.

3 Fluke Corporation A first look at DSOs

Trigger position. Sets the trig-

ger position to the 50 % point of

the vertical amplitude. Depend-

ing on the signal characteristics,

this action may or may not result

in a stable display.

At this point you should see

a trace that 1) lies within the

vertical range of the display,

2) shows at least three periods

of a waveform, and 3) is stable

enough to allow you to recognize

the overall characteristics of the

waveform. Next, start fine-tuning

the settings.

2) Adjust vertical and

horizontal settings

Start by adjusting the horizontal

timing, increasing the time per

division so that you see a wide

time span of the unknown wave-

form. From that view, reverse the

adjustment as required to narrow

the view to just what you want

to display.

Now adjust the vertical sensi-

tivity, expanding the waveform

vertically but ensuring that the

high and low points of the wave-

form do not exceed the vertical

span.

3) Adjust trigger settings

If needed, adjust the trigger set-

tings to stabilize the waveform

display. Or, you may want to

adjust the trigger delay to see

pre- or post-trigger details on the

waveform. Always start with the

trigger-level setting, adjusting

it so that it falls on a repetitive,

unique point on the rising or fall-

ing edge of a waveform.

As an example, with the oscil-

loscope trigger set to the rising

edge and the level set to the 50

% point, the following figures

illustrate the cause of an unsta-

ble waveform display.

After two successive updates

based on triggers 1 and 2, the

resultant trace will appear

unstable.

However, simply by manually

adjusting the trigger point to a

repetitive, unique point on the

edge, you can solve this problem

and produce a stable waveform

display.

When working with complex

signals like a series of pulses,

pulse width triggering may be

required. With this technique,

both the trigger-level setting

and the next falling edge of the

signal must occur within a speci-

fied time span. Once these two

conditions are met, the oscillo-

scope triggers.

Another technique is single-

shot triggering, by which the

oscilloscope will display a trace

only when the input signal meets

the set trigger conditions. Once

the trigger conditions are met,

the oscilloscope acquires and

updates the display, and then

freezes the display to hold the

trace.

Trig

Figure 5a. Trigger point set to 50 % point

on trace.

Trig

Figure 5b. Trigger point is set to the 50 %

point but due to the aberration on the lead-

ing edge in the second period, an additional

trigger results in an unstable display.

1st Trig 2nd

Trig

Figure 6a. At the first update, the scope

triggers on the first edge. On the second

update, the scope may trigger on the

second trigger point indicated.

Trig

Figure 6b. Unstable waveform display

caused by incorrect trigger level setting.

Trig

Figure 6c. Trigger level adjusted to a unique

repetitive position, outside the aberration on

the second period.

Trigger

Level (V)

Pulse Width (t)

Figure 7. Pulse width triggering will allow

you to set up the oscilloscope to trigger on a

specific pulse defined by level and time.

4 Fluke Corporation A first look at DSOs

Understanding and

reading waveforms

The majority of electronic wave-

forms encountered are periodic

and repetitive, and they conform

to a known shape. Here are the

factors to consider in analyzing

waveforms:

Shape. Repetitive waveforms

should be symmetrical. That is,

if you were to print the traces

and cut them in two like-sized

pieces, the two sides should be

identical. A point of difference

could indicate a problem.

Rising and falling edges.

Particularly with square waves

and pulses, the rising or fall-

ing edges of the waveform can

greatly affect the timing in digi-

tal circuits. It may be necessary

to decrease the time per division

to see the edge with greater

resolution.

Amplitude. Verify that the

level is within the operating

specifications of the circuit.

Also check for consistency, from

one period to the next. Monitor

the waveform for an extended

period of time, watching for any

changes in amplitude.

Amplitude offsets. DC-couple

the input and determine where

the ground reference marker

is. Evaluate any dc offset and

observe if this offset remains

stable or fluctuates.

Periodic waveshape. Oscil-

lators and other circuits will

produce waveforms with con-

stant repeating periods. Evaluate

each period in time using cursors

to spot inconsistencies.

1

2

Figure 8. If the two components of the

waveform are not symmetrical, there may be

a problem with the signal.

t t

Figure 9. Use cursors and the graticule

marks to evaluate the rise and fall times

of the leading and trailing edges of a

waveform.

Figure 10. Use horizontal cursors to identify

amplitude fluctuations.

F i g u r e 11. Evaluate waveform DC offsets.

t t

Figure 12. Evaluate period-to-period time

changes.

Waveform anomalies

Here are typical anomalies that

may appear on a waveform,

along with the typical sources of

such anomalies.

Transients or glitches. When

waveforms are derived from

active devices such as transistors

or switches, transients or other

anomalies can result from timing

errors, propagation delays, bad

contacts or other phenomena.

Noise. Noise can be caused

by faulty power supply circuits,

circuit overdrive, crosstalk, or

interference from adjacent cables.

Or, noise can be induced exter-

nally from sources such as dc-dc

converters, lighting systems and

high-energy electrical circuits.

Figure 13. A transient is occurring on the

rising edge of a pulse.

Figure 14. A ground reference-point mea-

surement showing induced random noise.

5 Fluke Corporation A first look at DSOs

Ringing. Ringing can be seen

mostly in digital circuits and in

radar and pulse-width-modula-

tion applications. Ringing shows

up at the transition from a rising

or falling edge to a flat dc level.

Check for excessive ringing,

adjusting the time base to give a

clear depiction of the transition-

ing wave or pulse.

Momentary fluctuation.

Momentary changes in the

measured signal generally result

from an external influence such

as a sag or surge in the mains

voltage, activation of a high-

power device that is connected

to the same electrical grid, or a

loose connection. Use the DSO’s

slowest time-base setting or

the paperless recording or “roll”

mode. Start at the input and

watch the acquired waveform

over long time spans to track

down the source of the problem.

Drift. Drift—or minute changes

in a signal’s voltage over time—

can be tedious to diagnose. Often

the change is so slow that it is

difficult to detect. Temperature

changes and aging can affect

passive electronic components

such as resistors, capacitors and

crystal oscillators. One problem-

atical fault to diagnose is drift in

a reference dc voltage supply or

oscillator circuit. Often the only

solution is to monitor the mea-

sured value (V dc, Hz, etc.) over

an extended time.

Figure 15. Excessive ringing occurring on

the top of the square wave.

Figure 16a. A momentary change of

approximately 1.5 cycles in the amplitude of

the sinewave.

110 Vac

Figure 16b. Using an oscilloscope with a

paperless recorder mode allows plotting of

the amplitude (voltage level) over time.

1. 01 kHz

Figure 17. Performing a frequency

measurement on a crystal oscillator that

has been trend-plotted over an extended

period (days or even weeks) can highlight

the affect of drift caused by temperature

changes and aging.

Diagnosing problems

Although successful trouble-

shooting is both an art and a

science, adopting a troubleshoot-

ing methodology and relying on

the functionality of an advanced

DSO can greatly simplify the

process.

Good troubleshooting practices

will save time and frustra-

tion. The time-tested approach

known as KGU, Known Good Unit

comparison, accomplishes both

goals. KGU builds on a simple

principle: an electronic system

that is working properly exhibits

predictable waveforms at critical

nodes within its circuitry, and

these waveforms can be cap-

tured and stored. This reference

library can be stored right on the

DSO as an online resource, or

can be printed out to serve as a

hard-copy reference document. If

the system or an identical system

later exhibits a fault or failure,

waveforms can be captured from

the faulty system—called the

Device Under Test (DUT)—and

compared with their counterparts

in the KGU. Consequently, the

DUT can either be repaired or

replaced.

To build a reference library,

start by identifying appropriate

test points, or nodes, on the DUT.

Now, run the KGU through its

paces, capturing the waveform

from each node. Annotate each

waveform as required.

Get into the habit of always

documenting key waveforms

and measurements. Having a

reference to compare to will

prove invaluable during future

troubleshooting.

6 Fluke Corporation A first look at DSOs

Troubleshooting

Whichever troubleshooting

scenario below is appropriate

at the time, remember that it’s

important to inspect waveforms

for fast-moving transients or

glitches, even if a spot check of

the waveform reveals no anoma-

lies. These events can be difficult

to spot, but the high sampling

rate of today’s DSOs, together

with effective triggering, makes

it possible.

DUT with KGU. This approach

assumes that you have access to

a KGU and a reference library.

1. Make sure the DUT and KGU

are set up in identical operat-

ing modes.

2. Starting at a high-level point

in the system or block dia-

gram, use the DSO to look

for the presence or absence

of fundamental signals. For

example, look for a line-

voltage supply, as well as

subsequent dc supply voltages

to the various subsystems.

This requires probing the

major input and output signals

at major nodes in the system.

3. Compare signals at key nodes

while changing the operat-

ing mode to see if a failure

occurs. With signals from both

devices available, you have

two options:

• Display the live waveform

from the KGU on Channel 1

of the DSO, and display the

live waveform from the DUT

on Channel 2.

• Capture a trace from the

KGU and overlay it with a

trace from the DUT. Perform

a waveform-compare or

pass/fail test.

• The ScopeMeter 190 Series

II has 4 input channels

which means that 4 signals

from 4 different sources can

be compared.

4. Continue with this process

until you note a variance

between the DUT and KGU

waveforms.

DUT with Circuit Diagrams.

This approach assumes that no

KGU and no waveform reference

library for the DUT are available,

but that circuit diagrams of the

DUT can be located.

1. Review the circuit diagrams to

understand the basic opera-

tion of the DUT.

• Analog circuits such as

oscillators, amplifiers and

signal conditioners (attenu-

ators, filters and dividers)

should exhibit uniform

waveform patterns.

• Digital circuits such as

gates, switches and pro-

cessors should display

waveforms with predictable

amplitudes, pulse periods

and even pulse patterns.

2. Starting at a high-level point

in the system or block dia-

gram, use the DSO to look

for the presence or absence

of fundamental signals. For

example, look for a line-

voltage supply, as well as

subsequent dc supply voltages

to the various subsystems.

This requires probing the

major input and output signals

at major nodes in the system.

3. Use the storage capability

of the DSO to capture and

compare waveforms while

changing the operating mode

of the DUT.

• Visualize a theoreti-

cal “good” waveform and

compare it to the waveform

displayed on the DSO. Try

to identify any obvious

anomalies.

4. Use the horizontal or vertical

cursors to quickly evaluate if

the time or amplitude of the

trace falls within the time or

amplitude ranges suggested

by the circuit design.

Complex DUT, No Circuit

Diagrams. This approach

assumes that the DUT is a fairly

complex system, that no KGU is

available, and that only limited

DUT documentation is available.

1. Study the circuit cards, look-

ing for common components

and circuits, and identify

high-level test points in the

system and check for the

presence or absence of funda-

mental signals. As before, start

at one point and work your

way backwards probing the

major input and output signals

at major nodes in the system.

2. Compare waveforms at key

nodes while changing the

operating modes to see if a

failure occurs. Store these

waveforms.

3. If an examination of the DUT

and analysis of waveforms at

key circuit nodes reveal no

obvious faults, use the DSO’s

storage capability to solicit the

aid of peers.

• Identify “suspect” wave-

forms from the DUT.

• Use the DSO’s Extract or

Output mode to save these

waveform files in a bitmap

(.bmp) format.

• Email the files to a peer or

factory expert anywhere in

the world for aid in trouble-

shooting the circuit.

4. Using outside experts, go

through each key node and

one by one eliminate the obvi-

ous good nodes, eventually

narrowing your focus to obvi-

ous faulty or suspect nodes.

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