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INSTRUCTION MANUAL

DT835A

DIGITAL

UHF TRANSMITTER

AXCERA, LLC

103 FREEDOM DRIVE P.O. BOX 525 LAWRENCE, PA 15055-0525 USA

(724) 873-8100 • FAX (724) 873-8105

www.axcera.com • info@axcera.com

Digital UHF Transmitter Table of Contents

DT835A, Rev. 1 i July 23, 2003

TABLE OF CONTENTS

CHAPTER 1 INTRODUCTION

Section Page

1.1 Manual Overview..................................................................................1-1

1.2 Safety.................................................................................................1-1

1.3 Maintenance ........................................................................................1-2

1.4 Material Return Procedure .....................................................................1-2

1.5 Limited One-Year Warranty for Axcera Products.......................................1-3

CHAPTER 2 SYSTEM DESCRIPTION AND CUSTOMER REMOTE CONNECTIONS

2.1 System Overview .................................................................................2-1

2.1.1 Single UHF Exciter Assembly ........................................................2-1

2.1.2 (Optional) 10 MHz Reference Distribution Amplifier Tray..................2-2

2.1.3 DM8-R Modulator Tray.................................................................2-2

2.1.4 Amplifier Array Assemblies...........................................................2-2

2.1.5 Transmitter Output Assemblies.....................................................2-3

2.1.6 Control and Status ......................................................................2-3

2.1.7 Metering Panel............................................................................2-4

2.1.8 UHF Exciter Tray .........................................................................2-5

2.1.9 Variable Gain/Phase Tray .............................................................2-6

2.1.10 UHF Amplifier Tray .....................................................................2-6

2.1.11 Modulator front panel menus and indicators ..................................2-7

2.1.12 Main AC Input............................................................................2-8

2.1.13 Customer Remote Interface Connections.............................. 2-8

CHAPTER 3 INSTALLATION AND SETUP PROCEDURES

3.1 Site Considerations................................................................................3-1

3.2 Unpacking the Cabinets and Trays...........................................................3-4

3.3 Installing the Cabinets and Trays ............................................................3-4

3.3.1 DM8-R Modulator Installation ........................................................3-5

3.4 Main AC Inputs......................................................................................3-6

3.5 Output Connections...............................................................................3-6

3.6 Setup and Operation Procedures.............................................................3-7

3.6.1 DM8-R Modulator Setup Procedures ...............................................3-8

CHAPTER 4 CIRCUIT DESCRIPTIONS

4.1 UHF Exciter Assembly ............................................................................4-1

4.1.1 Control and Status .......................................................................4-1

4.1.2 Digital Input and Remote Connections............................................4-1

4.1.3 AC Input .....................................................................................4-2

4.2 DM8-R Modulator Tray...........................................................................4-2

4.2.1 DM8 Digital Modulator Board .........................................................4-2

4.2.2 DM8-R Front Panel Board..............................................................4-3

4.3 UHF Exciter Tray ..................................................................................4-3

4.3.1 UHF Filter...................................................................................4-3

4.3.2 UHF Generator Board...................................................................4-4

4.3.3 (Optional) 10-MHz Reference Oscillator Board ................................4-4

Digital UHF Transmitter Table of Contents

DT835A, Rev. 1 ii July 23, 2003

TABLE OF CONTENTS

CHAPTER 4 CIRCUIT DESCRIPTIONS - Continued

Section Page

4.3.4 PLL Board...................................................................................4-5

4.3.5 Power Entry Module.....................................................................4-7

4.3.6 IF Phase Corrector Board .............................................................4-7

4.3.7 ALC Board ..................................................................................4-9

4.3.8 Visual/Aural Metering Board .......................................................4-15

4.3.9 UHF Upconverter Board..............................................................4-16

4.3.10 Transmitter Control Board .........................................................4-17

4.3.11 +12V(4A)/-12V(1A) Power Supply Board....................................4-22

4.3.12 5-Section Delay Equalizer Board ................................................4-23

4.4 Variable Phase/Gain Tray ....................................................................4-23

4.4.1 Variable Gain/Phase Board ..........................................................4-23

4.4.2 Metering Board ..........................................................................4-24

4.4.3 Power Entry Module....................................................................4-24

4.4.4 +15V/-12V Power Supply Board ..................................................4-24

4.5 Metering Panel ...................................................................................4-24

4.5.1 Splitter Board ............................................................................4-25

4.5.2 Visual/Aural Metering Board ........................................................4-25

4.5.3 Dual Peak Detectors ...................................................................4-25

4.5.4 Detector Threshold Board............................................................4-26

4.5.5 Power Entry Module....................................................................4-26

4.5.6 +/-12V Power Supply Board........................................................4-26

4.6 AC Distribution Assembly, UHF Exciter..................................................4-27

4.7 2-3kW Amplifier Array.........................................................................4-27

4.8 UHF Amplifier Trays............................................................................4-29

4.8.1 UHF Filter.................................................................................4-29

4.8.2 1-Watt Amplifier Board Assembly................................................4-29

4.8.3 1-Watt Amplifier Board ..............................................................4-30

4.8.4 Single Stage Amplifier Assembly.................................................4-30

4.8.5 Coupler Board...........................................................................4-34

4.8.6 Dual Peak Detector Enclosure.....................................................4-34

4.8.7 Dual Peak Detector Board ..........................................................4-34

4.8.8 4-Way Splitter Assembly............................................................4-35

4.8.9 4-Way Splitter Board .................................................................4-35

4.8.10 Dual Stage Amplifier Assembly...................................................4-35

4.8.11 Amplifier Protection Board .........................................................4-39

4.8.12 Amplifier Control Board.............................................................4-40

4.8.13 Variable Gain/Phase Enclosure...................................................4-42

4.8.14 Variable Gain/Phase Board ........................................................4-42

4.8.15 4-Way Combiner Assembly........................................................4-43

4.9 AC Distribution Assembly, Single Phase Amplifier Array..........................4-44

4.10 AC Distribution Assembly, Three Phase Amplifier Array..........................4-45

4.11 Combiner Reject Load Assembly..........................................................4-45

4.11.1 Combiner Reject Load Board......................................................4-46

Digital UHF Transmitter Table of Contents

DT835A, Rev. 1 iii July 23, 2003

TABLE OF CONTENTS - (continued)

Section Page

CHAPTER 5 DETAILED ALIGNMENT PROCEDURES

5.1 DM8-R Modulator ..................................................................................5-1

5.1.1 DM8-R Modulator Board Configuration .........................................5-1

5.2 UHF Exciter Tray ...................................................................................5-2

5.2.1 Delay Equalization Adjustment......................................................5-3

5.2.2 IF Phase Corrector Adjustment .....................................................5-3

5.2.3 UHF Generator Board...................................................................5-3

5.2.4 10-MHz Reference Generator Board ..............................................5-4

5.2.5 PLL Board...................................................................................5-4

5.3 UHF Amplifier Trays...............................................................................5-4

5.3.1 Variable Gain/Phase Board ...........................................................5-4

5.3.2 1-Watt UHF Amplifier Board .........................................................5-4

5.3.3 UHF Filter...................................................................................5-4

5.3.4 Single Stage Amplifier Assembly...................................................5-4

5.3.5 Coupler Board.............................................................................5-4

5.3.6 Dual Stage Amplifier Assembly .....................................................5-5

5.3.7 4-Way Splitter Assembly..............................................................5-5

5.3.8 Dual Stage Amplifier Assembly .....................................................5-5

5.3.9 4-Way Combiner Assembly...........................................................5-6

5.3.10 Circulator...................................................................................5-6

5.3.11 Dual Peak Detector Board............................................................5-6

5.3.12 Amplifier Protection Board ...........................................................5-6

5.3.13 Amplifier Control Board...............................................................5-6

5.3.14 Switching Power Supply ..............................................................5-6

5.3.15 Calibration of Output and Reflected Power and VSWR Cutback ........5-6

5.4 Phase and Gain Adjustment of the UHF Amplifier Trays in each Array..........5-7

5.5 Calibration of the Side A and Side B Arrays Forward Output Power .............5-7

5.6 Calibration of the Side A and Side B Arrays Reflected Output Power............5-7

5.7 Phase and Gain Adjustment for the entire Amplifier Array..........................5-8

5.8 Calibration of the Forward Output Power Level of the Transmitter...............5-8

5.9 Calibration of the Reflected Output Power Level of the Transmitter .............5-8

5.10 Calibration of the Reject Output Power Level of the Transmitter................5-9

APPENDICES

APPENDIX A SAMPLE LOG REPORT SHEET

APPENDIX B TYPICAL OPERATIONAL READINGS

APPENDIX C ASSEMBLY DRAWINGS AND PARTS LISTS

APPENDIX D SUBASSEMBLY DRAWINGS AND PARTS LISTS

APPENDIX E DT835A SYSTEM SPECIFICATIONS

Digital UHF Transmitter Table of Contents

DT835A, Rev. 1 iv July 23, 2003

LIST OF FIGURES

2-1 Exciter Remote Interface Assembly ................................................2-10

2-2 Amplifier Array Remote Interface Panel ..........................................2-14

3-1 1 kW Minimum Ventilation Configuration...........................................3-4

3-2 Tray Installation Instructions...........................................................3-6

5-1 Typical Digital Spectrum ...............................................................5-12

Digital UHF Transmitter Table of Contents

DT835A, Rev. 1 v July 23, 2003

LIST OF TABLES

2-1 DT835A Major Assemblies ...............................................................2-1

2-2 Metering Panel Meters.....................................................................2-4

2-3 UHF Exciter Tray Meters..................................................................2-5

2-4 UHF Exciter Tray Switches...............................................................2-5

2-5 UHF Exciter Tray Fault Indicators.....................................................2-5

2-6 UHF Exciter Tray Samples ...............................................................2-6

2-7 Variable Phase/Gain Tray Meters......................................................2-6

2-8 Variable Phase/Gain Tray Sample.....................................................2-6

2-9 UHF Amplifier Tray Meters...............................................................2-6

2-10 UHF Amplifier Tray Status Indicators................................................2-7

2-11 UHF Amplifier Tray Control Adjustments ...........................................2-7

2-12 UHF Amplifier Tray Sample..............................................................2-7

2-13 Modulator Front Panel LEDs.............................................................2-8

2-14 Customer Remote Control Interface Connections.................... 2-12

3-1 MPEG and 10 MHz Input Information................................................3-6

3-2 Jumper Positions in the Gain 1 and Gain 2 Circuits

For an Input Level of Between –6 and –26 dBm.................................3-8

4-1 Fuses, Idling Currents and Voltage Settings for Amplifier Devices......4-42

5-1 8-VSB Modulator Connections..........................................................5-1

5-2 Jumper Configurations for the Symbol Generator Board......................5-3

5-3 Jumper Configurations for the VSB Filter Board .................................5-5

5-4 DIP Switch Settings........................................................................5-6

5-5 Jumper Positions on the 6 dB Pad ..................................................5-10

5-6 Center Frequencies for the Delay Equalizer Sections.........................5-11

5-7 PLL Board Switch Positions............................................................5-13

5-8 Switch Positions for Channel Frequencies........................................5-14

Digital UHF Transmitter Chapter 1, Introduction

DT835A, Rev. 1 1-1

Chapter 1

Introduction

This manual explains the installation,

setup, alignment, and maintenance

procedures for the DT835A Digital UHF

Transmitter. It is important that you

read all of the instructions, especially

the safety information in this chapter,

before you begin to install or operate

the unit.

1.1 Manual Overview

This instruction manual is divided into

four chapters and supporting appendices.

Chapter 1, Introduction, contains

information on safety, maintenance,

return procedures, and warranties. The

Chapter 2, System Description and

Customer Remote Interface Connections,

describes the transmitter and includes

discussions of system control and status

indicators and remote control

connections. Chapter 3, Installation and

Set up Procedures, explains how to

unpack, install, setup, and operate the

transmitter. Chapter 4, Circuit

Descriptions, provides information on the

boards and subassemblies that make up

the DT835A. Chapter 5, Detailed

Alignment Procedures, provides

information on adjusting the system

assemblies for optimal operation The

appendices contain sample log sheets,

typical operational readings, assembly

and subassembly drawings and parts list,

and system specifications.

1.2 Safety

The digital UHF transmitters

manufactured by Axcera are designed for

ease of use and repair while providing

protection from electrical and mechanical

hazards. Listed throughout the manual

are notes, cautions, and warnings

concerning possible safety hazards that

may be encountered while operating or

servicing the transmitter. Please review

these warnings and familiarize yourself

with the operation and servicing

procedures before working on the

transmitter.

Read All Instructions – All of the

operating and safety instructions should

be read and understood before operating

this equipment.

Retain Manuals – The manuals for the

transmitter should be retained at the

transmitter site for future reference. We

provide two sets of manuals for this

purpose; one set can be left at the office

while one set can be kept at the site.

Heed Notes, Warnings, and

Cautions – All of the notes, warnings,

and cautions listed in this safety section

and throughout the manual must be

followed.

Follow Instructions – All of the

operating and use instructions for the

transmitter should be followed.

Cleaning – Unplug or otherwise

disconnect power from the equipment

before cleaning. Do not use liquid or

aerosol cleaners. Use a damp cloth for

cleaning.

Ventilation – Openings in the cabinets

and tray front panels are provided for

ventilation. To ensure reliable operation,

and to protect the unit from overheating,

these openings must not be blocked.

Servicing – Do not attempt to service

this product yourself until becoming

familiar with the equipment. If in doubt,

refer all servicing questions to qualified

Axcera service personnel.

Replacement Parts – When

replacement parts are used, be sure that

the parts have the same functional and

performance characteristics as the

original part. Unauthorized substitutions

may result in fire, electric shock, or other

Digital UHF Transmitter Chapter 1, Introduction

DT835A, Rev. 1 1-2

hazards. Please contact the Axcera

Technical Service Department if you have

any questions regarding service or

replacement parts.

1.3 Maintenance

The DT835A is designed with

components that require little or no

periodic maintenance except for the

routine cleaning of the fans and the front

panels of the trays.

The amount of time between cleanings

depends on the conditions within the

transmitter room. While the electronics

have been designed to function even if

covered with dust, a heavy buildup of

dust, dirt, or insects will affect the

cooling of the components. This could

lead to a thermal shutdown or premature

failure of the affected trays.

When the front panels of the trays

become dust covered, the top covers

should be removed and any accumulated

foreign material removed. A vacuum

cleaner, utilizing a small wand-type

attachment, is an excellent way to

suction out the dirt. Alcohol and other

cleaning agents should not be used

unless you are certain that the solvents

will not damage components or the silk-

screened markings on the trays and

boards. Water-based cleaners can be

used, but do not saturate the

components. The fans and heatsinks

should be cleaned of all dust or dirt to

permit the free flow of air for cooling

purposes.

It is recommended that the operating

parameters of the transmitter be

recorded from the meters on the trays

and the system metering control panel at

least once a month. It is suggested that

this data be retained in a rugged folder

or envelope. A sample format for a log

sheet is provided in Appendix A.

Photocopies of the log sheet should be

made for continued data entries.

1.4 Material Return Procedure

To insure the efficient handling of

equipment or components that have been

returned for repair, Axcera requests that

each returned item be accompanied by a

Material Return Authorization Number

(MRA#).

An MRA# can be obtained from any

Axcera Field Service Engineer by calling

the Axcera Field Service Department, at

(724) 873-8100. This procedure applies

to all items sent to the Field Service

Department regardless of whether the

item was originally manufactured by

Axcera.

When equipment is sent to the field on

loan, an MRA# is included with the unit.

The MRA# is intended to be used for the

return of the unit to Axcera. In addition,

all shipping material should be retained

for the return of the unit to Axcera.

Replacement assemblies are also sent

with an MRA# to allow for the proper

routing of the exchanged hardware.

Failure to close out this type of MRA# will

normally result in invoicing for the value

of the loaner item or the exchange

assembly.

When shipping an item to Axcera, please

include the MRA# on the packing list and

on the shipping container. The packing

slip should also include contact

information and a brief description of why

the unit is being returned.

Please forward all MRA items to:

Axcera

103 Freedom Drive

P.O. Box 525

Lawrence, PA 15055-0525 USA

For more information concerning this

procedure, call the Axcera Field Service

Department, at (724) 873-8100 or by

fax at (724) 873-8105.

Axcera can also be contacted through e-

mail at service@axcera.com and on the

Web at www.axcera.com.

Digital UHF Transmitter Chapter 1, Introduction

DT835A, Rev. 1 1-3

1.5 Limited One-Year Warranty for

Axcera Products

Axcera warrants each new product that

it has manufactured and sold against

defects in material and workmanship

under normal use and service for a

period of one (1) year from the date of

shipment from Axcera's plant, when

operated in accordance with Axcera's

operating instructions. This warranty

shall not apply to tubes, fuses,

batteries, or bulbs.

Warranties are valid only when and if

(a) Axcera receives prompt written

notice of breach within the period of

warranty, (b) the defective product is

properly packed and returned by the

buyer (transportation and insurance

prepaid), and (c) Axcera determines, in

its sole judgment, that the product is

defective and not subject to any misuse,

neglect, improper installation,

negligence, accident, or (unless

authorized in writing by Axcera) repair

or alteration. Axcera’s exclusive liability

for any personal and/or property

damage (including direct, consequential,

or incidental) caused by the breach of

any or all warranties, shall be limited to

the following: (a) repairing or replacing

(in Axcera’s sole discretion) any

defective parts free of charge (F.O.B.

Axcera’s plant) and/or (b) crediting (in

Axcera’s sole discretion) all or a portion

of the purchase price to the buyer.

Equipment furnished by Axcera, but not

bearing its trade name, shall bear no

warranties other than the special hours-

of-use or other warranties extended by

or enforceable against the manufacturer

at the time of delivery to the buyer.

NO WARRANTIES, WHETHER

STATUTORY, EXPRESSED, OR

IMPLIED, AND NO WARRANTIES OF

MERCHANTABILITY, FITNESS FOR

ANY PARTICULAR PURPOSE, OR

FREEDOM FROM INFRINGEMENT,

OR THE LIKE, OTHER THAN AS

SPECIFIED IN PATENT LIABILITY

ARTICLES, AND IN THIS ARTICLE,

SHALL APPLY TO THE EQUIPMENT

FURNISHED HEREUNDER.

Digital UHF Transmitter Chapter 2, System Description and

Customer Remote Connections

DT835A, Rev. 1 2-1

Chapter 2

System Description and Customer Remote Connections

MAJOR ASSEMBLY NUMBER TRAY/ASSEMBLY NAME

A1 Single UHF exciter assembly

A2 and A3 two 2-3kW amplifier array assemblies

A4 Hybrid combiner assembly

A7 Directional coupler assembly

Output coupler assembly

DM8-R digital modulator tray

Table 2-1: DT835A Major Assemblies

2.1 System Overview

The DT835A is a complete UHF solid-

state television transmitter that operates

at an output power of 2000 watts to

3000 watts digital, depending on the

digital application. The DT835A is made

up of three cabinets: a UHF exciter

cabinet and two amplifier array cabinets.

The DT835A is made up of the trays and

assemblies shown in Table 2-1.

2.1.1 (A1) Single UHF Exciter

Assembly

The single UHF exciter assembly

contains (A1) a UHF exciter tray, a DM8-

R digital modulator tray, (A10) an

(Optional) 10 MHz reference generator

tray, (A3) a splitter, (A4 and A5) two

variable phase/gain trays, (A6) a

metering panel, (A8) an AC distribution

assembly, and (A9) a remote interface

assembly.

The DM8-R modulator tray can have an

external 10 MHz reference connected to

J3 on the rear panel or it will generate

and internal 10 MHz reference if no

external reference is supplied. The

Digital input connects to J2 on the

remote interface assembly that is cabled

to J2 on the DM8-R modulator. The

Output from the DM8-R modulator at J4

connects to J6 on the rear of the UHF

exciter tray. The Digital IF is wired to

J18 on the 5 section delay equalizer

board, 44 MHz (1072090). The

processed digital IF signal is then cabled

to the J32 on the ALC board (1265-

1305). The digital IF input must be

selected as the input. This is

accomplished by connecting the jumper

W11 on J29 between pins 2 and 3 on the

ALC board. The digital IF is wired

through the K3 and K4 relays to the rest

of the ALC board. The LED DS5, digital

IF modulator enable, should be lit. The

ALC board has the capability of

switching between two different IF

inputs, but in this case only the digital IF

input is used.

The output of the ALC board (-10 dBm)

connects to (A11) the UHF upconverter

board (1265-1310) in the upconverter

section of the UHF digital exciter. The

upconverter takes the LO and

heterodynes it with the IF; the signal is

then filtered to produce the RF on-

channel output.

The (A15-A1) UHF generator board

(1565-1109) is mounted in the UHF

Generator Enclosure (1519-1144) for EMI

and RFI protection. The board contains a

VCXO circuit and additional circuitry to

multiply the VCXO frequency by eight.

The output is split and provides an input

to the x8 multiplier circuitry as well as a

sample for the PLL board. The amplified

eighth harmonic is then fed to the SMA

output jack of the board at J3. Typical

output level of the signal is +16 dBm

nominal.

The (Optional) (A14-A1) 10-MHz

reference generator board (1519-1126)

Digital UHF Transmitter Chapter 2, System Description and

Customer Remote Connections

DT835A, Rev. 1 2-2

is located in the (Optional) (A10) 10-MHz

reference kit (1286-1108). The board

contains a high-stability crystal oscillator

that provides a 10-MHz output that is

used as reference frequency for the

transmitter. The board is mounted within

an enclosed assembly that helps to

maintain the operating temperature of

the oscillator board.

The (A13) PLL board (1286-1104) is part

of the phase lock loop (PLL) circuit, which

provides the automatic frequency control

(AFC) voltage, that connects to the VCXO

assembly, on the UHF Generator board,

and maintains the accurate output

frequency of the VCXO. The AFC is

generated by comparing a sample of the

10-MHz reference to a sample of the

VCXO frequency. The PLL board uses an

external 10-MHz signal as the reference

unless it is missing, then an internally

generated 10-MHz signal is used. A

sample of the signal from the UHF

generator board connects to SMA jack J9,

the sample input on the board. The signal

is amplified by U8 and coupled to U9, a

divide by 20/21 IC. A sample of the

signal at the output of U8 is connected to

J10, the sample output jack on the

board, which is typically connected to the

front panel of the tray.

The selected 10-MHz reference connects

to amplifier IC U1 whose output is split.

A sample of the 10-MHz reference is

cabled to jack J3, the 10-MHz output

jack, which is connected to J5 on the rear

of the tray.

If the 50 kHz from the 10-MHz reference

and the 50-kHz from the UHF generator

board become unlocked, the red Unlock

LED, lights and the Lock LED, located on

the LED display board, is extinguished.

The RF output of the UHF exciter at J15

is split two ways by the (A3) splitter.

The RF output #1 connects to the input

of (A4) one of the variable phase/gain

trays and the RF output #2 connects to

the input of (A5) the other variable

phase/gain tray. The output of the (A4)

variable phase/gain tray connects to

(A2) the side A amplifier array

assembly. The output of the (A5)

variable phase/gain tray connects to

(A3) the side B amplifier array

assembly.

2.1.2 (Optional) 10 MHz Reference

Distribution Amplifier Tray

The (Optional) 10 MHz reference

distribution amplifier tray provides a 10

MHz reference at a suitable output level

for distribution.

The system is made up of an internal 10

MHz reference source, a reference

distribution amplifier, and splitter kit(s)

without precise offset IF to distribute the

reference signal to each transmitter.

The reference distribution amplifier tray

utilizes an internal 10 MHz reference

generator assembly to generate a 10

MHz reference signal. This signal is then

divided into five equivalent signals and

then amplified and delivered to the

output of the tray.

2.1.3 DM8-R Digital Modulator Tray

The DM8-R modulator Tray is an ATSC

compliant 8 VSB modulator that mounts

in a standard 19” rack. The DM8-R

modulator accepts a SMPTE-310 MPEG

data stream and outputs a 6 MHz wide

IF output centered at 44 MHz with a

pilot carrier at 46.69 MHz. The DM8-R

modulator provides linear and nonlinear

correction capability for the transmission

path as well as internal test sources that

are used during initial transmitter

installation. All of the functions of the

DM8-R modulator are controlled from

the LX Controller LCD display and

pushbuttons.

2.1.4 Amplifier Array Assemblies

The amplifier array assemblies each

contain (A9-A1) an 8-way splitter

(ZFSC-8-43) with four to six connections

used depending on the power level of

the transmitter, four to six (A1, A2, A3,

Digital UHF Transmitter Chapter 2, System Description and

Customer Remote Connections

DT835A, Rev. 1 2-3

A4, A5 & A6) UHF amplifier trays, (A7) a

4 to 6-way combiner, (A8) an output

coupler, (A10) an AC distribution

assembly, and (A11) an interface panel.

The RF input from the variable

phase/gain tray connects to J1 on (A11)

the interface panel in the amplifier array

assembly. The RF is connected to the

COM input of the 8-way splitter, which

splits it eight ways, with six of the

outputs connected to J1, the RF input on

each of the UHF amplifier trays and the

other two outputs terminated with 50Ω.

The UHF amplifier trays amplify the RF

signals to the power needed to produce

a maximum of 300 watts 64 QAM or 8-

VSB digital per tray.

The outputs of the four, five or six UHF

amplifier trays are combined in the 4, 5

or 6-way combiner. This provides

approximately half of the power needed

to generate the full 8-VSB digital output

of the transmitter. The RF output is

connected to the output coupler

assembly. The output coupler assembly

supplies a forward and a reflected power

sample of the output from the amplifier

assembly to the metering panel in the

single exciter assembly.

In each UHF amplifier tray, the forward

power and reflected power samples from

the 4-way combiner board are

connected to the dual peak detector

board, single supply, that provides peak

detected samples to the amplifier control

board. The amplifier control board then

supplies DC voltages to the front panel

meter of the UHF amplifier tray. Before

exiting each UHF amplifier tray, the RF

is fed through a circulator for the

protection of the tray from high VSWR

conditions.

2.1.5 Transmitter Output

Assemblies

The outputs of the amplifier array

assemblies connect through (A5 and A6)

1-5/8” to 3-1/8” adapters to (A4) a

hybrid combiner (55223A) that

combines the output from the amplifier

assemblies into a single output. The

reject output of the hybrid combiner is

connected to (A7) an output coupler that

provides a reject sample from J3 to the

metering panel in the single UHF exciter

assembly for monitoring purposes. The

output of the (A7) directional coupler

connects to (A8) a 2500-watt reject load

that dissipates any reject power

produced during the combining process.

A (A8-A1) thermal switch is mounted on

the 2500-watt reject load that connects

to the overtemperature fault circuit on

the transmitter control board in the UHF

exciter tray. This fault circuit shuts down

the transmitter if an overtemperature

fault occurs. The overtemperature is

normally caused by a large reject power

being dissipated by the reject load due

to the malfunctioning of one of the

amplifier arrays.

The output of the (A4) hybrid combiner

at J3 is fed to (A11) the output coupler

assembly. The filtered signal is

connected to (A11) an output coupler

assembly that provides combined

forward and reflected power samples to

the metering panel in the single UHF

exciter assembly. The forward sample is

processed to provide peak detected

visual and aural power output samples

to the front panel meter on the metering

panel. The reflected power sample is

also peak detected and wired to the

front panel meter.

2.1.6 Control and Status

Information on the control and status of

the transmitter is provided by the

indicators on the metering panel and the

variable phase/gain trays. There are also

control, status, and LED indicators on

the front panel of the UHF exciter tray.

The functions of these control and status

indicators are shown in the following

tables. The switches and LED indicators,

which are mounted so that the switches

and LEDs can be operated or viewed

from the front panel of the UHF exciter,

are part of the transmitter control board.

Digital UHF Transmitter Chapter 2, System Description and

Customer Remote Connections

DT835A, Rev. 1 2-4

On the UHF exciter tray, switch (S1) is

an Operate/Standby switch that

provides the Operate command

(Enable), when in Operate, to the each

of the amplifier arrays. The Enable to

each amplifier array is split four, five or

six ways and then applied to the UHF

amplifier trays. The Enable is needed to

turn on the switching power supplies

located in the UHF amplifier trays.

When the UHF exciter is in Operate, the

green LED (DS2) is on and when the

exciter is in Standby, the amber LED

(DS1) is on.

NOTE: If the transmitter does not

switch to Operate when S1 is switched

to Operate, check that a dummy jumper

plug, with a jumper between pins 1 and

2, is connected to jack J7 on (A9) the

remote interface assembly in the single

UHF exciter assembly. This jumper

provides the interlock needed for the

operation of the transmitter. If the

interlock is present, the green LED

(DS5) on the transmitter control board

should be lit.

The operation of the transmitter is

controlled by the front panel switches on

the UHF exciter tray. During normal

operation of the transmitter, switch S2

should be in the Auto position. The front

panel of the UHF exciter also has LEDs

that indicate a Modulation Fault (Loss),

red LED (DS9), and a VSWR Cutback,

amber LED (DS7).

2.1.7 Metering Panel

Table 2-2: Metering Panel Meters

METER FUNCTION

Combination meter

(A13) Displays power in terms of a percentage of the calibrated

transmitter output level on the scale; a full-scale reading is

120%

Switch (S1), meter Selects the desired % Forward, % Reject, or % Reflected

reading.

% Forward (0 - 120 %) Displays the output power level (≈100%)

% Reject (0 - 120 %) Displays the % Reject Output Power (<5%)

% Reflected (0 - 120 %)

Displays the % Reflected Output Power (<5%)

Power meter side A

(A15) Displays power in terms of a percentage of the calibrated

amplifier array output level on the scale; a full-scale reading

is 120%

Switch (S2), meter Selects the desired % Forward or % Reflected reading

Forward (0 - 120 %) Displays the output level of the array (≈ 95%)

% Reflected (0 - 120 %)

Displays the % Reflected Output Power of the array (<5%)

Power meter side B

(A16) Displays power in terms of a percentage of the calibrated

amplifier array output level on the scale; a full-scale reading

is 120%

Switch (S3), meter Selects the desired % Forward or % Reflected reading

Forward (0 - 120 %) Displays the output level of the array (≈ 95%)

% Reflected (0 - 120 %)

Displays the % Reflected Output Power of the array (< 5%)

Digital UHF Transmitter Chapter 2, System Description and

Customer Remote Connections

DT835A, Rev. 1 2-5

2.1.8 UHF Exciter Tray

Table 2-3: UHF Exciter Tray Meters

METER FUNCTION

Meter (A18) Displays power in terms of a percentage of the calibrated

exciter output level on the upper scale. The voltage level is

read on one of the two bottom scales. A full-scale reading on

the top scale is 120%. 100% equals +10 dBm 8-VSB digital.

Also reads the ALC voltage level.

Switch (S3), meter Selects the desired % Exciter or the ALC voltage reading

ALC (0 - 1 V) Displays the ALC voltage level (≈0.85 VDC)

% Exciter Displays the % Exciter Output Power. This is the level needed

to attain (0 -100) which is the desired output power of the

transmitter (≈ 30 %).

Table 2-4: UHF Exciter Tray Switches

SWITCH FUNCTION

Transmitter (S1)

Operate/Standby

The momentary contact switch (S1) applies a ground to K1, a

latching relay, on the transmitter control board. K1 will switch

either to Operate or to Standby depending on which direction

S1 is pushed. When switched to Operate, a low, Enable

command, is applied to the UHF amplifier trays mounted in the

amplifier arrays. These Enables will turn on the UHF amplifier

trays. The opposite occurs when the transmitter is switched to

Standby.

Mode Select (S2)

Auto/Manual The momentary contact switch (S2) applies a ground to K2, a

latching relay, on the transmitter control board. K2 will switch

the transmitter to Automatic or Manual depending on which

direction S2 is pushed. In Automatic, the Modulation Fault

command from the ALC board will control the operation of the

transmitter. The transmitter will switch to Standby, after a

slight delay, if the input modulation is lost and will quickly

switch back to Operate when the modulation is restored. In

Manual, the transmitter is controlled by the operator using the

front panel Operate/Standby switch or by remote control.

Table 2-5: UHF Exciter Tray Fault Indicators

FAULT INDICATOR FUNCTION

Modulation loss (DS9

red) Indicates that the input has been lost to the transmitter. The

fault is generated on the ALC board in the UHF exciter tray.

VSWR cutback (DS7

amber) Indicates that the reflected power level of the transmitter

has increased above 20%. This will automatically cut back

the output power level to 20%. The fault is generated on the

transmitter control board in the UHF exciter tray.

Digital UHF Transmitter Chapter 2, System Description and

Customer Remote Connections

DT835A, Rev. 1 2-6

Table 2-6: UHF Exciter Tray Samples

SAMPLE DESCRIPTION

f(s) A sample of the channel oscillator output taken from

the sample jack of the PLL board

Exciter O/P An output power sample of the exciter taken from the

UHF upconverter board

2.1.9 Variable Phase/Gain Trays

Table 2-7: Variable Phase/Gain Tray Meters

METER FUNCTION

Switch (S1), meter Selects the desired % Power or the ALC voltage

reading

% Output Power Displays the % Output Power of the amplifier array

(100%=+15 dBm 8- VSB digital ≈45%)

ALC Displays the ALC voltage from the UHF exciter that

connects to the amplifier trays in the amplifier array

(.6 - 1 VDC typical)

Table 2-8: Variable Phase/Gain Tray Sample

SAMPLE DESCRIPTION

Output (J4), (0 dBm) A front panel sample of the RF output of the

phase/gain tray, which connects to the amplifier

array, taken from the variable gain/phase board

2.1.10 UHF Amplifier Trays

Table 2-9: UHF Amplifier Tray Meters

METER FUNCTION

Meter (A9) Displays power in terms of a percent of the calibrated power

output value. A full-scale reading is 100%, which is equivalent to

300 watts 8-VSB digital. Also reads % Reflected Power, power

supply voltage levels, and AGC voltage levels.

Switch (S2),

meter Selects the desired % Power or the voltage reading

% Output pwr Displays the % Output Power of the tray (100%=300 watts 8-VSB

digital)

% Refl

(Reflected) Displays the % Reflected Output Power of the tray (<5% with all

amplifier trays operating as measured on the top scale)

Power supply Displays the power supply voltage, +26.5 VDC, on the middle

scale

AGC voltage Displays the AGC voltage level, +1 - +2 VDC, on the bottom scale

Digital UHF Transmitter Chapter 2, System Description and

Customer Remote Connections

DT835A, Rev. 1 2-7

Table 2-10: UHF Amplifier Tray Status Indicators

INDICATOR FUNCTION

Enable (DS4 green) Indicates that an Enable, Operate command, is

applied to the UHF amplifier tray from the selected

UHF exciter tray

Overdrive (DS2 red) Indicates that the level of the drive is too high. The

protection circuit will limit the drive to the set

threshold. The fault is generated on the amplifier

control board.

VSWR cutback (DS1 red) Indicates that the reflected power level of the tray has

increased above 50%. This will automatically cut back

the output power level to 50%. The fault is generated

on the amplifier control board.

Overtemp (DS3 red) Indicates that the temperature of (A5-A6- A3 and A5-

A6-A4), one or both of two thermal switches mounted

on the heatsink assembly for the output amplifiers, is

above 175° F. When this fault occurs, the Enable to

the switching power supply in the affected amplifier

tray is immediately removed and it will shut down.

Input fault (DS5 red) Indicates that the input RF level to the amplifier trays

dropped below the 0 dBm range

Table 2-11: UHF Amplifier Tray Control Adjustments

ADJUSTMENT FUNCTION

Phase (A10-R5) Adjusts the phase of the RF output by approximately

70°

Gain (A11-R6) Adjusts the gain of the RF output when the amplifier

control board is in the AGC mode

Table 2-12: UHF Amplifier Tray Sample

SAMPLE DESCRIPTION

Module O/P (0 dBm) A sample of the combined output of the four dual-

stage amplifier boards taken from the dual-peak

detector board

2.1.11 DM8-R Modulator Front Panel LED Indicators

The front panel LEDs are described in Table 2-13.

Table 2-13: Modulator Front Panel LEDs

LED Indicator Function

PWR Indicates that the DC power supply is operating

MPEG

LED Indicates the presence of a valid MPEG stream at the

J1-2B input

PLL A

LED Indicates that the DM8 symbol clock is locked to the

frequency of the 10 MHz reference.

PLL B

LED Indicates that the pilot frequency is locked to the

incoming 10 MHz reference.

Digital UHF Transmitter Chapter 2, System Description and

Customer Remote Connections

DT835A, Rev. 1 2-8

2.1.12 Main AC Input

The transmitter needs an AC input of

208/240 VAC at 100 amps for each

amplifier array assembly and at 20 amps

for the exciter assembly.

The 208/240 VAC input to each amplifier

assembly connects to (A10) the AC

distribution assembly, amplifier array

(1278-1200), on the right, center of

each cabinet. The assembly contains the

terminal block (TB1) that connects to

the 208/240 VAC. Line 1 is connected to

TB1-1A, line 2 to TB1-3A, and safety

ground to TB1-4A.

The distribution assembly contains nine

circuit breakers that supply the AC to

the rest of the amplifier assembly. The

input AC from TB1 is connected to (CB1)

the main AC circuit breaker (100 amps)

that distributes the 208/240 VAC to the

other eight circuit breakers. The output

of CB1 has three MOVs, VR3, VR4, and

VR6, mounted to it: VR4 is connected

from line 1 to ground, VR6 is connected

from line 2 to ground and VR3 is

mounted across the two lines. The

switched input AC is wired through the

six circuit breakers, CB2-CB7, to the six

UHF amplifier trays in the amplifier

cabinets. CB2 is a 20-amp circuit

breaker that supplies the AC voltage to

the (A1) UHF amplifier tray. CB3 is a 20-

amp circuit breaker that supplies the AC

voltage to the (A2) UHF amplifier tray.

CB4 is a 20-amp circuit breaker that

supplies the AC voltage to the (A3) UHF

amplifier tray. CB5 is a 20-amp circuit

breaker that supplies the AC voltage to

the (A4) UHF amplifier tray. CB6 is a 20-

amp circuit breaker that supplies the AC

voltage to the (A5) UHF amplifier tray.

CB7 is a 20-amp circuit breaker that

supplies the AC voltage to the (A6) UHF

amplifier tray. The circuit breakers CB8

and CB9 control the AC to the A12-A3

and A12-A5 fans mounted on the

(optional) (A12) reject load assembly

that only used as part of the 2.5 and 3

kW amplifier array assemblies.

The 208/240 VAC input to the single

UHF exciter assembly connects to (A8)

the AC distribution assembly, single UHF

exciter assembly (1245-1500), in the

right, center rear of the cabinet. The

assembly contains the terminal block

(TB1) that connects to the 208/240

VAC. Line 1 is connected to TB1-1A, line

2 to TB1-3A, and ground to TB1-2A. The

AC distribution panel contains (CB1) the

main circuit breaker (20 amps) that

supplies the AC to the rest of the single

exciter assembly. The output of CB1 has

three MOVs, VR1, VR2, and VR3,

connected to it: VR1 from line 1 to

ground, VR3 from Line 2 to ground and

VR2 across the two lines. The AC output

of CB1 connects to A1 and A2, the IEC

outlet strips, into which the external

trays connect. The (A1) UHF exciter tray

and the (A4) variable phase/gain tray

plug into the (A1) IEC outlet strip. The

(A5) variable phase/gain tray, the (A6)

metering panel, the (optional) (A7)

receiver tray, the 8 VSB modulator tray

or other optional trays plug into the (A2)

IEC outlet strip. When the circuit

breaker CB1 on the single UHF exciter

assembly is switched on, +12 VDC from

the UHF exciter tray is supplied to each

of the amplifier array cabinets. The +12

VDC is split six ways in the amplifier

array and connected to each of the UHF

amplifier trays to operate the LED status

indicators in the tray. Also +12V is

connected to the (Optional) external

exhaust kit, if present.

2.1.13 Customer Remote Interface

Connections

The remote monitoring and operation of

the transmitter is provided through the

jacks (J8, J9, and J10) on (A9) the

remote interface assembly mounted on

the rear of the single UHF exciter

assembly (see Figure 2-1).

Digital UHF Transmitter Chapter 2, System Description and

Customer Remote Connections

DT835A, Rev. 1 2-9

Figure 2-1: Exciter Remote Interface Assembly

Jack (J7) should have a dummy plug

connected to it, with a jumper

connected between pins 1 and 2, to

provide the interlock to the exciter

needed to operate the transmitter. If the

jumper is missing, the transmitter will

not switch to Operate. If remote

connections are made to the

transmitter, they should be made

through the plugs provided in the

installation material for the transmitter

and wired as indicated on the

interconnect drawing for the single UHF

exciter assembly (11056919). The

remote control interface connections for

this transmitter are shown in

Table 2-14.

Digital UHF Transmitter Chapter 2, System Description and

Customer Remote Connections

DT835A, Rev. 1 2-10

Table 2-14: Remote Control Interface Connections

FUNCTION REMOTE JACK/PIN

NUMBER INTERFACE TYPE

Exciter Enable Interlock J7-1

Exciter Enable Interlock Rtn J7-2 J7-1 and 2 must be

jumpered together for

normal operation.

Transmitter Remote Metering

Combined Output Power J10-7

Combined Output Power Rtn J10-8 1V full scale at 1kΩ source

resistance

Combined Reflected Power J10-9

Combined Reflected Power

Rtn J10-10 1V full scale at 1kΩ source

resistance

Exciter Output J9-26

Exciter Output Rtn J9-27 1V full scale at 1kΩ source

resistance

Exciter Remote Control Commands

Exciter Operate (Enable) J8-3 Contact closure

Exciter Standby/Operate Rtn J8-2

Exciter Standby (Disable) J8-1 Contact closure

Exciter Auto J8-6 Contact closure

Exciter Auto/Manual Rtn J8-5

Exciter Manual J8-4 Contact closure

Power Raise (optional) J9-6 Contact closure

Raise/Lower Rtn (optional) J9-5

Power Lower (optional) J9-4 Contact closure

Exc Modulator Select

(optional) J8-10 Contact closure

Exc Modulator Select Rtn

(optional) J8-11

Exciter Remote Status Indications

Exciter Auto Mode Indicator J9-30 50 mA max current sink

Auto/Manual Mode Ind Rtn J9-31

Exciter Manual Mode

Indicator J9-32 50 mA max current sink

Operate Indicator J9-1 50 mA max current sink

Operate/Standby Ind Rtn J9-2

Standby Indicator J9-3 50 mA max current sink

Digital UHF Transmitter Chapter 2, System Description and

Customer Remote Connections

DT835A, Rev. 1 2-11

FUNCTION REMOTE JACK/PIN

NUMBER INTERFACE TYPE

Exciter VSWR Cutback

Indicator J8-7 50 mA max current sink

Video Loss Indicator J9-7 50 mA max current sink

Video Loss Indicator Rtn J9-8

Exc Receiver Fault Ind

(optional) J8-9 50 mA max current sink

Side A Driver Remote Metering

Side A Driver Output Power J10-1

Side A Driver Output Power

Rtn J10-2 1V full scale at 1kΩ source

resistance

Side B Driver Remote Metering

Side B Driver Output Power J10-3

Side B Driver Output Power

Rtn J10-4 1V full scale at 1kΩ

source resistance

Side A Remote Metering

Side A Forward Power J10-14

Side A Forward Power Rtn J10-15 1V full scale at 1kΩ source

resistance

Side A Reflected Power J10-16

Side A Reflected Power Rtn J10-17 1V full scale at 1kΩ source

resistance

Side B Remote Metering

Side B Forward Power J10-20

Side B Forward Power Rtn J10-21 1V full scale at 1kΩ

source resistance

Side B Reflected Power J10-18

Side B Reflected Power Rtn J10-19 1V full scale at 1kΩ source

resistance

Side A + B Remote Metering

Reject Power J10-11

Reject Power Rtn J10-12 1V full scale at 1kΩ source

resistance

The above connections are made to jack (J9), the 37-position “D” connector; to jack

(J10), the 25-position “D” connector; to jack (J8), the 25-position “D” connector; or

to jack (J7), the 9-position “D” connector on (A9) the remote interface assembly

mounted at the top, rear of the single UHF exciter cabinet.

Digital UHF Transmitter Chapter 2, System Description and

Customer Remote Connections

DT835A, Rev. 1 2-12

Figure 2-2: Amplifier Array Remote Interface Panel

Amplifier Array Assembly Remote Metering

(A1) UHF Amp Tray Refl Pwr J5-2

(A1) Reflected Power Rtn J5-1 1V full scale at 1kΩ source

resistance

(A1) UHF Amp Tray Fwrd Pwr J5-4

(A1) Forward Power Rtn J5-3 1V full scale at 1kΩ source

resistance

(A2) UHF Amp Tray Refl Pwr J5-6

(A2) Reflected Power Rtn J5-5 1V full scale at 1kΩ source

resistance

(A2) UHF Amp Tray Fwrd Pwr J5-8

(A2) Forward Power Rtn J5-7 1V full scale at 1kΩ source

resistance

(A3) UHF Amp Tray Refl Pwr J5-9

(A3) Reflected Power Rtn J5-10 1V full scale at 1kΩ source

resistance

(A3) UHF Amp Tray Fwrd Pwr J5-11

(A3) Forward Power Rtn J5-12 1V full scale at 1kΩ source

resistance

(A4) UHF Amp Tray Fwrd Pwr J5-13

(A4) Forward Power Rtn J5-14 1V full scale at 1kΩ source

resistance

(A4) UHF Amp Tray Refl Pwr J5-15

(A4) Reflected Power Rtn J5-16 1V full scale at 1kΩ source

resistance

(A5) UHF Amp Tray Fwrd Pwr J5-18

(A5) Forward Power Rtn J5-17 1V full scale at 1kΩ source

resistance

(A5) UHF Amp Tray Refl Pwr J5-21

(A5) Reflected Power Rtn J5-20 1V full scale at 1kΩ source

resistance

The above connections are made to jack (J5), the 37-position “D” connector on

(A11) the remote interface panel (Figure 2-2) mounted at the top, rear of each of

the amplifier array cabinets.

Digital UHF Transmitter Chapter 3, Installation and Setup Procedures

DT835A, Rev. 0 3-1

Chapter 3

Installation and Setup Procedures

There are special considerations that

need to be taken into account before the

DT835A can be installed. For example, if

the installation is completed during cool

weather, a heat-related problem may not

surface for many months, suddenly

appearing during the heat of summer.

This section provides planning

information for the installation and set up

of the transmitter.

3.1 Site Considerations

The transmitter requires an AC input line

of 220 VAC with a rating of 20 amps for

the exciter cabinet and 100 amps for

each amplifier cabinet. Make sure that

the proposed site for the transmitter has

the voltage requirements that are

needed.

The DT835A is designed and built to

provide long life with a minimum of

maintenance. The environment in which

it is placed is important and certain

precautions must be taken. The three

greatest dangers to the transmitter are

heat, dirt, and moisture. Heat is usually

the greatest problem, followed by dirt,

and then moisture. Over-temperature

can cause heat-related problems such as

thermal runaway and component failure.

Each amplifier tray in the transmitter

contains a thermal interlock protection

circuit that will shut down that tray until

the temperature drops to an acceptable

level.

A suitable environment for the

transmitter can enhance the overall

performance and reliability of the

transmitter and maximize revenues by

minimizing down time. A properly

designed facility will have an adequate

supply of cool, clean air, free of airborne

particulates of any kind, and no

excessive humidity. An ideal environment

will require temperature in the range of

40° F to 70° F throughout the year,

reasonably low humidity, and a dust-free

room. It should be noted that this is

rarely if ever attainable in the real world.

However, the closer the environment is

to this design, the greater the operating

capacity of the transmitter.

The fans and blowers designed and built

into the transmitter will remove the heat

from within the trays, but additional

means are required for removing this

heat from the building. To achieve this, a

few considerations should be taken into

account. The first step is to determine

the amount of heat to be removed. There

are generally three sources of heat that

must be considered. The first and most

obvious is the heat from the transmitter

itself. This can be determined by

subtracting the average power to the

antenna (1500 watts) from the AC input

power (15,000 watts). This number in

watts (13,500) is then multiplied by

3.41, which gives 46,035, the BTUs to be

removed every hour. 12,000 BTUs per

hour equals one ton, so a 4-ton air

conditioner will cool a 1.5 kW digital

transmitter.

The second source of heat is other

equipment in the same room. This

number is calculated in the same way as

the equation for BTUs. The third source

of heat is equally obvious but not as

simple to calculate. This is the heat

coming through the walls, roof, and

windows on a hot summer day. Unless

the underside is exposed, the floor is

usually not a problem. Determining this

number is usually best left up to a

qualified HVAC technician. There are far

too many variables to even estimate this

number without detailed drawings of the

site showing all construction details. The

sum of these three sources is the total

amount of heat that must be removed.

There may be other sources of heat, such

as personnel, and all should be taken into

account.

Digital UHF Transmitter Chapter 3, Installation and Setup Procedures

DT835A, Rev. 0 3-2

Now that the amount of heat that must

be removed is known, the next step is to

determine how to accomplish this. The

options are air conditioning, ventilation,

or a combination of the two. Air

conditioning is always the preferred

method and is the only way to create

anything close to an ideal environment.

Ventilation will work quite well if the

ambient air temperature is below 100° F,

or about 38° C, and the humidity is be

kept at a reasonable level. In addition,

the air stream must be adequately

filtered to ensure that no airborne

particulate of any kind will be carried into

the transmitter. The combination of air

conditioning for summer and ventilation

during the cooler months is acceptable

when the proper cooling cannot be

obtained through the use of ventilation

alone and using air conditioning

throughout the year is not feasible.

Caution: The operation of air

conditioning and ventilation

simultaneously is not recommended.

This can cause condensation in

transmitters. For tube type

transmitters, this can be especially

serious if the condensation forms in

the tube cavity and creates

damaging arcs.

The following precautions should be

observed regarding air conditioning

systems:

1. Air conditioners have an ARI

nominal cooling capacity rating. In

selecting an air conditioner, do not

assume that this number can be

equated to the requirements of

the site. Make certain that the

contractor uses the actual

conditions that are to be

maintained at the site in

determining the size of the air

conditioning unit. With the desired

conditioned room temperature

under 80° F, the unit must be

derated, possibly by a substantial

amount.

2. Do not have the air conditioner

blowing directly onto the

transmitter. Condensation may

occur on, or worse in, the

transmitter under certain

conditions.

3. Do not isolate the front of the

transmitter from the back with the

thought of air conditioning only

the front of the unit. Cooling air is

drawn in at the front of all

transmitters and in the front and

back of others. Any attempt to

isolate the front from the rear will

adversely affect the cooling air

flow.

4. Interlocking the transmitter with

the air conditioner is

recommended to keep the

transmitter from operating without

the necessary cooling.

5. The periodic cleaning of all filters

is a must.

When using ventilation alone, the

following general statements apply:

1. The blower, with attendant filters,

should be on the inlet, thereby

pressurizing the room and

preventing dirt from entering the

transmitter.

2. The inlet and outlet vents should

be on the same side of the

building, preferably the leeward

side. As a result, the pressure

differential created by wind will be

minimized. Only the outlet vent

may be released through the roof.

3. The inlet and outlet vents should

be screened with 1/8" hardware

cloth (preferred) or galvanized

hardware cloth (acceptable).

4. Cooling air should enter the room

as low as practical but in no case

higher than four feet above the

floor. The inlet must be located

Digital UHF Transmitter Chapter 3, Installation and Setup Procedures

DT835A, Rev. 0 3-3

where dirt, leaves or snow will not

be carried in with the cooling air.

5. The exhaust should be located as

high as possible. Some ducting is

usually required to insure the

complete flushing of heated air

with no stagnant areas.

6. The filter area must be adequate

to insure a maximum air velocity

of 300 feet per minute through the

filter. This is not a conservative

number but a never-exceed

number. In a dusty or remote

location, this number should be

reduced to 150 CFM.

7. The inlet and outlet(s) must have

automatic dampers that close any

time the ventilation blower is off.

8. In those cases in which

transmitters are regularly off for a

portion of each day, a

temperature-differential sensor

that controls a small heater must

be installed. This sensor will

monitor inside and outside

temperatures simultaneously. If

the inside temperature falls to

within 5° F of the outside

temperature, the heater will come

on. This will prevent condensation

when the ventilation blower comes

on and applies even in the

summer.

9. A controlled-air bypass system

must be installed to prevent the

temperature in the room from

falling below 40° F during

transmitter operation.

10. The blower should have two

speeds, which are thermostatically

controlled, and interlocked with

the transmitter.

11. The blower on high speed must be

capable of moving the required

volume of air into a half inch of

water pressure at the required

elevation. The free air delivery

method must not be used.

12. Regular maintenance of any filters

can not be overemphasized.

13. Tube transmitters should not rely

on the internal blower to exhaust

cooling air at elevations above

4000 feet. For external venting,

the air vent on the cabinet top

must be increased to an 8"

diameter for a 1 kW transmitter

and to 10" for 5 kW and 10 kW

transmitters. An equivalent

rectangular duct may be used but,

in all cases, the outlet must be

increased in area by 50% through

the outlet screen.

14. It is recommended that a site plan

be submitted to Axcera for

comments before installation

commences.

In calculating the blower requirements,

filter size, and exhaust size, if the total

load is known in watts, 2000 CFM into

1/2" of water will be required for each

5000 watts. If the load is known in BTUs,

2000 CFM into 1/2" of water will be

required for each 17,000 BTUs. The inlet

filter must be a minimum of seven

square feet, larger for dusty and remote

locations, for each 5000 watts or 17,000

BTUs. The exhaust must be at least four

square feet at the exhaust screen for

each 5000 watts or 17,000 BTUs.

The information presented in this section

is intended to serve only as a general

guide and may need to be modified for

unusually severe conditions. A

combination of air conditioning and

ventilation should not be difficult to

design (see Figure 3-1). System

interlocking and thermostat settings

should be reviewed with Axcera. As with

any equipment installation, it is always

good practice to consult the

manufacturer when questions arise.

Axcera can be contacted at

(724) 873-8100.

Digital UHF Transmitter Chapter 3, Installation and Setup Procedures

DT835A, Rev. 0 3-4

Figure 3-1. 1 kW Minimum Ventilation Configuration

3.2 Unpacking the Cabinets and

Trays

Note: Air conditioning and any

related heat exhaust ducts should be

in place before continuing with the

installation of the transmitter.

Thoroughly inspect the cabinets and all

other materials upon their arrival. Axcera

certifies that upon leaving our facility the

equipment was undamaged and in proper

working order. The shipping containers

should be inspected for obvious damage

that indicates rough handling. Check for

dents and scratches or broken switches,

meters, or connectors. Any claims

against in-transit damage should be

directed to the carrier. Inform Axcera as

to the extent of any damage as soon as

possible.

Remove the cabinets, trays, hybrid

combiner, bandpass filter, trap filter, and

output couplers, along with any

installation material, from the crates and

boxes. Remove the straps that hold the

cabinets to the shipping skids and slide

the cabinets from the skids. Remove the

plastic wrap and foam protection from

around the cabinets. Do not remove any

labeling or tags from any cables or

connectors. These are identification

markers that make reassembly of the

transmitter much easier.

3.3 Installing the Cabinets and Trays

There are three cabinets: (A1) the single

UHF exciter assembly, (A2) the side A

amplifier array assembly, and (A3) the

side B amplifier array assembly. With the

operator facing the cabinets, they should

be arranged from left to right with the

single UHF exciter cabinet on the left, the

side A amplifier cabinet in the center, and

the side B amplifier cabinet on the right.

The cabinets should be positioned with

consideration given to adequate air

intake and exhaust, the opening of the

rear door, access to the trays (including

sliding them out for testing), the AC

hookup, and the installation of the output

transmission line. The cabinets should be

grounded using copper strapping

material and should also be permanently

mounted to the floor of the site using the

holes in the bottom of the cabinets.

Digital UHF Transmitter Chapter 3, Installation and Setup Procedures

DT835A, Rev. 1 3-5

If trays are pre-mounted in the exciter

cabinet, remove the two L-brackets,

mounted on the front panel rails of the

single exciter cabinet, which hold the

trays in place during shipment. The UHF

amplifier trays are shipped separately

from the amplifier array cabinets because

of the weight of each tray. The trays are

mounted in the cabinet using Chassis

Trak cabinet slides. The tray slides are on

the top and bottom of the UHF amplifier

trays and on the sides of the UHF exciter

tray and the variable phase/gain trays.

Inspect the trays for any loose hardware

or connectors, tightening where needed.

Open the rear door; the key to unlock

the door, if the door has the optional

lock, is found in a tan envelope taped to

the door. Inspect the interior for packing

material and carefully remove any

packing material that is found. Slowly

slide each tray in and out to verify that

they do not rub against each other and

have no restrictions to free movement.

Note: The UHF amplifier trays must

be placed into the cabinets in the

proper location, and according to the

labeling on each tray, or the gain and

the phasing will not be maximized.

The UHF amplifier trays are labeled to

indicate where they are positioned in the

side A or the side B amplifier cabinets.

They are mounted in each cabinet from

bottom left to top right, with A1 the

bottom, left tray and A6 (if used) the top,

right tray. Locate the tray labeled A1 and

slide it into the side A cabinet in the

bottom, left position. Locate the tray

labeled A2 and slide it into the side A

cabinet in the bottom, right position.

Locate the tray labeled A3 and slide it

into the side A cabinet in the middle, left

position. Locate the tray labeled A4 and

slide it into the side A cabinet in the

middle, right position. Locate the tray

labeled A5 and slide it into the side A

cabinet in the top, left position.

Locate the UHF amplifier tray labeled B1

and slide it into the side B cabinet in the

bottom, left position. Locate the tray

labeled B2 and slide it into the side B

cabinet in the bottom, right position.

Locate the tray labeled B3 and slide it

into the side B cabinet in the middle, left

position. Locate the tray labeled B4 and

slide it into the side B cabinet in the

middle, right position. Locate the tray

labeled B5 and slide it into the side B

cabinet in the top, left position. All of the

amplifier trays should now be installed.

Slowly slide each tray in and out to verify

that they do not rub against each other

and have no restrictions to free

movement.

Caution: Each UHF amplifier tray has

a hard-line coaxial cable connected

to the rear panel. The tray will not

slide out without first removing this

connection.

In order to pull out the tray for test

purposes or other reasons, use the test

cable included in the installation material

kit for connections from the tray to the

output cable.

It may be necessary to make

adjustments to the positions of the trays.

This can be accomplished by loosening

the cabinet slide mounting bolts that hold

the front of the slide to the mounting

frame of the cabinet and moving the tray

up or down or left or right, as needed, to

correct for any misalignment.

3.3.1 DM8-R Modulator Installation

1. If not pre-mounted, remove the

modulator tray from the shipping box

and inspect it for any damage that

may have occurred during shipment.

Remove all packing material used to

protect the unit during shipment.

2. The tray mounts using four clearance

holes into a standard 19" rack or

cabinet. Mount the tray into the

cabinet and adjust or align the tray

so that it does not interfere with

cabling or other trays.

Digital UHF Transmitter Chapter 3, Installation and Setup Procedures

DT835A, Rev. 1 3-6

3. Verify that the circuit breaker (CB1),

located on the rear of the tray, and

the main circuit breaker for the

system are off.

4. Connect the AC power cord, provided

in the installation kit, to J1, the AC

input jack on the rear of the tray.

5. Connect the MPEG input to J2, the IF

output from J4, and the 10 MHz

reference input, if supplied, to J3 on

the rear of the tray.

6. Connect the IF output at J4 to the

digital IF input to the upconverter of

the system.

This completes the installation procedure

for the DM8-R modulator.

3.4 Main AC Inputs

Once the cabinets are in place, and the

trays have been checked for damage and

alignment, the main AC hookup is ready

to be made.

Caution: Before connecting the

208/240 VAC, make certain that all

of the circuit breakers associated

with the transmitter are switched

off.

There are three AC input circuits to the

DT835A transmitter: one is 20 amps and

the two others are 100 amps single

phase. The input AC to the exciter

cabinet should be a 20 amp, 208/240

VAC line. The input AC to each amplifier

cabinet should be a single-phase, 100

amp, 208/240 VAC line.

The 20-amp, 208/240 VAC input

connections are made to the terminal

block A1-TB1, part of the AC distribution

assembly, exciter cabinet (1245-1500),

near the center, right-hand side, rear

portion of cabinet #1. Connect terminal 1

to line 1, terminal 3 to line 2 (208/240

VAC), and terminal 2 to the chassis

ground.

For single phase - Connect one of the

100-amp, 208/240 VAC inputs to the

terminal block A3-TB1, part of the AC

distribution assembly, amplifier

assembly (1278-1100), near the center,

right-hand side, rear portion of cabinet

#2. Connect line 1 to TB1-1A, line 2 to

TB1-3A and the safety ground to TB1-

4A. Follow the same procedure for

cabinet #3.

3.5 Output Connections

The RF output of the (A2) amplifier

assembly connects through (A5) a 1-5/8"

to 3-1/8" adapter to J1 on (A4) the

hybrid combiner. The RF output of the

(A3) amplifier assembly connects

through (A6) a 1-5/8" to 3-1/8" adapter

to J2 on (A4) the hybrid combiner. The

reject output of the combiner at J4

connects through (A12) a 1-5/8” to 3-

1/8” adapter to (A7) a directional coupler

(1016-1043) and then to (A8) the 2500-

watt reject load. The combined RF output

of the combiner at J3 connects through

(A9) the bandpass filter to (A10) the

output trap filter assembly. The filtered

output connects through (A11) the

output coupler assembly (1020-1002) to

the transmission line that is connected to

the antenna.

This completes the unpacking and

installation procedures for the DT835A

transmitter. Refer to the setup and

operation procedures that follow before

applying power to the transmitter.

Digital UHF Transmitter Chapter 3, Installation and Setup Procedures

DT835A, Rev. 1 3-7

Table 3-2: Jumper Positions in the Gain 1 and Gain 2 Circuits for an

Input Level of Between –6 and –26 dBm

IF Input Range W3 on J4 and W4 on J5 W5 on J6 and W6 on J7

-11 to -6 dBm Low Low

-16 to -11 dBm Low High

-21 to -16 dBm High Low

-26 to -21 dBm High High

3.6 Setup and Operation Procedures

The transmitter should initially be turned

on with the RF output at J2 of the (A11)

output coupler assembly terminated into

a dummy load. If a load is not available,

check that the output of the coupler

assembly at J2 is connected to the

antenna. Connect the digital IF input to

J6 on the rear of the UHF exciter tray. On

(A5) the 5 Section delay equalizer board,

44 MHz, set the input matching jumpers

J2 and J3 to positions 1 and 2 for a 50Ω

input, or to positions 2 and 3 for a 75Ω

input, as needed. Depending on the input

level, two gain circuits on the delay

equalizer board, 44 MHz, are adjusted

using jumpers, W3-W6, to set the gain

through the board. Table 3-2 shows the

jumper positions in the gain 1 and gain 2

circuits for an input level of between -6

to -26 dBm.

Switch on the main AC circuit breaker on

the AC distribution assembly mounted

toward the rear of the single UHF exciter

assembly. Switch on the main AC circuit

breakers on the AC distribution

assemblies mounted toward the rear of

the amplifier cabinets. Switch on the

CB2-CB6 circuit breakers for the

individual UHF amplifier trays on the AC

distribution assemblies mounted in each

amplifier array cabinet.

Switch the Operate/Standby switch on

the UHF exciter to standby and the

Auto/Manual switch, also on the UHF

exciter, to Auto. Normal operation of the

transmitter is with the switch in

Automatic. Automatic operation of the

exciter uses the digital input to the

exciter as an Operate/Standby switch. In

Auto, if the input is lost, the exciter will

automatically cause the transmitter to

revert to Standby. When the input signal

is restored, the exciter will automatically

return the transmitter to Operate.

Move the Operate/Standby switch on the

exciter to Operate. Observe the power

supply readings on the front panel

meters of the UHF amplifier trays in each

amplifier cabinet assembly; the reading

should be +26.5 VDC.

Note: If the transmitter does not

switch to Operate when the

Operate/Standby switch is switched

to Operate, check that there is an

external interlock plug connected to

jack (J7) on the remote interface

assembly mounted in the top, rear of

the single UHF exciter cabinet. Make

sure that the plug has a jumper from

pins 1 to 2 to provide the interlock

for the exciter. The interlock must be

present for the transmitter to

operate.

Observe the front panel meter on the

metering panel with the switch in the

Combined Visual Output Power position;

it should read 100%. If needed, adjust

the power adjust screwdriver pot on the

front panel of the UHF exciter to attain a

100% reading on the front panel meter.

At the same time that the output power

is being observed, check the meter

readings on the metering panel and the

combined meter, in the % Reflected

Power position, for the side A and side B

amplifier assemblies. If the % Reflected

Power for any of the readings is very

high, above 50%, a problem is present

with the output coaxial lines in the side A

amplifier assembly or in the output lines

Digital UHF Transmitter Chapter 3, Installation and Setup Procedures

DT835A, Rev. 1 3-8

for the system and needs to be checked

and corrected. A center bullet missing

from the 1-5/8" or 3-1/8" rigid coax lines

or loose bolts on the connections can

cause this problem. Return the

Operate/Standby switch to Standby.

Observe the % Exciter Power reading of

the meter on the exciter; it should be the

same as on the Test Data Sheet for the

transmitter, typically 70%. Move the

Operate/Standby switch on the exciter to

Standby.

The gain and phase controls on the front

panels of the individual UHF amplifier

trays were adjusted at the factory to

attain 100% output of the transmitter

and should not need to be readjusted.

The forward meter readings for side A

and side B may not be the same, but

should be the same as the values written

on the Test Data Sheet. Side A and side

B are combined to give the total output,

which is 100% in the combined output

power position.

The readings on each of the individual

UHF amplifier trays may not be the

same. Refer to the Test Data Sheet and

compare the final readings from the

factory with the readings on each of the

trays after the setup. They should be

very similar. If a reading is way off, refer

to the phasing and power adjustment

procedures for the UHF amplifier trays in

Chapter 5, Detailed Alignment

Procedures, of this manual before trying

to make any adjustments.

If a dummy load is connected to the

transmitter, switch the transmitter to

Standby and switch off the main AC

circuit breakers found on the AC

distribution panels in each cabinet.

Remove the dummy load and make all of

the connections needed to connect the

transmitter to the antenna. Switch the

main AC circuit breakers on and the

Operate/Standby switch to Operate.

Adjust the exciter power adjust pot to

attain 100% combined visual output.

If the transmitter is already connected to

the antenna, check that the combined

power output is 100%. If necessary,

adjust the power adjust pot located on

the UHF exciter for 100%.

3.6.1 DM8-R Modulator Setup

Procedures

NOTE: The modulator was setup at the

factory to meet specifications for digital

mask and corrections and should not

need adjusted.

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-1

Chapter 4

Circuit Descriptions

4.1 (A1) UHF Exciter Assembly

The (A1) UHF Exciter Assembly

contains (A1) a UHF Exciter Tray

(1142445), (A4 & A5) two Variable

Gain/Phase Trays (1245-1200), (A6) a

Metering Panel (1061456), (A8) an AC

Distribution Assembly, Exciter (1245-

1500) and (A9) a Remote Interface

Assembly (1061198).

The (A1) UHF Exciter Tray (1142445),

takes the Digital IF Input that is

upconverted to the On Channel RF by

mixing it with a L.O. signal generated

by the Channel Oscillator Assembly.

The RF Output of the UHF Exciter at J15

connects to (A3) a Splitter which splits

the RF two ways with the RF Output #1

connected to the input of the (A5)

Variable Gain/Phase Tray (1245-1200)

and the RF Output #2 connected to the

input of the (A4) Variable Gain/Phase

Tray (1245-1200). The output of the

(A4) Variable Gain/Phase Tray connects

to (A2) the Side A Amplifier Assembly

(1278-1300). The output of the (A5)

Variable Gain/Phase Tray connects to

(A3) the Side B Amplifier Assembly

(1278-1300).

4.1.1 Control and Status

The Control and Status of the

Transmitter are provided by the Meter

indications on the Metering Panel and

the Variable Gain Phase Trays. There

are also Control, Status and LED

Indications located on the front panel of

the UHF Exciter Tray. The switches and

LED indicators, which are mounted so

that the switches and LEDs are

operated or viewed from the front

Panel of the UHF Exciter, are part of the

Transmitter Control Board (1245-

1101). On the UHF Exciter Tray, switch

(S1) is an Operate/Standby Switch that

provides the Operate Commands

(Enables), when in Operate, to each of

the Amplifier Arrays. The Enable is

needed to turn on the Switching Power

Supplies located in each of the UHF

Amplifier Trays. When the UHF Exciter

is in Operate, the Green LED (DS2) is

On and when in Standby the Amber

LED (DS1) is On. NOTE: If the

Transmitter does not switch to Operate,

when S1 is switched to Operate, check

that a Dummy Jumper Plug is

connected to Jack J7, with a Jumper

between Pins 1 & 2, located on (A9) the

Remote Interface Assembly in the

Single UHF Exciter Assembly. The

Jumper provides the Interlock to the

UHF Exciter needed for the operation of

the Transmitter. If the Interlock is

present, the Green LED (DS5), located

on the Transmitter Control Board,

should be lit.

Operation of the Transmitter is

controlled by the front panel switches

located on the UHF Exciter Tray.

During Normal operation of the

Transmitter, Switch S2 should be in the

Auto position. The front panel of the

UHF Exciter also has LEDs that indicate

a Input Fault (Loss), Red LED (DS9)

and a VSWR Cutback, Amber LED

(DS7).

4.1.2 Digital Input and Remote

Connections

The Digital Input to the Transmitter,

connects to the (A9) Remote Interface

Panel located on the rear of the Single

UHF Exciter Assembly. The Digital

Input connects to Jack J2 that is cabled

to J2 on the DM8-R Modulator.

Remote Monitoring and Operation of

the Transmitter is provided through the

Jacks (J8, J9 & J10) located on (A9) the

Remote Interface Assembly mounted

toward the rear of the Single UHF

Exciter Assembly. Jack (J7) should

have a dummy plug connected to it,

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-2

which has a jumper connected between

Pins 1 & 2, that provides the Interlock

to the Exciter needed to operate the

Transmitter. If the Jumper is missing,

the Transmitter will not switch to

Operate. If remote connections are

made to the Transmitter, they should

be made through the plugs provided in

the Installation Material as noted on the

Interconnect Drawing (1056919 or

1278-8400) for the Single UHF Exciter.

4.1.3 AC Input

The Single UHF Exciter Assembly needs

an AC input of 208/240 VAC at 20

Amps. The AC Input to the Single UHF

Exciter Assembly connects to (A8) the

AC Distribution Assembly, UHF Exciter

Assembly (1245-1500) located in the

right, center rear of the Cabinet. The

Assembly contains the 4 Terminal Block

(TB1) to which the 208/240 VAC

connects. Line 1 to TB1-1, Line 2 to

TB1-3 and Safety Ground to TB1-2.

The AC Distribution Panel contains one

Circuit Breaker that supplies the AC to

the rest of the Single Exciter Assembly.

The Input AC is connected to (CB1) the

Main AC Circuit Breaker (20 Amps).

The output of CB1 has three MOVs,

VR1, VR2 and VR3, connected to it, one

connected from each leg of the Input

AC to ground and one across the two

legs. The AC output of CB1 connects to

A1 and A2 which are IEC Outlet Strips.

The (A1) Exciter 1 and the (A4)

Variable Gain/Phase Tray plug into the

(A1) IEC Outlet Strip. The (A5)

Variable Gain/Phase Tray, the (A8)

Metering Panel and the 8 VSB

Modualtor Tray plug into the (A2) IEC

Outlet Strip.

When the Circuit Breaker CB1,

mounted on the AC Distribution

Assembly, is switched On, +12 VDC

from the Exciter, is supplied to each of

the Amplifier Array Cabinets for the

operation of the LED Status Indicators

in each of the UHF Amplifier Trays.

4.2 (A19) DM8-R Digital Modulator

(1306978; Appendix C)

4.2.1 (A1) DM8 Modulator Board

(1304883; Appendix D)

4.2.1.1 SMPTE-310 Input

The DM8-R modulator accepts a

SMPTE-310 input the BNC Jack J2

located on the rear panel of the tray.

This input is connected to J42 on the

Digital Modulator Board via a RG-179

cable. This input is applied to a high

speed window comparator that adjusts

the level to a low voltage TTL signal to

be used by the Altera FPGA, U3. The

SMPTE-310 signal is input to the FPGA

to recover the clock and the data. A

portion of the clock and recovery

circuit is performed by a high speed

comparator, U17, that functions as an

external delay circuit.

4.2.1.2 Channel Coder

The FPGA subsequently uses the

SMPTE-310 clock and data as the input

to the channel coder contained inside

the FPGA. The channel coder is a

series of DSP blocks defined by the

ATSC standard for 8 VSB data

transmission. These blocks include the

data randomizer, Reed Solomon

Encoder, data interleaver, trellis coder,

and sync insertion. The channel

coder portion inside the FPGA

generates the 8 distinct levels in an 8

VSB transmitter. These levels are

subsequently input to a linear

equalizer that provides for frequency

response correction in the

transmission path. The linear

equalizer is a 67 tap FIR filter that is

loaded with tap values from the

microntroller, U1, located on this

board. The output of the linear

equalizer is then input to two pulse

shaping filters, an in phase (I) and a

quadrature (Q) filter that are also

located inside the FPGA. The pulse

shaping filters are FIR filters that have

fixed tap values that are preset inside

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-3

the FPGA. The output of the pulse

shaping filters is then applied to a Pre-

Distortion Linearizer chip, U4, which

can be used to correct for

nonlinearities in the data transmission

path. The output of the Pre-Distortion

chip is gain scaled and output to a dual

D/A converter that output a baseband

I and Q analog signal.

4.2.1.3 Analog Output Section

The baseband I and Q signals from the

D/A converter are applied to

differential analog filters that remove

some of digital artifacts from the D/A

conversion process. The output of the

I channel filter is then mixed with the

pilot frequency, 46.69 MHz, using

mixer U30. The output of the Q filter

is mixed with the pilot frequency that

is phase shifted 90 degrees using

mixer U34. The mixers are current

driven devices so that when the

outputs of U30 and U34 are connected

together, they provide a combined

output. This combined output is

subsequently input to a final

differential output filter which provides

the final IF output at the SMA

connector, J38. This output is

connected through a RG-174 cable to

the BNC connector J4, the IF output

jack, located on the rear panel of the

tray.

4.2.1.4 Pilot Frequency Generation

The 46.69 MHz pilot signal, which is

used in the mixing process, is

generated from a 46.69 MHz VCXO

that is phase locked to a 10 MHz

reference. The VCXO and the 10 MHz

are divided down to a common

frequency, which is then compared

internal to the FPGA. The FPGA

subsequently provides error signals to

a analog phase locked implemented

with op amp stages U45-A, B and C.

The output of these compensation

stages is used as the control voltage to

the VCXO, U37. The phase locked

output of U37 is applied to a analog

filter to remove harmonics of the pilot

and then input to quadrature splitter

Z1. The outputs of Z1 are used as the

inputs to the mixers in the analog

output section.

4.2.2 (A2) DM8-R Front Panel

Board (1307113; Appendix D)

The front panel board contains four

LEDs, Power, MPEG, PLL A and PLL B,

which are viewable on the front panel.

J2 connects to the Modulator Board,

which controls the operation of the

LEDs. When the system is operation

normally, all LEDs will be Green. If a

problem occurs, that LED will not be

lit.

4.3 (A4) UHF Exciter Tray (1142445

or 1294-1111; Appendix C)

4.3.1 (A12 and A18) UHF Filter

(1007-1101; Appendix D)

The UHF filter is a tunable two-section

cavity filter that is typically tuned for a

bandwidth of 5 MHz and has a loss of -

1 dB through the filter.

4.3.2 (A15-A1) UHF Generator

Board (1565-1109; Appendix D)

The UHF generator board is mounted in

the UHF Generator Enclosure (1519-

1144) for EMI and RFI protection. The

board contains a VCXO circuit and

additional circuitry to multiply the VCXO

frequency by eight. The VCXO produces

an output of ≈ 67 MHz to 132 MHz,

depending on the desired channel

frequency. Course adjustment to the

frequency is made by C11, while fine

adjustments are accomplished by the

AFC voltage from (A11) the PLL board

(1286-1104). The VCXO frequency level

is adjusted by C6, L2, and L4. The

output is split and provides an input to

the x8 multiplier circuitry as well as a

sample for the PLL board.

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-4

The x8 circuitry consists of three

identical x2 broadband frequency

doublers. The input signal at the

fundamental frequency is fed through a

6-dB pad consisting of R21, R24, and

R25 to amplifier U3. The output of the

amplifier stage is directed through a

bandpass filter consisting of L8 and

C32, which is tuned to the fundamental

frequency (67 MHz to 132 MHz). The

voltage measured at TP1 is typically +.6

VDC. The first doubler stage consists of

Z1 with bandpass filter L9 and C34

tuned to the second harmonic (134 MHz

to 264 MHz). The harmonic is amplified

by U4 and again bandpass filtered at

the second harmonic by C38 and L11

(134 MHz to 264 MHz). The voltage

measured at TP2 is typically +1.2 VDC.

The next doubler stage consists of Z2

with bandpass filter C40 and L12 tuned

to the fourth harmonic of the

fundamental frequency (268 MHz to

528 MHz). The fourth harmonic is then

amplified by U5 and fed through

another bandpass filter tuned to the

fourth harmonic consisting of L14 and

C44 (268 MHz to 528 MHz). The voltage

measured at TP3 is typically +2.0 VDC.

The final doubler stage consists of Z3

with bandpass filter C46 and L15 tuned

to the eighth harmonic of the

fundamental frequency (536 MHz to

1056 MHz). The signal is amplified by

U6 and U7 to a typical value of from +2

to +4 VDC as measured at TP4. The

amplified eighth harmonic is then fed to

the SMA output jack of the board at J3.

Typical output level of the signal is +16

dBm nominal.

The +12 VDC for the board enters

through jack J4-3 and is filtered by L22

and C54-C58 before being distributed to

the circuits on the board.

4.3.3 (Optional) (A14-A1) 10-MHz

Reference Generator Board (1519-

1126; Appendix D)

The 10-MHz reference generator board

is located in (A10) the (Optional) 10-

MHz reference kit (1286-1108). The

board contains a high-stability crystal

oscillator that provides a 10-MHz

output that is used as reference

frequency for the transmitter. The

board is mounted within an enclosed

assembly that helps to maintain the

operating temperature of the oscillator

board.

The oscillator operates at 10 MHz.

Transistor Q1 is the oscillating

transistor with the frequency of

oscillation set by the crystal Y1. L2, C2,

and C3 have second-order effects on

the frequency, with C2 and C3 used to

pull the oscillator exactly on frequency.

Capacitors C4 and C5 provide the

positive feedback necessary for

oscillation.

The output of the oscillator is buffered

by Q2 and Q3. L4 and C12 form a 40-

MHz bandpass filter. The 40-MHz signal

passes through a digital divider IC U4,

which divides the signal down to 10

MHz. Transistor Q5 provides a buffered,

10-MHz output at jack J1.

The crystal is heated in an enclosed

crystal oven (HR-1) that is internally

set at 60° C. The oscillator board is

heated by a separate oven that is set at

50° C. U1 is a temperature

sensor/controller IC that monitors the

temperature of the oscillator assembly

and controls the operation of Q4 and

U2. The operating temperature of the

assembly is set by adjusting R15. If the

temperature of the assembly falls

below 50° C, U1 will bias Q4 on, which

in turn increases the amount of current

flow through U2.

The flange of U2 is thermally connected

to the heatsink of the assembly. The

temperature of the heatsink will

increase as the current through U2

increases. As a result, U2 will dissipate

more power in the form of heat, and

the temperature of the assembly will

increase. If the assembly temperature

rises above 50° C, the opposite action

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-5

will occur, thus lowering the

temperature of the assembly.

The +12 VDC enters the board at J2

and is filtered by L1 and C1 before it is

applied to the remaining circuits on the

board.

4.3.4 (A13) PLL Board (1286-1104;

Appendix D)

The PLL board is part of the phase lock

loop (PLL) circuit, which provides the

automatic frequency control (AFC)

voltage, that connects to the VCXO

assembly, and maintains the accurate

output frequency of the VCXO. The AFC

is generated by comparing a sample of

the 10-MHz reference to a sample of

the VCXO frequency. The PLL board

uses an external 10-MHz signal as the

reference unless it is missing, then an

internally generated 10-MHz signal is

used. The two 10-MHz reference

signals are connected to the K1 relay

and the selected reference to U1. The

switching between the two references

is accomplished by the K1 relay which,

when energized, applies the external

10-MHz reference to U1, as long as an

externally generated 10-MHz reference

signal is present and an interlock is

connected to J8, pin 1.

If the interlock is removed or the

external 10-MHz reference is missing,

the relay is de-energized and the

internal 10-MHz reference is applied

through the relay to U1. The internally

generated 10-MHz reference connects

from J7 to pins 3 and 6 of relay K1. The

externally generated 10-MHz reference

connects from J2 to pins 2 and 5 of

relay K1. The unused 10-MHz reference

is connected through the relay to R10,

a 51-Ω load.

With the relay energized, the internally

generated 10-MHz reference from J7

connects through the closed contact of

the relay from pin 6 to pin 7 to R10,

the 51-Ω load. The externally

generated 10-MHz from jack J2

connects through the closed contact of

the relay from pin 2 to pin 1 to

amplifier U1. With the relay not

energized, the internally generated 10-

MHz reference from J7 connects

through the closed contact of the relay

from pin 3 to pin 1 to amplifier U1. The

externally generated 10-MHz from jack

J2 connects through the closed contact

of the relay from pin 5 to pin 7 to R10,

the 51-Ω load.

External 10-MHz Reference Present

Circuitry

The external 10-MHz reference signal

enters the board at J2 and is filtered by

C4, L2, and C5 before it is connected to

the K1 relay. A sample of the 10 MHz is

rectified by CR3 and connected to U3A.

If the sample level of the external 10

MHz is above the reference set by R13

and R14, which is connected to pin 2 of

U3A, the output of U3A stays high. The

high connects to gates of Q4 and Q9,

which are biased on and cause their

drains to go low. The low from the

drain of Q9 is wired to J8, pin 6, for

connection to a remote external 10-

MHz present indicator. The low from

the drain of Q4 connects to the green

LED DS2 which lights to indicate that

an external 10-MHz reference is

present. The low from the drain of Q4

also connects to the gate of Q5, biasing

it off and causing its drain to go high.

This high reverse biases CR4 and allows

a high to be applied to the gates of Q6,

Q8, and Q3, if an interlock, low, is

present at J8, pin 1. The high to the

gate of Q6 biases it on and causing its

drain to go low; the low is connected to

the green LED DS3, which lights,

indicating that an external 10-MHz

reference is selected. The high to the

gate of Q8 biases it on and applies a

low to J8, pin 7, for connection to a

remote reference select indicator. The

high that is applied to the gate of Q3

biases it on and causes its drain to go

low, which energizes the K1 relay and

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-6

applies the external 10-MHz reference

signal to U1 for use in the PLL circuits.

Internal 10-MHz Reference Circuitry

The internally generated 10-MHz

reference signal connects from jack J7

on the board to pins 3 and 6 of relay

K1. If the external 10-MHz reference is

missing, or the interlock is not present

at J8, pin 1, the relay is de-energized.

The internally generated 10-MHz

reference connects through the closed

contact of the relay from pin 3 to pin 1

to amplifier U1. The externally

generated 10-MHz signal from jack J2

connects through the closed contact of

the relay from pin 5 to pin 7 to R10,

the 51-Ω load.

Sample Input Circuitry

A sample of the signal from the UHF

generator board connects to SMA jack

J9, the sample input on the board. The

signal is amplified by U8 and coupled to

U9, a divide by 20/21 IC. A sample of

the output of U8 is connected to J10,

the sample output jack on the board,

which is typically connected to the front

panel of the tray.

Comparator Phase Lock Loop Circuit

The selected 10-MHz reference

connects to amplifier IC U1 whose

output is split by the circuit consisting

of L3, L4, and R8. A sample of the 10-

MHz reference is cabled to jack J3, the

10-MHz output jack, which is connected

to J5 on the rear of the tray. The 10-

MHz reference connects to IC U4, a

divider PLL chip. The divided-down

sample of the 10-MHz reference, which

is measurable at TP2, connects from

pin 10 of U4 to pin 27 of U5.

The divided-down sample from U9 is

connected to U5, a divider comparator

IC, which divides this signal to a 50-

kHz reference. The 50-kHz references

are compared in the U5 IC to a 50-kHz

sample of the 10-MHz signal that is

generated from an external 10-MHz

reference input or internally from a 10-

MHz reference kit. The three DIP

switches, SW1, SW2, and SW3, are set

up to divide down the reference sample

input generated by the VCXO to 50

kHz. The reference is then compared to

the 50-kHz sample from the 10-MHz

input in the U5 IC.

The output of U5 at pins 7 and 8

connect to U6A, a differential

comparator, whose output is the

difference between the two 50-kHz

references which is the AFC voltage.

The AFC voltage is amplified by U6B

and connected to jack J4. W1 on J4

must be in the AFC auto position,

between pins 1 and 2, for the PLL

circuit to operate. With jumper W1

between pins 2 and 3 on J6, fixed bias,

the AFC bias is set by R12. The AFC

output at J6 on the board connects to

the VCXO on the UHF generator board.

The PLL circuit will maintain the very

accurate VCXO output because any

change in frequency will be corrected

by the AFC error voltage.

Lock Detector Circuit

IC chip U2 contains an internal lock

detector that indicates the status of the

PLL circuit. When U2 is in a locked

state, pin 3 and pin 7 go low; the low is

applied to Q1, which is biased off. With

Q1 off, pin 1 of J1 goes high and is

connected to the Lock LED on the LED

display board that lights. The low

applied to DS1, the Red Unlock LED,

causes it not to light.

If the 50 kHz from the 10-MHz

reference and the 50-kHz from the UHF

generator board become unlocked, out

of the capture range of the PLL, pins 2

and 6 of U2 go to a logic low; this

causes U2, pins 3 and 7, to go high.

The high connects to DS1, the red

Unlock LED, which lights, and to Q1

and Q2, which are biased on. When Q2

is biased on, it connects a low remote

unlock to jack J1, pin 4. With Q1 biased

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-7

On, the drain goes low and removes

the high to pin 1 of J1, which is

connected to the Lock LED, on the LED

display board, which is extinguished.

Voltage Requirements

The ±12 VDC needed for the operation

of the board enters through jack J11.

The +12 VDC is connected to J11-3,

which is filtered and isolated by L6 and

C38 before it is connected to the rest of

the board. The -12 VDC is connected to

J11-5, which is filtered and isolated by

L7 and C39 before it is connected to

the rest of the board.

The +12 VDC is connected to U7, a 5-

volt regulator IC, that provides the +5

VDC operating voltage to the U2, U4,

U5, and U9 ICs.

4.3.5 (A1) Power Entry Module

Assembly (1227-1206; Appendix

The power entry module assembly

provides overvoltage and surge

protection for the input AC lines that

connect to the exciter tray. The AC

input plug connects to J14; J14 is part

of the power entry module on the rear

panel of the exciter tray.

The module assembly contains two

130-VAC varistors and one 250-VAC

varistor that connect across the AC

lines and to ground. The module also

contains two 4-amp fuses, one in each

input line for overcurrent protection.

4.3.6 (A9) IF Phase Corrector

Board (1227-1250; Appendix D)

The IF phase corrector board has

adjustments that pre-correct for any IF

phase modulation distortion that might

occur in output amplifier devices such

as Klystron power tubes and solid-state

amplifiers. Two separate, adjustable IF

paths are on the board: a quadrature

IF path and an in-phase IF path. The

quadrature IF is 90° out of phase and

much larger in amplitude than the in-

phase IF. When these paths are

combined in Z1, they provide the

required adjustable phase correction to

the IF signal.

The IF input signal enters at J1 and is

capacitively coupled to U1. U1 amplifies

the IF before it is connected to Z1, a

splitter, to create two, equal IF outputs.

IF output #1 is connected to J2 and IF

output #2 is connected to J3. IF output

#1 at J2 is jumpered through coaxial

cable W4 to jack J6, the quadrature

input, on the board. IF output #2 at J3

is jumpered through coaxial cable W5

to jack J7, the in-phase input, on the

board.

4.3.6.1 Phase Corrector Circuit

The phase corrector circuit adjusts for

any amplitude nonlinearities of the IF

signal. It is designed to work at IF and

has three stages of correction. Each

stage has a variable threshold and

magnitude control. The threshold

control determines the point where the

gain is changed and the magnitude

control determines the gain change

once the breakpoint is reached. The

second stage has a jumper that

determines the direction of the

correction so that the gain can be made

to increase either above or below the

threshold and either black or white

stretch can be accomplished in stage

two.

When the phase corrector circuit is

operating, the IF signal from J6 is

applied to transformer T1, which

doubles the voltage swing using a 1:4

impedance transformation. Resistors

R8, R61, R9, and R48 form an L-pad

that attenuates the signal. This

attenuation is adjusted by adding R7, a

variable resistor, in parallel with the L-

pad. R7 is only in parallel when the

signal reaches a level large enough to

bias on CR1 and CR2 and allow current

to flow through R7. When R7 is put in

parallel with the L-pad, the attenuation

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-8

through the L-pad is lowered, causing

black stretch.

Two reference voltages are utilized in

the corrector stages and both are

derived from the +12 VDC line. Zener

diode VR1, with R46 as a dropping

resistor, provides +6.8 VDC from the

+12 VDC line. Diodes CR11 and CR12

provide a .9-VDC reference that

temperature compensates the corrector

circuits from the affects of the two

diodes in each corrector stage. The

threshold for the first corrector stage is

set by controlling where CR1 and CR2

turn on. This is accomplished by

adjusting R3 and forming a voltage

divider from +6.8 VDC to ground. The

voltage at the wiper of R3 is buffered

by U9C, a unity-gain amplifier, and

applied to CR1. The .9-VDC reference is

connected to U9D, a unity-gain

amplifier, whose output is wired to

CR2. These two references are

connected to diodes CR1 and CR2

through chokes L2 and L3. The two

chokes form a high impedance for RF

that isolates the op-amps from the RF.

The adjusted signal is next applied to

amplifier U2, which compensates for

the loss through the L-pad. U2 is

powered through L4 and R10 from the

+12 VDC line. After the signal is

amplified by U2, it is applied to the

second corrector stage through T2 and

then to a third corrector stage through

T3. The two other corrector stages

operate the same as the first; they are

independent and do not interact with

each other.

When jumper W1 on J8 is connected

from center to ground, R15 is put in

series with ground. In this

configuration, black stretch (white

compression) is applied to the IF signal

by controlling the attenuation through

the path. When W1 is connected from

the center pin to the end that connects

to T2, R15 is put in parallel with the L-

pad. In this configuration, black

compression (white stretch) is applied

to the IF signal by controlling the

attenuation through the path.

The phase correctors can be bypassed

by moving jumper W2 on J9 to the

Disable position. This will move all of

the threshold points past sync tip so

that they will have no affect. R68 can

be adjusted and set for the required

correction range. TP2 is a test point

that gives the operator a place to

measure the level of the quadrature IF

signal that is connected to pin 6 on

combiner Z2.

4.3.6.2 Amplitude Corrector Circuit

The amplitude corrector circuit adjusts

for any amplitude nonlinearities of the

IF signal using one stage of correction.

The stage has a variable threshold

control, R31, and a variable magnitude

control, R35. The threshold control

determines the point where the gain is

changed and the magnitude control

determines the amount of gain change

once the breakpoint is reached.

Two reference voltages are needed for

the operation of the corrector circuit.

Zener diode VR1 with R46 provides

+6.8 VDC and diodes CR11 and CR12

provide a .9-VDC reference voltage that

is used to temperature compensate for

the two diodes in the corrector stage.

When the amplitude corrector circuit is

operating, the IF signal from J7 is

applied to transformer T4, which

doubles the voltage swing by means of

a 1:4 impedance transformation.

Resistors R36, R55, R56, and R37 form

an L-pad that lowers the level of the

signal. The amount that the level is

lowered can be adjusted by adding

more or less resistance, using R35, in

parallel with the L-pad resistors. R35 is

only in parallel when the signal reaches

a level large enough to turn on diodes

CR8 and CR9. When the diodes turn on,

current flows through R35, putting it in

parallel with the L-pad. When R35 is in

parallel with the resistors, the

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-9

attenuation through the L-pad is

lowered, causing signal stretch, which

is the amount of stretch determined by

the adjustment of R35.

The signal is next applied to amplifier

U5 to compensate for the loss in level

through the L-pad. The breakpoint, cut-

in, for the corrector stage is set by

controlling where CR8 and CR9 turn on.

This is accomplished by adjusting cut-in

resistor R31, which forms a voltage

divider from +6.8 VDC to ground. The

voltage at the wiper arm of R31 is

buffered by unity-gain amplifier U8B.

This voltage is then applied to R34,

through L11, and to diode CR9. The .9-

VDC reference created by CR11 and

CR12 is applied to unity-gain amplifier

U8A. C36 keeps the reference from

sagging during the vertical interval. The

reference voltage is then connected to

diode CR8 through choke L12. Chokes

L11 and L12 form a high impedance for

RF that isolates the op-amp ICs from

the IF.

After the signal is amplified by U5, it is

applied to a second stage through T5.

The transformer doubles the voltage

swing by means of a 1:4 impedance

transformation. Resistors R39, R57,

R58, and R40 form an L-pad that

lowers the level of the signal. The

signal is applied to amplifier U6 to

compensate for the loss in level

through the L-pad. After the signal is

amplified by U6, it is applied to a third

stage through T6. The transformer

doubles the voltage swing by means of

a 1:4 impedance transformation.

Resistors R42, R59, R60, and R43 form

an L-pad that lowers the level of the

signal. The signal is applied to amplifier

U7 to compensate for the loss in level

through the L-pad.

TP1 is a test point that gives the

operator a place to measure the level

of the in-phase IF signal that is

connected to mixer stage Z2. The

amplitude corrector can be disabled by

moving jumper W3 on J10 to the

Disable position; this will move the

breakpoint past sync tip so that it will

have no affect on the signal.

4.3.6.3 Output Circuit

The phase-corrected signal from pin 1

on combiner Z2 exits the board at IF

output jack J4 after passing through a

matching network consisting of six

resistors.

4.3.7 (A8) ALC Board (1265-1305;

Appendix D)

The ALC board provides for automatic

level control (ALC) and amplitude

linearity correction of the IF signal. The

ALC adjusts the level of the IF signal

through the ALC board which, in turn,

controls the output power of the

transmitter.

The visual + aural IF input (0 dBm)

signal from the modulator enters the

board at modulator IF input jack J32

and, if the (optional) receiver tray is

present, the visual + aural IF input (0

dBm) from the receiver tray connects

to J1, the receiver IF input jack. The

modulator IF input connects to relay K3

and the receiver IF input connects to

relay K4. The two relays are controlled

by the Modulator Select command that

is connected to J30 on the board.

Modulator select enable/disable jumper

W11 on J29 controls whether the

Modulator Select command at J30

controls the operation of the relays.

With jumper W11 on J29, pins 1 and 2,

the Modulator Select command at J30

controls the operation of the relays but,

with jumper W11 on J29, pins 2 and 3,

the modulator is selected all of the

time.

4.3.7.1 Modulator Selected

With the modulator selected, J11-10

and J11-28 on the rear of the UHF

exciter tray are connected together;

this makes J30 low and causes relays

K3 and K4 to de-energize. When K4 is

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-10

de-energized, it connects the receiver

IF input at J1, if present, to 50 watts.

When K3 is de-energized, it connects to

the modulator IF input at J32 and from

there to the rest of the board. At this

point, the Modulator Enable LED DS5

will be lit.

4.3.7.2 Receiver Selected

With the receiver selected, J11-10 and

J11-28, which are on the rear of the

UHF exciter tray and connect to J30 on

the board, are not connected together;

relays K3 and K4 are energized. When

K4 is energized, it connects the

receiver IF input at J1, if present, to

the rest of the board. When K3 is

energized, it connects to the modulator

IF input at J32 to 50 watts. At this

point, the Modulator Enable LED DS5

will not be lit.

4.3.7.3 Main IF Signal Path (Part 1 of 3)

The selected visual + aural IF input (0

dBm) signal is split, with one half

entering a bandpass filter consisting of

L3, L4, C4, L5, and L6. This bandpass

filter, which can be tuned with C4, is

substantially broader than the IF signal

bandwidth. It is used to slightly steer

the frequency response of the IF and

make up for any small discrepancies in

the frequency response in the stages

that precede this point. The filter also

serves the additional function of

rejecting unwanted frequencies that

may occur if the tray cover is off and

the tray is in a high RF environment.

This allows for the servicing of this

transmitter with the tray cover off in

spite of being in the presence of other

RF signals. The filtered IF signal is fed

through a pi-type matching pad,

consisting of R2, R3, and R4, to the

pin-diode attenuator circuit consisting

of CR1, CR2, and CR3.

4.3.7.4 Input Level Detector Circuit

The other part of the split IF input is

connected through L2 and C44 to U7,

an IC amplifier, which is the input to

the input level detector circuit. The

amplified IF is fed to T4, a step-up

transformer, that feeds diode detector

CR14. The positive-going detected

signal is then low-pass filtered by C49,

L18, and C50; this allows only the

video with positive sync to be applied

through emitter follower Q1. The signal

is then connected to detector CR15,

which produces a peak-sync voltage

that is applied to op-amp U9A. There is

a test point at TP3 that provides a

voltage reference check of the input

level. The detector serves the dual

function of providing a reference that

determines the input IF signal level to

the board and acting as an input

threshold detector.

The input threshold detector prevents

the automatic level control from

reducing the attenuation of the pin-

diode attenuator to minimum

(maximum signal) if the IF input to the

board is removed. The ALC, video loss

cutback, and the threshold detector

circuits will only operate when jumper

W3 on jack J6 is in the Auto position,

between pins 1 and 2. Without the

threshold detector, with the pin-diode

attenuator at minimum when the signal

is restored, the stages following this

board will be overdriven.

On the threshold detector, the

minimum IF input level at TP3 is fed

through detector CR15 to op-amp IC

U9A, pin 2. The reference voltage for

the op-amp is determined by the

voltage divider consisting of R50 and

R51 off of the +12 VDC line. When the

detected input signal level at U9A, pin

2, falls below this reference threshold,

approximately 10 dB below the normal

input level, the output of U9A, pin 1,

goes to the +12 VDC rail. This high is

connected to the base of Q2, which is

forward biased, and creates a current

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-11

path from the -12 VDC line, through

the red Input Level Fault Indicator LED

DS1 (which lights), resistor R54, and

transistor Q2 to +12 VDC. The high

from U9A also connects through diode

CR16 to U9B, pin 5, whose output at

pin 7 goes high. The high connects

through range adjust pot R74 to J20,

which connects to the front panel-

mounted power adjust pot. This high

connects to U10A, pin 2, which causes

it to go low at output U10A, pin 1. The

low is applied through jumper W3 on J6

to the pin-diode attenuator circuit that

cuts back the IF level and also cuts

back the output power level to 0. When

the input signal level increases above

the threshold level, the output power

will rise as the input level increases

until normal output power is reached.

The video input level at TP3 is also fed

to a sync-separator circuit, consisting

of IC U8, CR17, Q3, and associated

components, and then to a comparator

circuit made up of U9C and U9D. The

reference voltage for the comparators

are determined by the voltage divider,

consisting of R129, R64, R65, R66, and

R130, off of the -12 VDC line. When the

input signal level to the detector at TP3

falls below this reference threshold,

which acts as a loss of sync detector

circuit, the output of U9C and U9D goes

towards the -12 VDC rail, which is split,

with one part biasing on the transistor

Q5. A current path is then established

from the +12 VDC line through Q5,

resistors R69 and R137, and the red

Video Loss Indicator LED DS3, which

lights. When Q5 is on, it applies a high

to the gates of Q6 and Q7, causing

them to conduct; this applies video loss

fault pull-down outputs to J18, pins 5

and 2.

The other low output of U9C and U9D is

connected through CR20 to jack J5.

Jumper W2 on J5, in the cutback

enable position between pins 2 and 3,

connects the low to the base of Q4,

which is forward biased. If jumper W2

is in the Disable position, between pins

1 and 2, the automatic cutback will not

operate. With Q4 biased on, a level

determined by the setting of cutback

level pot R71, which is set at the

factory to cut back the output to

approximately 25%, is applied to U9B,

pin 5. The output of U9B, pin 7, goes

low and is applied through the power

adjust pot to U10A, pin 2, whose

output goes low. This low is applied to

the pin-diode attenuator, which will cut

back the level of the output to

approximately 25%.

4.3.7.5 Pin-Diode Attenuator Circuit

The input IF signal is fed to the pin-

diode attenuator circuit, consisting of

CR1 to CR3. Each of the pin diodes

contain a wide, intrinsic region, which

makes the diodes function as voltage-

variable resistors at this intermediate

frequency. The value of the resistance

is controlled by the DC bias supplied to

the diode. The pin diodes are

configured in a pi-type attenuator

configuration where CR1 is the first

shunt element, CR3 is the series

element, and CR2 is the second shunt

element. The control voltage, which can

be measured at TP1, originates either

from the ALC circuit, when jumper W3

on J6 is in the ALC Auto position

between pins 1 and 2, or from pot R87

when the jumper is in the Manual Gain

position.

In the pin-diode attenuator circuit, a

current path exists from J6 through R6

and then through the diodes of the pin

attenuator. Changing the amount of

current through the diodes by forward

biasing them changes the IF output

level of the board. There are two

extremes of attenuation ranges for the

pin-diode attenuators. In the minimum

attenuation case, the voltage is

measured as TP1 approaches the +12

VDC line. There is a current path

created through R6, through series

diode CR3, and finally through R9 to

ground. This path forward biases CR3

and causes it to act as a relatively low-

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-12

value resistor. The larger current flow

increases the voltage drop across R9

and tends to turn off the diodes CR1

and CR2, causing them to act as high-

value resistors. In this case, the shunt

elements act as high resistance and the

series element acts as low resistance,

which represents the minimum loss

condition of the attenuator, maximum

signal output.

The other extreme case occurs as the

voltage at TP1 is reduced, going

towards ground or even slightly

negative. This tends to turn off and

reverse bias diode CR3, the series

element, which causes it to act as a

high-value resistor. An existing fixed-

current path from the +12 VDC line

through R5, CR1, CR2, and R9 biases

the series element CR3 off and the

shunt elements, diodes CR1 and CR2,

on, causing them to act as relatively

low-value resistors. This represents the

maximum attenuation case of the pin

attenuator, minimum signal output. By

controlling the value of the voltage

applied to the pin diodes, the IF signal

level is maintained at the set level.

4.3.7.6 Main IF Signal Path (Part 2 of 3)

When the IF signal passes out of the

pin-diode attenuator through C11, it is

applied to modular amplifier U1. This

device includes the biasing and

impedance-matching circuits that

allows it operate as a wideband IF

amplifier. The output of U1 is available

at jack J2, as a sample of the pre-

correction IF, for troubleshooting

purposes and system set-up. The IF

signal is then connected to the linearity

corrector portion of the board.

4.3.7.7 Linearity Corrector Circuits

The linearity corrector circuits adjust

for any amplitude non-linearities of the

IF signal using three stages of

correction. Each stage has a variable

threshold control adjustment, R34,

R37, or R40, and a variable magnitude

control adjustment, R13, R18, or R23.

The threshold control determines the

point at which the gain is changed and

the magnitude control determines the

amount of gain change that occurs

once the breakpoint is reached. Two

reference voltages are needed for the

operation of the corrector circuits.

Zener diode VR1, with R33 and R135,

provides a +6.8-VDC reference and

diodes CR11 and CR12 provide a .9-

VDC reference that temperature

compensates for the two diodes in each

corrector stage.

When the linearity correctors are

operating, the IF signal is applied to

transformer T1, which doubles the

voltage swing by means of a 1:4

impedance transformation. Resistors

R14, R15, and R16 form an L-pad that

lowers the level of the signal. The

amount the level is lowered can be

adjusted with R13, in parallel with the

L-pad resistors. R13 is only in parallel

when the signal reaches a level large

enough to turn on diodes CR4 and CR5.

When the diodes turn on, current flows

through R13, putting it in parallel with

the L-pad. When R13 is put in parallel

with the resistors, the attenuation

through the L-pad is lowered, causing

signal stretch, the amount of which is

determined by the adjustment of R13.

The signal is next applied to amplifier

U2, which compensates for the loss

through the L-pad.

The breakpoint, cut-in, for the first

corrector is set by controlling where

CR4 and CR5 turn on. This is

accomplished by adjusting cut-in

resistor R34, which forms a voltage-

divider network from +6.8 VDC to

ground. The voltage at the wiper arm

of R34 is buffered by unity-gain

amplifier U5D. This reference voltage is

then applied to R35, R36, and C39,

through L12, to the CR4 diode. C39

keeps the reference from sagging

during the vertical interval. The .9-VDC

reference created by CR11 and CR12 is

applied to unity-gain amplifier U5B. The

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-13

reference voltage is then connected to

diode CR5 through choke L11. Chokes

L11 and L12 form a high impedance for

the RF that serves to isolate the

op-amp ICs from the IF.

After the signal is amplified by U2, it is

applied to the second corrector stage

through T2. This corrector and the third

corrector operate in the same fashion

as the first. All three corrector stages

are independent and do not interact

with each other.

The correctors can be disabled by

moving jumper W1 on J4 to the Disable

position, between pins 2 and 3; this

moves all of the breakpoints past the

tip of sync so that they will have no

affect. The IF signal exits the board at

IF output jack J3 after passing through

the three corrector stages and is

normally connected to an external IF

phase corrector board.

4.3.7.8 Main IF Signal Path (Part 3 of 3)

After the IF signal passes through the

external IF phase corrector board, it

returns to the ALC board at IF input

jack J7. The IF then passes through a

bandpass filter consisting of L20, C97,

C62, L21, C63, L22, L23, C64, and

C99. This bandpass filter is identical in

both form and function to the one

described at IF input jack J1. In this

case, the filter is intended to make up

for the small errors in frequency

response that are incurred by the signal

while it is being processed through the

linearity and incidental phase correction

circuits. Following the bandpass filter,

the signal is split using L24, L25, and

R89. The signal passing through L24 is

the main IF path through the board.

A sample of the corrected IF signal is

split off and connected to J10, the IF

sample jack. The IF connects to jacks

J27 and J28, which control whether a

6-dB pad is included in the circuit by

the positioning of jumpers W9 and

W10. The 6-dB pad is in when jumpers

W9 and W10 are connected between

pins 2 and 3 on J27 and J28. The 6-dB

pad is out when jumpers W9 and W10

are connected between pins 1 and 2 on

J27 and J28. Normally, the pad is out.

The IF signal is then applied to a two-

stage frequency response corrector

circuit that is adjusted as necessary.

Variable resistors R103 and R106

adjust the depth, or gain, of the

notches and variable caps C71 and C72

adjust the frequency, or position, of the

notches. The IF signal is amplified by

U13 and U14 before it is connected to

J12, the IF output jack of the board.

R99 is an output level adjustment that

is set to provide approximately 0 dBm

of IF output at J12. A sample of the IF

is fed to J11, which provides an IF

sample point that can be monitored

without breaking the signal path and

gives an indication of the IF signal after

the linearity and frequency response

correction takes place.

4.3.7.9 ALC Circuit

The other path of the corrected IF

signal is used in the ALC circuit. The IF

is wired out of the splitter through L25,

which is connected to op-amp U12. The

output of U12 is wired to jacks J8 and

J9; jumpers W4 and W8 control the

normal, or encoded, operation of the

ALC circuitry. For normal operation,

jumper W4 on J8 is between pins 1 and

2 and jumper W8 on J9 is between pins

1 and 2.

The IF signal is applied to transformer

T5, which doubles the voltage swing by

means of a 1:4 impedance

transformation before it is connected to

the ALC detector circuit on the board

and amplified by U10B. For normal

operation, jumper W7 on J26 is

between pins 1 and 2 and jumper W5

on J21 is between pins 1 and 2. The

detected ALC voltage is wired to U10A,

pin 2, where it is summed with the

front panel power control setting. The

output power adjustment for the

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-14

transmitter is accomplished, if the

(optional) remote power raise/lower kit

(1227-1039) is purchased, by R75, a

motor-driven pot controlled by switch

S1 on the board or screwdriver adjust

pot R1 on the front panel of the UHF

exciter tray. An external power

raise/lower switch can be used by

connecting it to jack J10, at J10-11

power raise, J10-13 power raise/lower

return, and J10-12 power lower on the

rear of the UHF exciter tray.

S1, or the remote switch, controls

relays K1 and K2 which, in turn, the

control motor M1 that moves variable

resistor R75. If the (optional) remote

power raise/lower kit is not purchased,

the ALC voltage is controlled only by

screwdriver adjust pot R1 on the front

panel of the UHF exciter tray. The ALC

voltage is set for .8 VDC at TP4 with 0

dBm output at J12 of the board. A

sample of the ALC at J19, pin 2, is

wired to the transmitter control board

where it is passed on to the front panel

meter and the AGC circuits.

The ALC voltage and the DC level

corresponding to the IF level after

signal correction are fed to U10A, pin 2,

whose output at pin 1 connects to the

ALC pin-diode attenuator circuit. If

there is a loss of gain somewhere in an

IF circuit, the output power of the

transmitter will drop, which the ALC

circuit senses at U10A, automatically

lowering the loss of the pin-diode

attenuator circuit and increasing the

gain to compensate.

The ALC action starts with the ALC

detector level that is monitored at TP4.

The detector output at TP4 is nominally

+.8 VDC and is applied through resistor

R77 to a summing point at op-amp

U10A, pin 2. The current available from

the ALC detector is offset, or

complimented, by current that is taken

away from the summing junction. In

normal operation, U10A, pin 2, is at 0

VDC when the loop is satisfied. If the

recovered or peak-detected IF signal at

IF input jack J7 of this board should

drop in level, which would normally

mean that the output power is

decreasing, the null condition would no

longer occur at U10A, pin 2. When the

level drops, the output of U10A at pin 1

will go more positive and, if jumper W3

on J6 is in the Automatic position, it will

cause ALC pin-diode attenuators CR1,

CR2, and CR3 to have less attenuation.

This will increase the IF level and

compensate for the decrease in level. If

the ALC cannot sufficiently increase the

input level to satisfy the ALC loop,

because of not enough range, an ALC

fault will occur. The fault is generated

because U10D, pin 12, increases above

the trip point set by R84 and R83 until

it conducts. This makes U10D, pin 14,

high, and lights the red ALC Fault LED

DS2 on the board.

4.3.7.10 Fault Command

The ALC board also has circuitry for an

external mute fault input at J19, pin 6.

This is a Mute command and, in most

systems, it is part of the interface for

the protection circuits of high-gain

output amplifier devices. The Mute

command is intended to protect the

amplifier devices against VSWR faults.

In this case, it is required that the

actions occur faster than just pulling

the ALC reference down. Two different

mechanisms are employed: one is a

fast-acting circuit to increase the

attenuation of pin-diode attenuator

CR3, CR1, and CR2, and the second, as

just described, is the reference voltage

being pulled away from the ALC

amplifier device. An external mute is a

pull-down applied to J19, pin 6, that

provides a current path from the +12

VDC line through R78 and R139, the

Mute Indicator LED DS4, and the LED

section of opto-isolator U11.

These actions turn on the transistor

section of U11 that applies -12 VDC

through CR21 to U10A, pin 3, which

pulls down the reference voltage. This

is a fairly slow action that is slowed

down by the low-pass filter function of

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-15

R81 and C61. When the transistor

section of U11 is on, -12 VDC is also

connected through CR22 to the pin-

diode attenuator circuit. This

establishes a very fast muting action,

by reverse biasing CR3, in the event of

an external VSWR fault.

4.3.7.11 ±12 VDC to Operate the

Board

The ±12 VDC connects to the board at

J14. +12 VDC connects to J14-3 and is

filtered by L30, L41, and C80 before it

is applied to the rest of the board. The

-12 VDC connects to J14-5 and is

filtered by L31 and C81 before it is

applied to the rest of the board.

The +12 VDC also connects to U16, a

5-VDC regulator IC, which produces the

+5 VDC needed to operate timing IC

U17.

4.3.8 (A19) Visual/Aural Metering

Board (1265-1309; Appendix D)

Note: This board was originally

designed for analog television signal

operation. For digital applications,

forward digital can be substituted for

forward visual and the aural circuits

along with the scrambling circuits are

not used.

The visual/aural metering board

provides detected outputs of the

forward digital and reflected output

samples that are used for monitoring

on the front panel meter. The board

also provides adjustments for the

calibration of the readings on the

meter. These readings are obtained

from the samples of the forward power

and reflected power outputs of the tray.

A forward power sample is applied to

SMA jack J1 of the board. The input

signal is split, with one path connected

to forward power sample SMA jack J2

for monitoring purposes. The other

path is connected through C1 to CR2,

R4, R5, R6, C4, and CR1, which make

up a detector circuit. The detected

signal is amplified by U6B and its

output is split. One amplified output of

U6B connects to the digital level circuit,

the other output is not used in digital

operation.

4.3.8.1 Digital Level Circuit

The detected digital level output from

U6B is connected to U1C. The

intercarrier notch L3 is not used with

digital. The digital output of U1C is fed

to a peak-detector circuit consisting of

CR5 and U2A and then fed through

R28, the digital calibration control that

is adjusted for a 100% digital reading,

to amplifier U2B. The amplified digital

output is connected to comparator U2C.

The other input to U2C is the level set

by aural null adjust R51, which is not

used with digital. Pots R51 and R20

should be set full CCW. The offset null

adjust R48 is adjusted for 0% digital

power with the transmitter in Standby.

The adjusted output is amplified by

U3D and connected to the other input

of U2C. The output of U2C connects to

J6, pins 2 and 3, which supplies the

forward digital level output to the front

panel meter for monitoring.

The scrambling circuits are not used

with digital operation.

4.3.8.2 Reflected Level Circuit

A reflected power sample is applied to

J3 of the board and is detected by

diode detector CR7 and U3B. The

detected output is fed through R39, the

reflected calibration pot, which can be

adjusted to control the gain of U3C.

The output of U3C connects to J6, pin

7, which supplies a reflected power

level output to the front panel meter.

4.1.8.3 Voltages for Circuit Operation

The ±12 VDC is applied to the board at

J5. +12 VDC is connected to J5, pin 3,

and is isolated and filtered by L4 and

C34 before it is connected to the rest of

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-16

the board. The +12 VDC also connects

to U5, a 5-VDC regulator that provides

the voltage needed to operate U4. The

-12 VDC is applied to J5, pin 1, and is

isolated and filtered by L5 and C35

before it is connected to the rest of the

board.

4.3.9 (A11) UHF Upconverter Board

(1265-1310; Appendix D)

The UHF upconverter board provides

upconversion processing by mixing the

IF and LO signals in mixer Z1 to

produce the desired RF frequency

output. The RF output is connected

through J3 to an external filter and

applied back to the board at J4 where

the gain is set by R10. The RF is

amplified and connected to the RF

output jack of the board at J5.

The IF signal (0 dBm) enters the board

at J1, an SMA connector, and is applied

through a filter circuit consisting of L10

and C25 to C28 to a matching pad. This

pad consists of R1, R2, and R3, which

presents a relatively good source

impedance, and feeds the signal to pins

3 and 4, the I input of mixer Z1. The

local oscillator signal (+13 dBm) from

the x8 multiplier connects to the board

at jack J2, an SMA connector, through

a UHF channel filter and is connected

directly to pin 8, the L input of the

mixer.

The frequency of the LO is the sum of

the IF frequency above the required

visual carrier. For instance, in system

M, the IF visual frequency is at 45.75

MHz and the relative location of the

aural would be 4.5 MHz lower, or 41.25

MHz. For digital applications, the LO is

the center frequency of the digital

channel added to the 44-MHz IF

frequency. By picking the local

oscillator to be 45.75 MHz above the

visual carrier, a conversion in frequency

occurs by selecting the difference

product. The difference product, the

local oscillator minus the IF, will be at

the required visual carrier frequency

output. There will also be other signals

present at the RF output connector J3

at a lower level. These are the sum

conversion product: the LO and the IF

frequencies. Usually, the output

product that is selected by the tuning

of the external filter is the difference

product: the LO minus the 45.75-MHz

IF. The difference product has its

sidebands flipped so that the visual

carrier is lower in frequency than the

aural carrier.

If a bad reactive load is connected to

the mixer, the LO signal that is fed

through it can be increased because

the mixer no longer serves as a double-

balanced mixer. The mixer has the

inherent property of suppressing

signals that may leak from one input

port to any of the other ports. This

property is enhanced by having inputs

and outputs of the mixer at 50Ω

impedance. The reactive filter that is

externally connected to J3 of the board

does not appear as a good 50-Ω load at

all frequencies. The pad, in the output

line of the board, consists of R5, R4,

R6, and R7. The pad buffers the bad

effects of the reactive filter load and

makes it appear as a 50-Ω impedance.

The RF signal is amplified by U1, a

modular amplifier, and includes within

it biasing and impedance matching

networks that makes U1 act as a

wideband-RF amplifier device. This

amplifier, in a 50-Ω system, has

approximately 12 dB of gain. U1 is

powered from the +12 VDC line

through RF decoupling components

R27, R28, C30, R8, and L1. Inductor L1

is a broadband-RF choke and is

resonance free through the UHF band.

The amplified RF connects to SMA RF

output jack J3 which is cabled to the

external filter.

The RF input signal from the external

filter re-enters the board at J4 (-11 to

-14 dBm) and is capacitively coupled to

the pin-diode attenuator circuit

consisting of CR1, CR2, and CR5. The

pin-diode attenuator acts as a voltage-

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-17

variable attenuator in which each pin

diode functions as a voltage-variable

resistor that depends on the DC bias

supplied to the diode for the resistance

value. The pin diodes, because of a

large, intrinsic region, cannot rectify

signals at this RF frequency; therefore,

they act as a linear voltage-variable

resistor.

The pin diodes are configured in shunt

configuration: CR1 is the first shunt

element, CR2 is the second shunt

element, and CR5 is the series

element. The manual gain AGC, W1 on

J10 between pins 1 and 2, is used in

most cases. The control voltage from

manual gain pot R10 sets up a current

path through R11 and the diodes in the

pin attenuator. The level-controlled RF

signal from the pin-diode attenuator

circuit is amplified by wideband-hybrid

amplifier IC U2 which is configured in

the same way as U1. The RF signal is

buffered by Q1 and applied to the

push-pull class A amplifier circuit

consisting of Q2 and Q3. At the input to

the transistors, the RF is converted to a

balanced, dual feed by balun L4, which

is made from a short length of UT-141

coaxial cable.

Capacitors C12 and C13 provide DC

blocking for the input signal to the

amplifier devices. The RF outputs at the

collectors of the transistors are applied

through C19 and C20, which provide

DC blocking for the output signals. The

RF signals connect to L7, which consists

of UT-141 coaxial cable. L7 combines

the RF back to a single-RF output at a

50-Ω impedance to L8, which provides

a sample of the RF. The main path

through L10 is to J5, the RF output jack

of the board (+10 to +20 dBm). The

sample of the RF connects to a splitter

that provides a sample output (0 dBm)

at J6 of the board. The other output of

the splitter connects to a peak-detector

circuit consisting of CR3 and U3, which

provides a DC level at J7 that

represents the RF output of the UHF

exciter to the front panel meter. R29

sets up the calibration of the front

panel meter for 100% in the UHF

exciter position when the output power

of the exciter is at +17 dBm peak

visual or +10 dBm average for digital

applications.

The board is powered by ±12 VDC that

is produced by an external power

supply. +12 VDC enters the board

through J8, pin 3, and is filtered and

isolated by RF choke L9 and shunt

capacitors C24 and C33. This circuit

isolates the RF signals from the board

away from those of other devices

connected to the same +12 VDC line

external to the UHF upconverter board.

The +12 VDC is then applied to the rest

of the board.

The -12 VDC enters the board through

J8, pin 5, and is filtered and isolated by

RF choke L11 and shunt capacitors C34

and C35. This circuit isolates the RF

signals from this board away from

those of other devices connected to the

same

-12 VDC line that is external to the UHF

upconverter board; the -12 VDC is then

applied to the rest of the board.

4.3.10 (A17) Transmitter Control

Board (1265-1311; Appendix D)

The transmitter control board provides

the system control functions and the

operational LED indications that can be

viewed on the front panel of the

transmitter. The main control functions

are for Operate/Standby and

Auto/Manual selection. When the

transmitter is switched to Operate, the

board supplies the enables to any

external amplifier trays. When the

transmitter is in Auto, the board also

performs the automatic switching of the

transmitter to Standby upon the loss of

the video input. The board contains a

VSWR cutback circuit. If the VSWR of

the transmitter increases above 20%,

the VSWR cutback circuit will operate

and cut back the output level of the

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-18

transmitter, as needed, to maintain a

maximum of 20% VSWR. An interlock

(low) must be present at J8-24 for the

transmitter to be switched to Operate

and, when the interlock is present, the

green Interlock LED DS5 will be lit.

4.3.10.1 Operate/Standby Switch

K1 is a magnetic latching relay that

controls the switching of the

transmitter from Operate and Standby.

When the Operate/Standby switch S1

on the front panel of the tray is

switched to Operate, one coil of relay

K1 energizes; this causes the contacts

to close and apply a low to U4B-9. If

the transmitter interlock is present, and

there are no overtemperature faults,

lows will also be applied to U4B-10,

U4B-11, and U4B-12. With all the

inputs to U4B low, the output at U4B-

13 will be low. This low biases off Q1,

which turns off the amber Standby LED

DS1 on the front panel, and applies a

high to Q2. The high turns on and lights

the green Operate LED DS2 also on the

front panel. When Q2 is biased on, it

connects a low to Q12, which biases it

off, and allows the ALC to be applied to

J1, which is connected to any external

amplifier trays. The low from U4B-13 is

also applied to Q4 and Q24, which are

biased off, and removes the disables

from J1-4 and J18-1. The low from

U4B-13 also connects to Q10, which is

biased on, connects a high to Q6, Q7,

Q8, and Q9, which are also biased on,

and applies -12 VDC enables at J8-2, 3,

4, and 5, which connect to any external

amplifier trays. The high applied to Q2

is also connected to Q5 and Q26, which

are biased on, and applies a low enable

to J1-3, which can connect to a remote

operate indicator. The transmitter is

now in Operate.

When the Operate/Standby switch S1 is

switched to Standby, the other coil of

relay K1 energizes, which causes the

contacts to open. A high, +12 VDC, is

applied to U4B-9. The high at the input

causes the output at U4B-13 to go

high. The high biases on Q1, which

applies a low to the amber Standby

LED DS1 on the front panel and turns it

on. Q1 also applies a low to Q2, which

turns off and extinguishes the green

Operate LED DS2. In addition, Q12 is

biased on and connects to U2C, whose

output goes low and pulls the ALC

voltages at J1 low, lowering the gain of

the external amplifier trays. The high

from U4B-13 is applied to Q4 and Q24,

which are biased on, and applies

disables at J1-4 and J18-1. The high

from U4B-13 also connects to Q10,

which is biased off. When Q10 is off, it

removes the high from Q6, Q7, Q8, and

Q9, which are biased off, and removes

the -12 VDC enables at J8-2, 3, 4, and

5 that connect to the external amplifier

trays. The low applied to Q2 is also

connected to Q5 and Q26, which are

biased off, and removes the remote

enable at J1-3. The transmitter is now

in Standby.

4.3.10.2 Automatic/Manual Switch

K2 is a magnetic latching relay that

switches the operation of the

transmitter to Automatic or Manual

using Auto/Manual switch S2 on the

front panel of the tray.

When the S2 switch is set to the Auto

position, the operation of the

transmitter is controlled by the fault

circuits and will stay in Operate even if

Operate/Standby switch S1 is switched

to Standby.

With switch S2 in Auto, a low is applied

to one coil in the relay that energizes

and closes the contacts. The closed

contacts apply a low to the green

Automatic LED DS3, causing it to light.

The low from the relay connects to

U5A, pin 2; U5D, pin 13; Q21; and

Q23. Q21 and Q23 are biased off,

which causes their outputs to go high.

The high from Q21 connects to the

amber Manual LED DS4 on the front

panel, biasing it off, and to Q22,

biasing it on. The drain of Q22 goes

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-19

low. The low is applied to J8-7 and this

will enable any remote auto indicator

that is connected to it. The low to Q23

biases it off and removes the enable to

any remote manual indicator connected

to J8-6.

When switch S2 is set to the Manual

position, the operation of the

transmitter is no longer controlled by

the fault circuits; it is controlled by

Operate/ Standby switch S1. With

switch S2 in Manual, a low is applied to

the other coil in the relay, which

energizes and opens the contacts. The

open contacts remove the low from the

green Automatic LED DS3 on the front

panel, causing it to not light. The high

connects to U5A, pin 2; U5D, pin 13;

Q21; and Q23. Q21 and Q23 are biased

on, causing their outputs to go low. The

low from Q21 connects to the amber

Manual LED DS4 on the front panel,

biasing it on, and to Q22, biasing it off.

The drain of Q22 goes high. This high is

applied to J8-7 and will disable any

remote auto indicator that is connected

to it. Q23 is biased on and applies a low

enable to any remote manual indicator

connected to J8-6.

4.3.10.3 Automatic Control of the

Transmitter

The transmitter control board also

allows the transmitter to be turned on

and off by the presence of modulation

to the transmitter when the transmitter

is in Auto. When a modulation fault

occurs due to the loss of the

modulation input, J7-5 goes low. The

low is applied through W1 on J10 to

Q16, which is biased off, and to the red

Modulation Loss Fault LED DS9 on the

front panel, causing it to light. The

drain of Q16 goes high and connects to

U5B, pin 5, causing the output at pin 4

to go low. The low connects to Q18,

which is biased off, causing the drain of

Q18 to go high. The high connects to

U3D, pin 12, whose output at pin 14

goes high. The high connects to U5C,

pins 8 and 9, which causes its output at

pin 10 to go low, and to U5A, pin 1,

causing its output at pin 3 to go low.

With S2 set to Automatic, a low is

applied to U5A, pin 2, and to U5D, pin

13. When U5A, pin 1, is high and U5A,

pin 2, is low, it causes the output at pin

3 to go low. When U5D, pin 12, and

U5D, pin 13, are both low, it causes the

output to go high. When U5A, pin 3, is

low, it biases off Q20 and removes any

pull-down to the Operate switch. A high

at U5D, pin 11, biases on Q19 and

applies a low enable to the Standby

switch; this places the transmitter in

the Standby mode.

When the modulation input is returned,

J7-5 goes high. The high is applied to

Q16, which is biased on, and to the red

Modulation Fault LED DS9, which goes

out. The output of Q16 goes low and

connects to U5B, pin 5. If there is no

receiver ALC fault, U5B, pin 6, is also

low, which causes the output at pin 4

to go high. This high connects to Q18,

which is biased on, and causes the

drain of Q18 to go low. The low

connects to U3D, pin 12, whose output

at pin 14 goes low. The low connects to

U5C, pins 8 and 9, which causes the

output at pin 10 to go high, and to

U5A, pin 1. With Auto/Manual switch

S2 in Auto, a low is applied to U5A, pin

2, and to U5D, pin 13. With U5A, pins 1

and 2, low, the output at pin 3 goes

high. With pin 12 of U5D high, the

output of U5D at pin 11 goes low.

When U5A, pin 3, is high, it biases on

Q20, which applies a pull-down enable

to the Operate switch. A low at U5D,

pin 11, biases off Q19, which removes

any pull-down to the Standby switch.

The transmitter is switched to Operate.

4.3.10.4 Faults

There are four possible faults that may

occur in the transmitter and are applied

to the board: modulation loss fault,

VSWR cutback fault, overtemperature

fault, and ALC fault. During normal

operations there are no faults to the

board. The receiver ALC fault circuit will

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-20

only function if a receiver tray is part of

the system. The overtemperature fault

is controlled by the temperature of the

reject load.

Modulation Loss Fault

If a modulation loss occurs while the

transmitter is in Auto, the system will

go to Standby after a few seconds until

the modulation is returned. The

transmitter will immediately revert to

Operate. An input loss fault applies a

low from the ALC board to the fault

input at J7-5 on the board.

With jumper W1 in place on J10, the

input fault is connected to the red Input

Loss Fault LED DS9, causing it to light,

and to Q16. Q16 is biased off, causing

its drain to go high. The high is wired to

U5B, pin 5, whose output at U5B, pin 4,

goes low. The low is wired to Q18,

which is biased off, and causes the

drain to go high. The high is connected

to U3D, pin 12, which causes the

output at U3D, pin 14, to go high. The

high connects to U5A, pin 1, and, if the

transmitter is in Auto, pin 2 of U5A is

low. With pin 1 high and pin 2 low, the

output of U5A goes low and reverse

biases Q20, shutting it off. The high at

U5C, pins 8 and 9, causes its output at

pin 10 to go low. This low is connected

to U5D, pin 12, and, if the transmitter

is in Auto, pin 13 of U5D is also low.

The lows on pins 12 and 13 cause the

output to go high and forward bias

Q19. The drain of Q19 goes low and

connects the coil in relay K1, causing it

to switch to Standby.

When the input returns, the input loss

fault is removed from the video fault

input at J7-5. With jumper W1 in place

on J10, the base of Q16 goes high. The

red Input Loss Fault LED DS9 on the

front panel will go out. Q16 is biased

on, causing the drain to go low. This

low is wired to U5B, pin 5, and U5B, pin

6, will be low if no ALC fault occurs. The

two lows at the inputs make the output

at U5B, pin 4, go high. The high is

wired to Q18, which is biased on, and

causes the drain to go low. The low is

connected to U3D, pin 12, which causes

its output at U3D, pin 14, to go low.

The low connects to U5A, pin 1, and, if

the transmitter is in Auto, pin 2 of U5A

is also low. With both inputs low, the

output of U5A at pin 3 goes high. The

high forward biases Q20, causing its

drain to go low. The low connects to

the Operate coil on the K1 relay that

switches the transmitter to Operate.

The low at U5C, pins 8 and 9, causes

the output at pin 10 to go high. This

high is connected to U5D, pin 12, and,

if the transmitter is in Auto, pin 13 of

U5D is low. The high on pin 12 causes

the output of U5D to go low and

reverse bias Q19. The drain of Q19

goes high and removes the low from

the Standby coil in relay K1.

Overtemperature Fault

The thermal switch A8-A1 on the

output dummy load A8 connects to J8-

1 on the board. If the temperature of

the thermal switch rises above 170° F,

it closes and applies a low to J8-1. The

low connects to Q3, which is biased off,

and to the red Overtemperature LED

DS6, which is biased on. The drain of

Q3 goes high and connects to pins 11

and 12 of U4B. The high at the input to

U4B causes it to go high and switches

the system to standby. This removes

the Operate enable commands to any

external amplifier trays.

VSWR Cutback Fault

The reflected power sample of the RF

output of the transmitter is connected

to J2, pin 9, of the board. The sample

connects to op-amp U1B, pin 5, which

buffers the signal before it is split. One

of the reflected samples that is split is

connected to J1-5, which is wired to

J10-5 on the rear of the tray, for

remote monitoring. The other reflected

sample that is split is connected to

position 3 on the front panel meter of

the tray. The final remote reflected

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-21

sample that is split is connected to

U2B, pin 5.

If the reflected sample level increases

above the level set by R22, the VSWR

cutback pot, the output of U2B at pin 7

goes high. The high is connected to

Q11 through CR11, which is biased on,

making U2C, pin 10, low and causing

U2C, pin 8, to go low. This low is split

and fed out of the tray at J1-6, J1-7,

J1-8, and J1-9; these are ALC outputs

to the amplifier trays that cut back the

output power of the amplifier trays. The

low from U2C, pin 8, is also fed through

coaxial jumper W2 on J13 and J14 to

R73. R73 is a bias adjust pot that sets

the level of the pin-attenuator bias that

is available as an output at J16.

The high at U2B, pin 7, is fed to the

base of Q14 and Q13, which are

forward biased, and produces a low at

the drains. The low connects to the

front panel amber VSWR Cutback LED

DS7 and causes it to light, indicating

that the tray is in cutback, and to

output jack J8-37 for a connection to a

remote VSWR cutback indicator.

(Optional) Receiver ALC Fault

If a receiver tray is part of the system,

a sample of the ALC voltage from the

receiver tray is connected to J8-11 on

the board. If the receiver is operating

normally, the ALC level applied to U3C,

pin 9, remains below the trip level set

by R35; the output at pin 13 stays

high. The high is applied to the red ALC

Fault LED DS8, which is off. The high

also connects to U3A, pin 2, and to

Q15. Q15 is biased on and the drain

goes low. The low connects to U5B, pin

6. U5B normally has a low that is

connected to U5B, pin 5, and produces

a high at the output pin 4. The high is

wired to Q18, which is biased on, and

makes its drain low. The low connects

to U3D, pin 12, which, because the

level is below the preset, its output at

U3D, pin 14, goes low. A low at this

point indicates a no-fault condition. The

high connected to U3A, pin 2, causing

its output to go low. The low is

connected to Q25, which is biased off.

The low is removed from J8-12, which

will not light any remote receiver fault

indicator that is connected to it.

If the receiver should malfunction, the

ALC level applied to U3C, pin 9, goes

high; because this is above the level

set by R35, the output at pin 13 will go

low. The low is applied to the red ALC

Fault LED DS8, which lights. The low

also connects to U3A, pin 2, and to

Q15. Q15 is biased off and the drain

goes high. The high connects to U5B,

pin 6, which produces a low at its

output at pin 4. The low is wired to

Q18, which is biased off, and makes its

drain high. The high connects to U3D,

pin 12; because the level is above the

preset, the output at U3D, pin 14, goes

high. A high at this point indicates a

fault condition that switches the

transmitter to Standby. The low

connected to U3A, pin 2, causes its

output to go high. The high is

connected to Q25, which is biased on,

causing its drain to go low. The low is

connected to J8-12, which can light any

remote receiver fault indicator that is

connected to it.

4.3.10.5 Metering

The front panel meter connects to J3-

1(-) and J3-2(+) on the board; this is

the output of switch S3. The front panel

meter has four metering positions that

are controlled by switch S3: % Forward

Power, % Reflected Power, % Exciter,

and ALC.

The reflected sample connects to the

board at J2-9 and is connected through

buffer amplifier U1B and 100-Ω resistor

R84 to position 3 of switch S3. The

forward sample connects to the board

at J2-5 and is connected through buffer

amplifier U1D and 100-Ω resistor R86

to position 4 of switch S3. The exciter

sample connects to the board at J2-3

and is connected through buffer

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-22

amplifier U1A and 100-Ω resistor R87

to position 2 of switch S3. The ALC

sample connects to the board at J6-1

and is connected through buffer

amplifier U2C; R15, the ALC calibration

pot that adjusts the output of U2A, pin

1; through 100-Ω resistor R18; and to

position 1 on switch S3.

Typical readings on the meter are:

• % Reflected = <5%

• % Forward Power = 100%

• % Exciter = The level on the meter

needed to attain 100% output

power from the transmitter

Refer to the test specifications sheet for

the transmitter for the actual readings

for the items shown below:

• ALC = .8 VDC

U6 is a temperature sensor IC that

gives the operator the ability to

measure the temperature inside the

tray by measuring the voltage at TP1.

The sensor is set up for +10 mV equals

1° F (for example, 750 mV equals 75°

F).

4.3.10.6 Operational Voltages

The +12 VDC needed for the operation

of the board enters the board at jack

J4, pin 3. C28, L1, and L3 are for the

filtering and isolation of the +12 VDC

before it is split and applied to the rest

of the board. The -12 VDC needed for

the operation of the board enters at

jack J4, pin 5. C29 and L2 are for the

filtering and isolation of the -12 VDC

before it is split and applied to the rest

of the board.

When the +12 VDC is connected to the

board, it is split. Four of the +12 VDC

outputs are fed out of the board at J8-

16, J8-17, J8-18, and J8-19 through

diode CR7, CR8, CR9, or CR10 and

resistor R50, R51, R52, or R53 to any

external amplifier trays for use in their

logic circuits. The resistors are for

current limiting and the diodes are to

prevent voltage feedback from the

external amplifier trays.

4.3.11 (A3)+12V(4A)/-12V(1A)

Power Supply Board (1265-1312;

Appendix D)

The ±12-volt power supply board

consists of three separate power

supplies, two of which produce the +12

VDC and one for the -12 VDC needed

to power the circuit boards in the

exciter tray.

4.3.11.1 +12 VDC Power Supply

The 18 VAC from the external step-

down toroid in the tray connects to J1,

pins 7 and 8. The 18 VAC connects

through 7-amp fuse F2, for over-

current protection, to two full-wave

bridge rectifier circuits.

CR1, CR2, CR3, and CR4 rectify the AC

that is then filtered by C2 and

connected to U3 and U4; U3 and U4

are voltage-regulator ICs for the +12

VDC. The voltage-regulated and

current-limited +12-VDC output of U3

connects to J4, pins 1, 2, 3, and 4,

which is one of the +12-VDC outputs of

the board. The green LED DS3 will be

lit if +12 VDC is present to J4; C7 and

C8 are bypass capacitors. The +12 VDC

output of U4 connects to J5, pins 1, 2,

3, and 4, which is another one of the

+12-VDC outputs of the board. The

green LED DS4 will be lit if +12 VDC is

present to J5; C9 and C10 are bypass

capacitors.

CR5, CR6, CR7, and CR8 rectify the AC

that is then filtered by C1 and

connected to U1 and U2; U1 and U2

are voltage-regulator ICs for the +12

VDC. The voltage-regulated and

current-limited +12-VDC output of U1

connects to J6, pins 1, 2, 3, and 4,

which is one of the +12-VDC outputs of

the board. The green LED DS1 will be

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-23

lit if +12 VDC is present to J6; C3 and

C4 are bypass capacitors. The +12-

VDC output of U2 connects to J3, pins

1, 2, 3, and 4, which is another one of

the +12-VDC outputs of the board. The

green LED DS2 will be lit if +12 VDC is

present to J3; C5 and C6 are bypass

capacitors.

4.3.11.2 -12 VDC Power Supply

The 18 VAC from an external step-

down toroid connects to J1, pins 1 and

4, of the board and is wired to CR9,

CR10, CR11, and CR12 which form a

full-wave bridge rectifier network for

the –12-VDC power supply. F1 is a 3-

amp fuse that provides over-current

protection. The rectified output is

filtered by C11 and fed to U5, the

voltage-regulator IC for the -12 VDC.

The voltage-regulated and current-

limited -12 VDC is connected to the

-12-VDC output jacks J7, pins 5

through 10, and J8, pins 5 through 10.

The green LED DS5 will be lit if -12

VDC is present to J7 and J8; C12 and

C13 are bypass capacitors.

4.3.12 (A21) 5-Section Delay

Equalizer Board, 44 MHz (1072090;

Appendix D)

The 5-section delay equalizer board, 44

MHz, is designed to correct for any

group delay errors that have been

created by the digital mask filter. The

board consists of five separate

delay/attenuation equalizer sections.

Each section is tuned across the 41 to

47 MHz band depending on where the

correction is needed. J9 on W3

determines the input impedance for the

IF source:

• J19-1, J19-2=50 Ω

• J19-2, J19-3=75 Ω

J23, J27 determines 6 dB attenuation;

J23-1, J23-2 and J27-1, J27-2 means

that there is no attenuation; and J23-

2, J23-3 and J27-2, J27-3 means that

there is 6-dB attenuation.

Delay equalizers 1 through 5 are each

tuned to a different frequency to

equalize the delay characteristics of

that frequency. Attenuation equalizers

1 through 5 are tuned to the same

frequency as their matched delay

equalizer section and are isolated from

them by a 3-dB pad. Each

equalizer/attenuation pair is followed

by a 12-dB gain stage with 6 dB of

attenuation between the sections.

Depending on where the jumpers are

positioned, each equalizer section can

be jumpered into the circuit or

bypassed by using a 2-dB pad.

4.4 (A4 and A5) Variable

Phase/Gain Trays (1245-1200;

Appendix C)

The Variable Phase/Gain Tray sets the

phase and gain adjustments of the RF

to provide maximum output when the

outputs of the two Amplifier Arrays are

combined. There are two Variable

Phase/Gain Trays in the 835A UHF

Transmitter, each controlling the phase

and gain of one of the two Amplifier

Arrays.

The Tray is made up of (A4) a Variable

Phase/Gain Assembly (1245-1204),

which contains (A4-A1) a Variable

Phase/Gain Board (1245-1201), (A5) a

Metering Board (1245-1202) and (A3)

a +15V/-12V Power Supply Board

(1245-1203).

4.4.1 (A4-A1) Variable Gain/Phase

Board (1245-1201; Appendix D)

The RF Input signal connects to the

rear panel of the Tray at J1 and is

cabled to J1 on (A4-A1) the Variable

Phase/Gain Board (1245-1201) that is

mounted inside (A4) the Variable

Phase/Gain Assembly (1245-1204) for

RFI and EMI protection. The Variable

Phase/Gain Board contains two main

circuits, one for ALC and one for Phase

adjustment of the RF signal. A front

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-24

panel mounted ALC adjust Pot (A8)

connects through (A5) the Metering

Board (1245-1202) to J5 on the Board

which controls the output level to the

tray. A front panel mounted Phase

adjust Pot (A7) connects through the

Metering Board to J4 on the Variable

Phase/Gain Board which controls the

phasing of the RF output. The Phasing

adjust is needed to phase the two RF

outputs of the Amplifier Arrays, when

they are combined, to achieve

maximum output. The phase and level

controlled RF signal connects to J2 of

the Variable Phase/Gain Board which

connects to the RF Output Jack J3

located on the rear panel of the Tray.

A sample of the RF from the Variable

Phase/Gain Board connects to J4 the

output Sample Jack located on the

front panel of the Tray.

4.4.2 (A5) Metering Board (1245-

1202; Appendix D)

A detected output sample level

connects to the Metering Board at J7

Pins 3 & 4 which is connected to the %

Power Level position on (A6) the front

panel meter of the Tray. S1 is a two

position front panel mounted switch

that allows the operator to view the

ALC Level, typically reads .6V - 1V, or

the % Power, which is calibrated for

100 % is +23 dBm output.

4.4.3 (A1) Power Entry Module

Assembly (1227-1206; Appendix

120/240 VAC connects to the Tray at J2

on (A1) the Power Entry Module

Assembly. Jumpers on the Power Entry

Module are set for the desired AC

Voltage Input. The assembly contains

an On/Off Switch, two 4 Amp Fuses

and three Varistors for AC line

protection. The AC is connected to

(A2) a step down Toroid that supplies

an 18 VAC output to the +15VDC/-

12VDC Power Supply Board.

4.4.4 (A3) +15V/-12V Power

Supply Board (1245-1203;

Appendix D)

The +15 VDC and -12 VDC needed to

operate the boards in the Tray are

generated on (A3) the +15 VDC/-12

VDC Power Supply Board (1245-1203).

The +15VDC/-12VDC Power Supply

Board supplies voltage regulated +15

VDC and -12 VDC Outputs to the rest

of the Tray. The Green LED DS1 is lit if

+15 VDC is present and the Green LED

DS2 is lit if -12 VDC is present at the

output of the +15V/-12V Power Supply

Board.

4.5 (A6) Metering Panel (1061456;

Appendix C)

The Metering Panel provides three

meters, one for Combined, one for Side

A Amplifier Array and one for Side B

Amplifier Array Outputs. The Side A

Meter (A15) displays the % Forward

and the % Reflected Power samples for

the Side A Amplifier Array. The Side B

Meter (A17) displays the % Forward

and the % Reflected Power samples for

the Side B Amplifier Array. The

Combined Meter (A13) displays the %

Reject, % Forward and the % Reflected

Power samples for the Combined

Output of the Transmitter. The

Metering Panel also takes the ALC

Reference Inputs from the (A4 & A5)

Variable Phase/Gain Trays and splits

them six ways which are then

connected to the UHF Amplifier Trays in

the two amplifier arrays.

The Metering Panel (A6) contains (A4,

A5, A6 & A11) four Splitter Boards

(1181-1002), (A7, A8 & A9) three Dual

Peak Detector Modules (1159965 or

1555-1271), (A10) a Visual/Aural

Metering Board (1265-1309), (A18) a

Detector Threshold Board (1245-1402)

and (A3) a ±12V Power Supply Board

(1062-1013).

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-25

The (A13) Front Panel Meter is for

Combined Output Power that is

controlled by (S1) the Meter Control

Switch, the (A15) Front Panel Meter is

for Side A Output Power with (S2) the

Meter Control Switch and the (A17)

Front Panel Meter is for Side B Output

Power with (S3) the Meter Control

Switch.

4.5.1 (A4, A5, A6 & A11) Splitter

Board (1181-1002; Appendix D)

The (A4) Splitter Board (1181-1002)

takes the ALC reference from the (A4)

Variable Phase/Gain Tray at J1-1 and

splits it six ways that connect to the

UHF Amplifier Trays mounted in the

Side A Amplifier Array.

The (A5) Splitter Board (1181-1002)

takes the ALC reference from the (A5)

Variable Phase/Gain Tray at J1-1 and

splits it six ways that connect to the

UHF Amplifier Trays mounted in the

Side B Amplifier Array.

The (A6) Splitter Board takes the

Combined Reflected Sample Input at

J1-1 and splits it. One of the split

Combined Samples connects to the

front panel Meter, one is used for

Remote Monitoring and the other two

connect back to the UHF Exciters for

use in the VSWR Cutback circuitry.

The (A11) Splitter Board (1181-1002)

takes the Combined Aural Sample

Input at J1-1 from the (A10)

Visual/Aural Metering Board and splits

it. One of the split Aural Samples

connects to the front panel Meter and

the other is used for Remote

Monitoring.

4.5.2 (A10) Visual/Aural Metering

Board (1265-1309; Appendix D)

This board was originally designed for

Analog operation only. When used in a

digital transmitter the visual references

and circuits become combined and the

aural references and circuits are not

used.

The (A10) Visual/Aural Metering Board

has a Combined Forward Sample from

the Output Coupler Assembly of the

Transmitter connected to J1 on the

Board where it is split. One Forward

Sample connects to J2 on the board

that is cabled to J9, the Transmitter

Output Sample Jack, located on the

front panel of the Metering Panel. The

Combined Forward Sample on the

Visual/Aural Metering Board is split

again with one Sample used to produce

a Combined Forward Sample, that

connects to the front panel Meter (A13)

and also to the rear panel for Remote

monitoring. The other Combined

Forward Sample produces a combined

Aural Sample that connects to the

(A11) Splitter Board.

4.5.3 (A7, A8 & A9) Dual Peak

Detectors (1159965; Appendix D)

The (A7) Dual Peak Detector takes the

Combined Reflected Sample Input from

the Output Coupler Module and splits it.

A peak detected output of the reflected

sample connects to the (A6) Splitter

Board. The (A7) Dual Peak Detector

also has a Reject Sample Input from

the Reject Output Coupler Module and

splits it. One of the peak detected split

Reject Samples connects to the front

panel Meter and the other is used for

Remote Monitoring.

The (A8) Dual Peak Detector Module

takes a Reflected Sample from the Side

B Amplifier Array that is peak detected

and connected to the (A17) front panel

meter. Another input to the (A8) Dual

Peak Detector Module is a Forward

Sample from the Side B Amplifier Array

that is peak detected and connected to

the (A17) front panel meter.

The (A9) Dual Peak Detector Module

receives the Reflected Sample from the

Side A Amplifier Array which is peak

detected and connected to the (A15)

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-26

front panel meter. Another input to the

(A9) Dual Peak Detector Module is the

Forward Sample from the Side A

Amplifier Array which is peak detected

and connected to the (A15) front panel

meter.

4.5.4 (A18) Detector Threshold

Board (1245-1402; Appendix D)

The Detector Threshold Board consists

of three identical signal paths. One

path uses a detected Reflected sample

from Side A, another the detected

Reflected sample from Side B and the

final path uses a detected Combined

Reflected sample. These samples are

compared to the threshold settings,

20% Reflected level, and provide an

output to cut back the transmitter if the

reflected level of any of the input

samples increase above 20%.

A Sample of the Side A Reflected level

enters the board at J1-1. The level is

split and one path connects to J1-2 to

be used as a Side A Remote Reflected

output. The other path connects to the

Operational Amplifier U1A whose turn

on level is controlled by the setting of

R5. R5 is set to cause U1A to conduct

if the reflected sample input increases

above the 20% level. The output, at

J2-1 and also J2-5 through the jumper

W1 on J4, connects to the Transmitter

Control Board, located in the UHF

Exciter, where it cuts back the

transmitter using the VSWR cut back

circuits.

A Sample of the Side B Reflected level

enters the board at J8-1. The level is

split and one path connects to J8-2 to

be used as a Side B Remote Reflected

output. The other path connects to the

Operational Amplifier U1B whose turn

on level is controlled by the setting of

R12. R12 is set to cause U1B to

conduct if the reflected sample input

increases above the 20% level. The

output, at J2-5 and also J2-1 through

the jumper W1 on J4 connects to the

Transmitter Control Board, located in

the UHF Exciter, where it cuts back the

transmitter using the VSWR cut back

circuits.

A Sample of the Combined Reflected

level enters the board at J7-1. The

level is split and one path connects to

J7-2 to be used as a Remote Combined

Reflected output. The other path

connects to the Operational Amplifier

U1C whose turn on level is controlled

by the setting of R19. R19 is set to

cause U1C to conduct if the combined

reflected input sample increases above

the 20% level. The output, at J6-1 and

also J2-5 through the jumper W2 on J5

connects to the Transmitter Control

Board, located in the UHF Exciter,

where it cuts back the transmitter using

the VSWR cut back circuits

4.5.5 (A1) Power Entry Module

Assembly (1227-1206; Appendix D)

120/240 VAC connects to the Tray at J8

on (A1) the Power Entry Module

Assembly. Jumpers on the Power Entry

Module are set for the desired AC

Voltage Input. The assembly contains

an On/Off Switch, two 4 Amp Fuses

and three Varistors for AC line

protection. The AC is connected to

(A2) a step down Toroid that supplies

an 18 VAC output thru TB1 to the ±12V

Power Supply Board.

4.5.6 (A3) ±12V Power Supply

Board (1062-1013; Appendix D)

The ±12 VDC needed to operate the

Boards in the Metering Panel are

provided by the ±12 VDC Power Supply

Board (1062-1013).

The 18 VAC, from an external step-

down Transformer, is directed to the

Board thru J5 Pins 1 & 4, with Pin 2

connected to the center tap of the

secondary of the Transformer.

The +12 VDC is produced by using a

Full-Wave rectifier circuit consisting of

CR1 and CR2 whose output is filtered

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-27

by capacitor C1 and bypassed by C2 to

prevent oscillations. The filtered +12

VDC is applied to U1 a 7812 IC voltage

regulator and current limiter. C3 is a

bypass capacitor to prevent oscillations

in the output DC lines. DS1 is a Green

LED which when lit indicates that the

+12VDC is present at the output of the

board. The +12 VDC is distributed thru

Pins 2 & 7 of each of the Output Jacks,

J1, J2, J3 & J4, to the rest of the

boards in the Tray.

The -12 VDC is produced by using a

Full-Wave rectifier circuit that consists

of CR3 and CR4 whose output is filtered

by capacitor C4 and bypassed by C5 to

prevent oscillations. The filtered -12

VDC is applied to U2 a 7912 IC voltage

regulator and current limiter. C6 is a

bypass capacitor to prevent oscillations

in the output DC lines. C7 is a holding

capacitor to maintain -12VDC for an

instance even if the supply is lost. This

is to allow the positive voltages to be

removed from GaAs FET Amplifier

devices to prevent damage to the

devices. DS2 is a Green LED which

when lit indicates that the -12 VDC is

present at the output of the board. The

-12VDC is distributed to the rest of the

Boards in the Tray thru Pins 4 & 5 of

each of the Output Jacks, J1, J2, J3 &

J4.

The ±12 VDC outputs of the board are

connected to the rest of the boards in

the Metering Panel.

4.6 (A8) AC Distribution Assembly

(1245-1500; Appendix C)

The AC Distribution Assembly

distributes the Main AC Input, 208/240

VAC @ 20 Amps, that connects to the

UHF Exciter Assembly Cabinet to the

Trays and Assemblies which make up

the Exciter.

The 208/240 VAC Main AC Input to the

Exciter Cabinet connects to the

Terminal Block TB1, Line 1 to TB1-1A,

Line 2 to TB1-3A and Ground to TB1-

2A.

The 208/240 VAC Main AC Input

connects from the Terminal Block TB1-

1B (Line 1) to the 20 Amp Circuit

Breaker CB1-1, TB1-2B (Ground) to

Chassis Ground & TB1-3B (Line 2) to

the Circuit Breaker CB1-3. Three

MOVs, for surge and transient

protection, are connected to the AC

Lines from the Circuit Breaker, VR1 and

VR3 from the AC Lines to Ground and

VR2 across the AC Lines.

The switched 208/240 VAC Main AC

Input is wired from the Circuit Breaker

Terminal 2 to the L inputs and Terminal

4 to the N Inputs of the (A1 & A2) IEC

Outlet Strips. The E Inputs to both IEC

Outlet Strips connect to Chassis

Ground.

There are four outputs for both the

(A1) and the (A2) IEC Outlet Strips.

On the A1 Outlet Strip, J1 connects to

J14 on the (A1) Exciter #1, J2 connects

to J14 on the (Optional) (A2) Exciter

#1, J3 connects to J8 on the (Optional)

(A3) Exciter Switcher Tray and J4

connects to J2 on the (A4) Variable

Phase/Gain Tray. On the A2 Outlet

Strip, J5 connects to J2 on the (A5)

Variable Phase/Gain Tray, J6 connects

to J8 on the (A6) Metering Panel, J7

connects to J2 on the (Optional) (A7)

Receiver Tray or to the AC Input Jack

on the (Optional) Digital Modulator and

J8 is not used in this System.

4.7 (A2 & A3) 2-3 kW Amplifier

Array (1278-1300; Appendix C)

There are two Amplifier Array

Assemblies in the DT835A 2-3 kW UHF

Solid State Internally Diplexed Digital

Television Transmitter. The (A2 & A3)

Amplifier Array Assemblies (1278-

1300) are identical and configured

according to the output power needed.

Each Amplifier Array contains (A9-A1)

an 8 Way Splitter (ZFSC-8-43), four to

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-28

six outputs of the splitter are used,

depending on output power, (A1 thru

A6) UHF Amplifier Trays (1281-1100),

the number used depending on the

output power needed, (A7) a 4, 5 or 6

Way Combiner, depending on the

output power needed, (A8) an Output

Coupler (1016-1043), (A10) an AC

Distribution Assembly (1278-1200) for

Single Phase AC Input or (1278-1100)

for Three Phase AC Input and (A11) an

Interface Panel. If the Amplifier Array

contains the 2.5 kW or 3 kW Output

Power Kit, the Array also has (A12) a

Reject Load Assembly (1278-1312).

In the 2 kW configuration no Reject

Load Assembly is needed.

The output of the (A4) Variable

Phase/Gain Tray, located in the Exciter

Assembly, connects to (A2) the Side A

Amplifier Assembly (1278-1300). The

output of the (A5) Variable Phase/Gain

Tray connects to (A3) the Side B

Amplifier Assembly (1278-1300). The

RF Input from the Variable Phase/Gain

Tray connects to the RF Input Jack J1

on (A11) the Interface Panel located in

the Amplifier Array Assembly. The RF

is cabled to the COM Input of (A9-A1)

the 8 Way Splitter which splits it eight

ways maximum. Four to Six of the RF

outputs of the Splitter, depending on

the output power needed, are

connected to J1, the RF Input Jack on

each of the up to six UHF Amplifier

Trays (1281-1100). The outputs of the

Splitter that are not used are

terminated with 50O. Each of the UHF

Amplifier Trays amplify the RF signals

to the power needed to produce a total

of 4000 - 6000 Watts Peak of Sync

after combining of the two Amplifier

Assemblies, with a maximum of 600

Watts Peak of Sync output per Tray.

The outputs of the UHF Amplifier Trays

are combined in (A7) the 4, 5 or 6 Way

Combiner that provides half of the

power needed to generate the output

power of the Transmitter. In Amplifier

Arrays which contain a 2.5 kW or 3.0

kW Amplifier Array Power Level Kit, the

Reject Outputs of the Combiner are

connected to (A12-A2) the Combiner

Reject. Load Board (1278-1311) in

(A12) the Reject Load Assembly (1278-

1312). In the 2 kW Amplifier Array

Power Level, the Combiner has a

different Reject Load Assembly. The RF

Output of the Combiner is cabled to

(A8) the Output Coupler Assembly

(1016-1043). The Output Coupler

Assembly supplies a Forward and a

Reflected Power Sample of the output

from the Amplifier Assembly which

connects through the SMA Connectors

J2 and J3 located on the Interface

Panel to the Metering Panel located in

the UHF Exciter Assembly.

In each UHF Amplifier Tray, a Forward

Power Sample and a Reflected Power

Sample, from the 4 Way Combiner

Board, are connected to the Dual Peak

Detector Board, Single Supply which

provides peak detected forward

samples to the Amplifier Control Board

that supplies the samples to the front

panel meter of the UHF Amplifier Tray

and also to the Interface Panel for

remote monitoring. Before exiting each

UHF Amplifier Tray the RF is fed

through a Circulator which protects the

Tray from high VSWR conditions. The

Reject Port of the Circulator provides a

Reject Sample to the 4 Way Combiner

Board which supplies the Reflected

Sample to the Dual Peak Detector

Board, Single Supply located in the UHF

Amplifier Tray that connects to the

front panel meter. The outputs of the

(A2 & A3) Amplifier Array Assemblies

connect to (A4) a Hybrid Combiner

(55223A) which combines the outputs

into a single output.

The two Enable (Operate) Commands

from the UHF Exciter Tray, located in

the UHF Exciter Assembly, connect to

the Terminal Block (A9-TB2) mounted

in each Amplifier Array. The terminal

block splits the Enables in each array

and supplies them to all of the UHF

Amplifier Trays. The Enable is needed

to turn on the Switching Power Supply

located in each UHF Amplifier Tray. If

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-29

the Amplifier Array contains an External

Exhaust Kit, then a Fan Enable is

supplied at J4-19 with +12V at J4-20

that connect to the fan mounted on the

roof which turns it on..

Main AC Input

Each Amplifier Assembly needs an AC

Input of 208/240 VAC at 100 Amps

Single Phase or 208/240 VAC at 55

Amps Three Phase wired to it. The

208/240 VAC Input to each Amplifier

Assembly connects to (A10) the AC

Distribution Assembly, Amplifier Array

(1278-1200 Single Phase or 1278-1100

Three Phase) located on the right side,

center rear of each Cabinet. Each AC

Distribution Assembly contains the

Terminal Block (TB1) to which the

208/240 VAC connects. For Single

Phase operation connect Line 1 to TB1-

1A, Line 2 to TB1-3A and Ground to

TB1-4A and for Three Phase operation,

connect Line 1 to TB1-1A, Line 2 to

TB1-2A, Line 3 to TB1-3A and Ground

to TB1-4A. The AC Distribution Panel

contains from five to seven Circuit

Breakers that supply the AC to the rest

of the Amplifier Assembly. The Input

AC from TB1 is connected to (CB1) the

Main AC Circuit Breaker (100 Amps for

Single Phase or 80 Amps for Three

Phase) which distributes the 220 VAC

to the other circuit breakers (CB2-

CB7). The CB6 and CB7 Circuit

Breakers are used if the extra Amplifier

Trays are needed. The output of CB1

has three MOVs, VR1, VR2 and VR3 for

Single Phase or six MOVs VR1-VR6 for

Three Phase, mounted to it, one

connected from each leg of the Input

AC to ground and one across the each

of the legs.

The switched Input AC is wired through

a maximum of six Circuit Breakers,

CB2-CB7, to the UHF Amplifier Trays in

the Amplifier Array Cabinets. CB2 is a

20 Amp Circuit Breaker that supplies

the AC voltage to the (A1) UHF

Amplifier Tray. CB3 is a 20 Amp Circuit

Breaker which supplies the AC voltage

to the (A2) UHF Amplifier Tray. CB4 is

a 20 Amp Circuit Breaker which

supplies the AC voltage to the (A3) UHF

Amplifier Tray. CB5 is a 20 Amp Circuit

Breaker which supplies the AC voltage

to the (A4) UHF Amplifier Tray. The

CB6 and CB7 circuit breakers are part

of the (Optional) 2.5 kW or 3 kW High

Power Kit and may not be present.

CB6 is a 20 Amp Circuit Breaker which

supplies the AC voltage to the (A5) UHF

Amplifier Tray. CB7 is a 20 Amp Circuit

Breaker which supplies the AC voltage

to the (A6) UHF Amplifier Tray. In the

(Optional) 2.5 kW and 3 kW Amplifier

Array Assemblies circuit breakers CB8

and CB9 apply AC to the (A12-A3 &

A12-A5) Fans located on (A12) the

Reject Load Assembly.

When the Circuit Breaker CB1 on the

AC Distribution Assembly in the UHF

Exciter Assembly is switched On, +12

VDC from the Exciter is supplied to

each of the Amplifier Array Cabinets.

In the Amplifier Array, the +12 VDC is

split and connected to each of the UHF

Amplifier Trays in the Amplifier

Assemblies for operation of the LED

Status Indicators in the Tray.

4.8 (A1, A2, A3 & A4 with A5 & A6

used in 2.5 & 3 kW Arrays) UHF

Amplifier Trays (1294-1112 low

band/1294-1113 mid band/1294-

1114 high band; Appendix C)

4.8.1 (A1) UHF Filter (1007-1101;

Appendix D)

The UHF filter is a tunable two-section

cavity filter that is typically tuned for a

bandwidth of 6 MHz and has a loss of

-1 dB through the filter.

4.8.2 (A3) 1-Watt Amplifier Board

Assembly (1227-1319; Appendix D)

The 1-watt UHF amplifier board

assembly provides radio frequency

interference (RFI) and electromagnetic

interference (EMI) protection, as well

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-30

as the heatsink, for the 1-watt UHF

amplifier board (1227-1303) that is

mounted inside the assembly.

Depending on the frequency of the

channel of operation, the assembly has

approximately 9 dB of gain.

The RF input to the assembly connects

to SMA jack J1. The amplified RF output

of the assembly is at SMA jack J2.

Typically, with an input signal of +21

dBm at J1 of the assembly, an output

of +30 dBm can be expected at J2.

The +28-VDC bias voltage connects

through FL1, an RF-bypass, feed-

through capacitor, that is mounted to

E1 on the amplifier board.

4.8.3 (A3-A1) 1-Watt Amplifier

Board (1227-1303; Appendix D)

The 1-watt UHF amplifier board is part

of the 1-watt UHF amplifier assembly

(1227-1319) and provides

approximately +10 dB of gain.

The UHF signal enters the board at J1,

an SMA connector, and is applied

through coupling capacitor C1 and a

stripline circuit to Q1, a UTV040F UHF

wideband-amplifier device. The base

and collector voltages needed to

operate the transistor are obtained

from the +26.5 VDC line that connects

to the board at E1. The collector

voltages are fed through VR1, VR2, R3,

and R4 and the base voltages are fed

through R2 and R1. The amplified UHF

output of Q1 is coupled through a

stripline circuit and C14 to J2, the

output SMA jack of the board.

The board is powered by the +26.5

VDC that is produced by an external

power supply in the tray. The +26.5

VDC enters through E1 and is fed

across R4 and R5, which drops

approximately 6 volts, to the collector

(+20 VDC). The voltage is filtered by

RF decoupling components L4, C11,

C12, and C15 before it is connected to

the collector. The bias voltage is

connected across R3, VR1, VR2, R1,

and R2, which sets the base bias

voltage at +.8 VDC and forward biases

Q1. C6, L1, and L2 provide RF

decoupling of the bias voltage before it

is connected to the base.

The board has a self-bias protection

circuit that uses zener diodes VR1 and

VR2. If the current draw of the device

increases, the voltage drop across R4

and R5 increases; this decreases the

voltage that is applied to VR1 and VR2.

These two diodes drop a fixed voltage

of 20 VDC across them. As a result, if

the voltage drop across R4 and R5

increases, the voltage available to the

base of Q1 decreases and the device

will eventually be shut off.

4.8.4 (A4-A1) Single Stage Amplifier

Assembly (1265-1418, low band

/1265-1416, mid band/1265-1417,

high band; Appendix D)

4.8.4.1 Low Band

The single stage UHF amplifier

assembly, Class A, low band, is made

from the generic single stage amplifier

board, Class A (1265-1415). The

assembly uses a single PTB20101

Ericsson device that is made up of two

transistors in parallel and operating

Class A, which amplify the signal by

approximately +11 dB. Bias adjust pot

R6 sets the operating current for Q1.

Q1 and Associated Circuitry

The RF input signal connects to SMA

jack J1 on the board. The RF input is

applied through AC coupling and DC

blocking capacitor C1 to L1 and

associated circuitry. This forms a balun

that converts the input signal from a

50-Ω unbalanced impedance to a 12.5-

Ω balanced impedance configuration.

The two outputs, applied to the bases

of Q1, are 180° out of phase with each

other. C3, C4, C6, and C5, which are

adjusted for peak output, are for

impedance matching to the input of the

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-31

parallel transistors that make up Q1.

The base circuit is RF bypassed by C2,

C7, C17, C28, and C29.

The collectors are impedance matched

to 12.5Ω by C22, C23, and C19, which

can be adjusted for peak output with

the best linearity. C25 provides AC

coupling and DC blocking for the output

signal to SMA RF output connector J2.

L2 and its associated circuitry form a

balun that transforms the two balanced

signals back to a single, unbalanced

50-Ω impedance output. The collector

circuit is RF bypassed by C9, C12 to

C15, C18, C21, C24, C26, C30, and

C31.

The +26 VDC needed for biasing Q1 is

applied to E1. E1 is the high side of A4-

A4, a .5-Ω/25-watt external metering

resistor that is mounted on the

heatsink next to the single stage

amplifier assembly. The metering

resistor is in the collector circuit of the

RF transistor, Q1, and provides the

main current path for Q1. The base

bias applied to Q1 is supplied through

R11, R4, R2, and R3.

The collector bias voltage drop across

the A4-A4 metering resistor is in

parallel with the branch consisting of

R10 and the green Current LED DS1;

as a result, it has the same voltage

across it. The collector bias voltage

drop biases on the green LED DS1. The

current flow through R10 and DS1

gives a visual indication of the current

draw of Q1 by the relative brightness of

DS1. The higher the collector current of

the transistor, the larger the voltage

drop across the metering resistor; this

action increases the voltage across DS1

and R10. This greater voltage level

increases the current flow through DS1

and R10, which increases the

brightness of DS1. The opposite occurs

when the collector current decreases.

The use of opto-isolator U1 allows no

direct connection between the base and

collector biasing circuits other than

R11, the 200-Ω/5-watt control resistor

that determines the actual base current

flowing in Q1. R11 provides the primary

current path from the collector circuit

to the base of Q1. If there is no current

flowing initially through Q1, R11

provides a substantial amount of base

drive. When the collector current of Q1

increases to the desired operating level,

the opto-isolator LED, which is across

U1, is turned on. This turn-on point, or

threshold, is set by the voltage-divider

network consisting of R7, R9 and

adjustable resistor R6. R6 is adjusted

to set up the operating current at 5

amps. When the opto-isolator LED

turns on, it causes the transistor

portion to also turn on. When the

transistor portion turns on, it biases on

Q2, which acts as a shunt regulator for

the base current of Q1.

Negative feedback for the circuit is

preset so that if Q1 draws more than

the desired amount of collector current,

the voltage drop across A4-A4, the

metering resistor, becomes greater

than normal. This increase causes the

voltage across the opto-isolator to

increase; as a result, the opto-isolator

LED is brighter than normal and

forward biases Q2 even more. This

action tends to make the available

current that was fed through R11 and

Q1 to flowing more through Q2, from

collector to the emitter ground, setting

up the negative feedback path. The

negative feedback tends to stabilize the

current flow through Q1. Capacitor C8

provides an active filtering action for

this current source, which provides an

improved response in Q1 during the

vertical interval average current

changes that take place.

4.8.4.2 Mid Band

The single stage UHF amplifier

assembly, Class A, mid band is made

from the generic single stage amplifier

board, Class A (1265-1415). The

assembly uses a single PTB20101

Ericsson device that is made up of two

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-32

transistors in parallel and operating

Class A, which amplify the signal by

approximately +11 dB. Bias adjust pot

R6 sets the operating current for Q1.

Q1 and Associated Circuitry

The RF input signal connects to SMA

jack J1 on the board. The RF input is

applied through AC coupling and DC

blocking capacitor C1 to L1 and

associated circuitry. This forms a balun

that converts the input signal from a

50-Ω unbalanced impedance to a 12.5-

Ω balanced impedance configuration

with the two outputs. These outputs

are applied to the bases of Q1 and are

180° out of phase with each other. C3,

C4, C6, and C5, which are adjusted for

peak output, are for impedance

matching to the input of the parallel

transistors that make up Q1. The base

circuit is RF bypassed by C2, C7, C17,

and C29.

The collectors are impedance matched

to 12.5Ω by C22, C23, and C19, which

are adjusted for peak output with the

best linearity. C25 provides AC coupling

and DC blocking for the output signal to

SMA RF output connector J2. L2 and its

associated circuitry form a balun that

transforms the two balanced signals

back to a single, unbalanced 50-Ω

impedance output. The collector circuit

is RF bypassed by C9, C12 to C15, C18,

C21, C24, C26, C30, and C31.

The +26 VDC needed for biasing Q1 is

applied to E1. E1 is the high side of A4-

A4, a .5-Ω/25-watt external metering

resistor that is mounted on the

heatsink next to the single stage

amplifier assembly. The metering

resistor is in the collector circuit of RF

transistor Q1 and provides the main

current path for Q1. The base bias

applied to Q1 is supplied through R11,

R4, R2, and R3.

The collector bias voltage drop across

the A4-A4 metering resistor is in

parallel with the branch consisting of

R10 and the green Current LED DS1;

as a result, it has the same voltage

across it. The collector bias voltage

drop biases on the green LED DS1. The

current flow through R10 and DS1

gives a visual indication of the current

draw of Q1 by the relative brightness of

DS1. The higher the collector current of

the transistor, the larger the voltage

drop across the metering resistor; this,

in turn, increases the voltage across

DS1 and R10. This greater voltage level

increases the current flow through DS1

and R10 and increases the brightness

of DS1. The opposite occurs when the

collector current decreases.

The use of opto-isolator U1 allows no

direct connection between the base and

collector biasing circuits other than

R11, the 200-Ω/5-watt control resistor

that determines the actual base current

flowing in Q1. R11 provides the primary

current path from the collector circuit

to the base of Q1. If there is no current

flowing initially through Q1, R11

provides a substantial amount of base

drive. When the collector current of Q1

increases to the needed operating

level, the opto-isolator LED, which is

across U1, is turned on. This turn-on

point, or threshold, is set by the

voltage-divider network consisting of

R7, R9, and adjustable resistor R6. R6

is adjusted to set up the operating

current, at 5 amps. When the

opto-isolator LED turns on, it causes

the transistor portion to also turn on.

When the transistor portion turns on, it

biases on Q2, which acts as a shunt

regulator for the base current of Q1.

Negative feedback for the circuit is

preset so that if Q1 draws more than

the desired amount of collector current,

the voltage drop across A4-A4, the

metering resistor, becomes greater

than normal. This increase causes the

voltage across the opto-isolator to

increase; as a result, the opto-isolator

LED is brighter than normal and

forward biases Q2 even more. This

action tends to make the available

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-33

current that was fed through R11 and

Q1 to flow more through Q2, from

collector to the emitter ground, and

setting up the negative feedback path.

The negative feedback tends to

stabilize the current flow through Q1.

Capacitor C8 provides an active filtering

action for this current source, which

provides an improved response in Q1

during the vertical interval average

current changes that take place.

4.8.4.3 High Band

The single stage UHF amplifier

assembly, Class A, high band is made

from the generic single stage amplifier

board, Class A (1265-1415). The

assembly uses a single PTB20101

Ericsson device that is made up of two

transistors in parallel and operating

Class A, which amplify the signal by

approximately +11 dB. Bias adjust pot

R6 sets the operating current for Q1.

Q1 and Associated Circuitry

The RF input signal connects to SMA

jack J1 on the board. The RF input is

applied through AC coupling and DC

blocking capacitor C1 to L1 and

associated circuitry. This forms a balun

that converts the input signal from a

50-Ω unbalanced impedance to a 12.5-

Ω balanced impedance configuration

with the two outputs. The two outputs

are applied to the bases of Q1 and are

180° out of phase with each other. C6

and C5, which are adjusted for peak

output, are for impedance matching to

the input of the parallel transistors that

make up Q1. The base circuit is RF

bypassed by C2, C7, C17, C28, and

C29.

The collectors are impedance matched

to 12.5Ω by C16, C22, C23, and C19,

which are adjusted for peak output with

the best linearity. C25 provides AC

coupling and DC blocking for the output

signal to SMA RF output connector J2.

L2 and its associated circuitry form a

balun that transforms the two balanced

signals back to a single, unbalanced

50-Ω impedance output. The collector

circuit is RF bypassed by C9, C12 to

C15, C18, C21, C24, C26, C30, and

C31.

The +26 VDC needed for biasing Q1 is

applied to E1. E1 is the high side of A4-

A4, a .5-Ω/25-watt external metering

resistor that is mounted on the

heatsink next to the single stage

amplifier assembly. The metering

resistor is in the collector circuit of RF

transistor Q1 and provides the main

current path for Q1. The base bias

applied to Q1 is supplied through R11,

R4, R2, and R3.

The collector bias voltage drop across

the A4-A4 metering resistor is in

parallel with the branch consisting of

R10 and the green Current LED DS1;

as a result, it has the same voltage

across it. The collector bias voltage

drop biases on the green LED DS1. The

current flow through R10 and DS1

gives a visual indication of the current

draw of Q1 by the relative brightness of

DS1. The higher the collector current of

the transistor, the larger the voltage

drop across the metering resistor,

which, in turn. increases the voltage

across DS1 and R10. This greater

voltage level increases the current flow

through DS1 and R10 and increases the

brightness of DS1. The opposite occurs

when the collector current decreases.

The use of opto-isolator U1 allows no

direct connection between the base and

collector biasing circuits other than

R11, the 200-Ω/5-watt control resistor

that determines the actual base current

flowing in Q1. R11 provides the primary

current path from the collector circuit

to the base of Q1. If there is no current

flowing initially through Q1, R11

provides a substantial amount of base

drive. When the collector current of Q1

increases to the desired operating level,

the opto-isolator LED, which is across

U1, is turned on. This turn-on point, or

threshold, is set by the voltage-divider

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-34

network consisting of R7, R9, and

adjustable resistor R6. R6 is adjusted

to set up the operating current at 5

amps. When the opto-isolator LED

turns on, it causes the transistor

portion to also turn on. When the

transistor portion turns on, it biases on

Q2, which acts as a shunt regulator for

the base current of Q1.

Negative feedback for the circuit is

preset so that if Q1 draws more than

the desired amount of collector current,

the voltage drop across A4-A4, the

metering resistor, becomes greater

than normal. This increase causes the

voltage across the opto-isolator to

increase; as a result, the opto-isolator

LED is brighter than normal and

forward biases Q2 even more. This

action tends to make the available

current that was fed through R11 and

Q1 to flow more through Q2, from

collector to the emitter ground, setting

up the negative feedback path. The

negative feedback tends to stabilize the

current flow through Q1. Capacitor C8

provides an active filtering action for

this current source, which provides an

improved response in Q1 during the

vertical interval average current

changes that takes place.

4.8.5 (A4-A2) Coupler Board

Assembly (1227-1316; Appendix D)

The UHF coupler assembly is mounted

in the UHF amplifier tray and provides a

forward power sample of the input

drive level to the dual stage amplifier

assembly, class AB. The drive-level

sample from J3 is cabled to the

amplifier control board where it

connects to the input of the overdrive-

protection circuit.

The RF input to the UHF coupler

assembly from the dual stage amplifier

assembly, class A, connects to SMA

jack J1. The RF is connected by a

stripline track to SMA output jack J2. A

hybrid-coupler circuit picks off a

forward sample that is connected to

SMA-type connector jack J3. R1 is a

dissipation load for the reject port of

the coupler.

4.8.6 (A6) Dual Peak Detector

Enclosure (1227-1317; Appendix D)

The dual peak detector enclosure

provides EMI and RFI protection for the

dual peak detector board, single supply

(1227-1333), which is mounted inside

the enclosure.

The module has two inputs: a forward

power sample at SMA jack J1 and a

reflected power sample at SMA jack J2.

The module has two peak-detected

sample outputs: a forward power

sample at FL3 from J4-4 on the board

and a reflected power sample at FL2

from J4-2 on the board. The module

also has a forward power sample

output at SMA jack J3.

The voltage, +28 VDC, needed to

operate the board connects to FL1 on

the assembly, which is wired to J4-7 on

the board.

4.8.7 (A6-A1) Dual Peak Detector

Board, Single Supply (1227-1333;

Appendix D)

The function of the dual peak detector

board is to detect forward and reflected

samples of visual or aural RF signals

and generate an output voltage

proportional to the power levels of the

sampled signals for metering purposes.

There are two identical signal paths on

the board: one for forward power and

one for reflected power. A sample of

forward output power enters the board

at SMA jack J1. Resistors R1, R2, and

R3 form an input impedance-matching

network of 50Ω. The forward power

signal is detected by CR1, R4, R5, R7,

R10, C1, and C2. The output is buffered

by operational amplifiers U3B and U1C

before it is connected to forward power

output jack J4-4. U3 has a very high

input impedance that makes the IC less

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-35

sensitive to changes in the video level.

A sample of the forward power is

tapped off by R6 and R8 and fed to J3,

the forward sample output jack. Diode

CR2 provides temperature

compensation for diode CR1. An input

signal level of approximately +17 dBm

is enough to give a 1-VDC level at the

output of U1C.

A reflected output power sample enters

the board at SMA jack J2. Resistors

R18, R19, and R20 form an input

impedance-matching network of 50Ω.

The reflected power signal is then

detected by CR3, R21, R22, R24, C5,

and C6 and the output is buffered by

operational amplifiers U3A and U1B

before it is connected to reflected

power output jack J4-2. U3 has a very

high input impedance that makes the

IC less sensitive to changes in the

video level. Diode CR4 provides

temperature compensation for diode

CR3. An input signal level of

approximately +17 dBm is enough to

give a 1-VDC level at the output of

U1B.

The +12 VDC needed for the operation

of U1 on the board is generated from

the +28 VDC which enters at J4, pin 7.

The +28 VDC is fed to U2, a voltage

regulator IC, that produces +12 VDC at

its output, which is connected to U1.

4.8.8 (A5-A1) 4-Way Splitter

Assembly (1265-1432; Appendix D)

The 4-way splitter assembly contains

(A5-A1-A1) a 4-way splitter board

(1227-1312) that is made up of three

2-way Wilkinson stripline splitters. One

RF input to the board provides four,

equal RF outputs.

The RF input to the board is connected

to the input of the first 2-way splitter

that contains R1. R1 is a balancing

resistor in which any RF due to

mismatching in the first splitter will be

dissipated. One of the two outputs from

the splitter connects to another 2-way

splitter that contains R2. R2 is a

balancing resistor in which any RF due

to mismatching in the splitter will be

dissipated. The other output of the first

splitter connects to the third 2-way

splitter that contains R3. R3 is a

balancing resistor in which any RF due

to mismatching in the splitter will be

dissipated. The two output splitters

provide four, equal RF outputs, two

each, which are connected to the inputs

of the external amplifier boards.

4.8.9 (A5-A1-A1) 4-Way Splitter

Board (1227-1312; Appendix D)

The 4-way splitter board is made up of

three 2-way Wilkinson stripline

splitters. One RF input to the board

provides four, equal RF outputs.

The RF input to the board is connected

to the input of the first 2-way splitter

that contains R1. R1 is a balancing

resistor in which any RF due to

mismatching in the first splitter will be

dissipated. One of the two outputs from

the splitter connects to another 2-way

splitter that contains R2. R2 is a

balancing resistor in which any RF due

to mismatching in the splitter will be

dissipated. The other output of the first

splitter connects to the third 2-way

splitter that contains R3. R3 is a

balancing resistor in which any RF due

to mismatching in the splitter will be

dissipated. The two output splitters

provide four, equal RF outputs, two

each, that are connected to the inputs

of the external amplifier boards.

4.8.10 (A4-A3, A5-A2, A5-A3, A5-

A4, A5-A5) Dual Stage Amplifier

Assemblies (1265-1413 and 1265-

1439, low band/1265-1411 and

1265-1440, mid band/1265-1420

and 1265-1441, high band;

Appendix D)

4.8.10.1 Low Band

The dual stage UHF amplifier assembly,

Class AB, low band is made using a

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-36

generic dual stage amplifier board,

Class AB (1265-1404). The board uses

two PTB20101 Ericsson transistors in

parallel-biased Class AB to amplify the

signal by approximately +9 dB. Bias

adjust R106 sets the idling current for

Q101 at 300 mA and bias adjust R206

sets the idling current for Q201 at 300

mA. Each dual amplifier device is

mounted in identical, parallel circuits.

These devices may be biased up to 600

mA depending on the linearity of the

tray.

Input Description

The input signal from J1 on the dual

stage amplifier assembly connects to

E1 on the board. The signal is split in a

2-way Wilkinson splitter, using R1,

which provides two equal inputs, one to

each identical amplifier side.

Q101 and Associated Circuitry

One of the outputs of the splitter is

applied through AC coupling and DC

blocking capacitor C101 to L101 and

associated circuitry. This forms a balun

that converts the input signal from a

50-Ω unbalanced impedance to a 12.5-

Ω balanced impedance configuration.

C103, C104, C106, and C105, which

are adjusted for peak output, are for

impedance matching to the input of the

parallel transistors that make up Q101.

The bias voltage to the bases of the

paralleled transistors in Q101 is applied

at E101. The transistors are protected

from overvoltage by Q102, Q103,

R104, R105, and R106, which can be

adjusted to set the bias, operating

currents of the transistors. The base

voltage is RF bypassed by C129, C102,

C107, C108, C109, and C110 and

applied to the bases through R102 and

R103.

The collectors are impedance matched

to 12.5Ω by C122, C123, and C119,

which are adjusted for peak output with

the best linearity and lowest current.

C125 provides AC coupling and DC

blocking for the output signal to the

combiner. L102 and associated circuitry

form a balun that transforms the signal

back to an unbalanced 50-Ω impedance

signal.

The collector voltage is applied at E101.

The collector voltage is connected

through R108 to the collectors on the

two devices that make up Q101. R106

can be adjusted to set up the operating

currents. The collector circuit is RF

bypassed by C112 to C115, C117,

C118, C121, C124, C130, and C131.

Q201 and Associated Circuitry

The other output of the splitter is

applied through AC coupling and DC

blocking capacitor C201 to L201 and

associated circuitry. This forms a balun

that converts the input signal from a

50-Ω unbalanced impedance to a 12.5-

Ω balanced impedance configuration.

C203, C204, C206, and C205, which

are adjusted for peak output, are for

impedance matching to the input of the

parallel transistors (Q201).

The bias voltage to the bases of the

paralleled transistors, Q201, is applied

at E201. The transistors are protected

from overvoltage by Q202, Q203,

R204, R205, and R206, which can be

adjusted to set the bias, operating

currents of the transistors. The base

voltage is RF bypassed by C229, C202,

C207, C208, C209, and C210 and

applied to the bases through R202 and

R203.

The collectors are impedance matched

to 12.5Ω by C222, C223, and C219,

which are adjusted for peak output with

the best linearity and lowest current.

C225 provides AC coupling and DC

blocking for the output signal to the

combiner. L202 and associated circuitry

form a balun that transforms the signal

back to an unbalanced 50-Ω impedance

signal.

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-37

The collector voltage is applied at E201.

The collector voltage is connected

through R208 to the collectors on the

two devices that make up Q201. R206

can be adjusted to set up the operating

currents. The collector circuit is RF

bypassed by C212 to C215, C217,

C218, C221, C224, C230, and C231.

Output Description

The outputs of the two sides are

combined by a 2-way Wilkinson

combiner, using R2, and applied to the

RF output of the board at E2, which is

connected to J2, the SMA output jack

on the dual stage amplifier assembly.

4.8.10.2 Mid Band

The dual stage UHF amplifier assembly,

Class AB, mid band is made using a

generic dual stage amplifier board,

Class AB (1265-1404). The board uses

two PTB20101 Ericsson transistors in

parallel-biased Class AB to amplify the

signal by approximately +9 dB. Bias

adjust R106 sets the idling current for

Q101 at 300 mA and bias adjust R206

sets the idling current for Q201 at 300

mA. Each dual amplifier device is

mounted in identical parallel circuits.

These devices may be biased up to 600

mA, depending on the linearity of the

tray.

Input Description

The input signal from J1 on the dual

stage amplifier assembly connects to

E1 on the board. The signal is split in a

2-way Wilkinson splitter, using R1,

which provides two equal inputs, one to

each identical amplifier side.

Q101 and Associated Circuitry

One of the outputs of the splitter is

applied through an AC coupling and DC

blocking capacitor C101 to L101 and

associated circuitry. This forms a balun

that converts the input signal from a

50-Ω unbalanced impedance to a 12.5-

Ω balanced impedance configuration.

C106 and C105, which are adjusted for

peak output, are for impedance

matching to the input of the parallel

transistors that make up Q101.

The bias voltage to the bases of the

paralleled transistors in Q101, which is

applied at E101. The transistors are

protected from overvoltage by Q102,

Q103, R104, R105, and R106, which

can be adjusted to set the bias,

operating currents of the transistors.

The base voltage is RF bypassed by

C129, C102, C107, C108, C109, and

C110 and applied to the bases through

R102 and R103.

The collectors are impedance matched

to 12.5Ω by C116, C120, C122, C123,

and C119, which are adjusted for peak

output with the best linearity and

lowest current. C125 provides AC

coupling and DC blocking for the output

signal to the combiner. L102 and

associated circuitry form a balun that

transforms the signal back to an

unbalanced 50-Ω impedance signal.

The collector voltage is applied at E101.

The collector voltage is connected

through R108 to the collectors on the

two devices that make up Q101. R106

can be adjusted to set up the operating

currents. The collector circuit is RF

bypassed by C112 to C115, C117,

C118, C121, C124, C130, and C131.

Q201 and Associated Circuitry

The other output of the splitter is

applied through AC coupling and DC

blocking capacitor C201 to L201 and

associated circuitry. This forms a balun

that converts the input signal from a

50-Ω unbalanced impedance to a 12.5-

Ω balanced impedance configuration.

C206 and C205, which can be adjusted

for peak output, are for impedance

matching to the input of the parallel

transistors (Q201).

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-38

The bias voltage to the bases of the

paralleled transistors, Q201, is applied

at E201. The transistors are protected

from overvoltage by Q202, Q203,

R204, R205, and R206, which can be

adjusted to set the bias, operating

currents of the transistors. The base

voltage is RF bypassed by C229, C202,

C207, C208, C209, and C210 and

applied to the bases through R202 and

R203.

The collectors are impedance matched

to 12.5Ω by C216, C220, C222, C223,

and C219, which are adjusted for peak

output with the best linearity and

lowest current. C225 provides AC

coupling and DC blocking for the output

signal to the combiner. L202 and

associated circuitry form a balun that

transforms the signal back to an

unbalanced 50-Ω impedance signal.

The collector voltage is applied at E201.

The collector voltage is connected

through R208 to the collectors on the

two devices that make up Q201. R206

can be adjusted to set up the operating

currents. The collector circuit is RF

bypassed by C212 to C215, C217,

C218, C221, C224, C230, and C231.

Output Description

The outputs of the two sides are

combined by a 2-way Wilkinson

combiner, using R2, and applied to the

RF output of the board at E2, which is

connected to J2, the SMA output jack

on the dual stage amplifier assembly.

4.8.10.3 High Band

The dual stage UHF amplifier assembly,

Class AB, high band is made using a

generic dual stage amplifier board,

Class AB (1265-1404). The board uses

two PTB20101 Ericsson transistors in

parallel-biased Class AB to amplify the

signal by approximately +9 dB. Bias

adjust R106 sets the idling current for

Q101 at 300 mA and bias adjust R206

sets the idling current for Q201 at 300

mA. Each dual amplifier device is

mounted in identical, parallel circuits.

These devices may be biased up to 600

mA depending on the linearity of the

tray.

Input Description

The input signal from J1 on the dual

stage amplifier assembly connects to

E1 on the board, which is split in a 2-

way Wilkinson splitter, using R1, and

provides two equal inputs, one to each

identical amplifier side.

Q101 and Associated Circuitry

One of the outputs of the splitter is

applied through AC coupling and DC

blocking capacitor C101 to L101 and

associated circuitry. This forms a balun

that converts the input signal from a

50-Ω unbalanced impedance to a 12.5-

Ω balanced impedance configuration.

C106 and C105, which are adjusted for

peak output, are for impedance

matching to the input of the parallel

transistors that make up Q101.

The bias voltage to the bases of the

paralleled transistors in Q101 is applied

at E101. The transistors are protected

from overvoltage by Q102, Q103,

R104, R105, and R106, which can be

adjusted to set the bias, static currents

with no RF drive applied, of the

transistors. The base voltage is RF

bypassed by C129, C102, C107, C108,

C109, and C110 and applied to the

bases through R102 and R103.

The collectors are impedance matched

to 12.5Ω by C116, C120, C123, and

C119, which are adjusted for peak

output with the best linearity and

lowest current. C125 provides AC

coupling and DC blocking for the output

signal to the combiner. L102 and

associated circuitry form a balun that

transforms the signal back to an

unbalanced 50-Ω impedance signal.

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-39

The collector voltage is applied at E101.

The collector voltage is connected

through R108 to the collectors on the

two devices that make up Q101. R106

can be adjusted to set up the operating

currents. The collector circuit is RF

bypassed by C112 to C115, C117,

C118, C121, C124, C130, and C131.

Q201 and Associated Circuitry

The other output of the splitter is

applied through AC coupling and DC

blocking capacitor C201 to L201 and

associated circuitry. This forms a balun

that converts the input signal from a

50-Ω unbalanced impedance to a 12.5-

Ω balanced impedance configuration.

C206 and C205, which are adjusted for

peak output, are for impedance

matching to the input of the parallel

transistors (Q201).

The bias voltage to the bases of the

paralleled transistors, Q201, is applied

at E201. The transistors are protected

from overvoltage by Q202, Q203,

R204, R205, and R206, which can be

adjusted to set the bias, operating

currents of the transistors. The base

voltage is RF bypassed by C229, C202,

C207, C208, C209, and C210 and

applied to the bases through R202 and

R203.

The collectors are impedance matched

to 12.5Ω by C216, C220, C223, and

C219, which are adjusted for peak

output with the best linearity and

lowest current. C225 provides AC

coupling and DC blocking for the output

signal to the combiner. L202 and

associated circuitry form a balun that

transforms the signal back to an

unbalanced 50-Ω impedance signal.

The collector voltage is applied at E201.

The collector voltage is connected

through R208 to the collectors on the

two devices that make up Q201. R206

can be adjusted to set up the static

currents, with no RF applied. The

collector circuit is RF bypassed by C212

to C215, C217, C218, C221, C224,

C230, and C231.

Output Description

The outputs of the two sides are

combined by a 2-way Wilkinson

combiner, using R2, and applied to the

RF output of the board at E2, which is

connected to J2, the SMA output jack

on the dual stage amplifier assembly.

4.8.11 (A7) Amplifier Protection

Board (1265-1412; Appendix D)

The amplifier protection board

distributes the biasing voltages to the

transistor amplifier devices that are

mounted on the amplifier boards in the

UHF amplifier tray. It also protects the

transistor devices from overcurrent

conditions using the board-mounted 7-

amp fuses F1 to F12. F13 is a board-

mounted 3-amp fuse that protects the

+26.5 VDC that is applied to the

amplifier control board and is needed

for the operation of the board. F14 and

F15 are 7-amp spare fuses.

The +26.5 VDC from the switching

power supply enters the board at TB1,

with the plus (+) connections sent to

pins 1 to 4 and the minus (-)

connections sent to pins 5 and 6. The

+26.5 VDC is connected across the .01-

W/3-Ω voltage-dropping resistors R14

to R26 that are used to set up the

idling currents for the transistor

devices; the fuses F1 to F13 that

protect the transistor devices during an

overcurrent condition through the

outputs of the board at TB2, TB3; and

also to output jack J1. Table 4-1

indicates the fuse, the amplifier device

it protects, and the idling current

settings for the class AB amplifier

devices.

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-40

Table 4-1: Fuses, Idling Currents, and Voltage Settings for the Class AB Amplifier

Devices

SWITCH

POSITION AMPLIFIER

MODULE TRANSISTOR

DEVICE

BIAS

ADJUST

POT

IDLING

CURRENT VOLTAGE

SETTING FUSE

I1 A4-A1 Q1 R6 *5 Amps 50 mV F1

I2 A4-A3 Q201 R206 600 mA 6.0 mV F2

I3 A4-A3 Q101 R106 600 mA 6.0 mV F3

I4 A5-A2 Q201 R206 300 mA 3.0 mV F4

I5 A5-A2 Q101 R106 300 mA 3.0 mV F5

I6 A5-A3 Q201 R206 300 mA 3.0 mV F6

I7 A5-A3 Q101 R106 300 mA 3.0 mV F7

I8 A5-A4 Q201 R206 300 mA 3.0 mV F8

I9 A5-A4 Q101 R106 300 mA 3.0 mV F9

I10 A5-A5 Q201 R206 300 mA 3.0 mV F10

I11 A5-A5 Q101 R106 300 mA 3.0 mV F11

*The A4-A1 transistor Q1 operates class A and is adjusted for 5 amps of operating

current.

The voltage drop across the selected

resistor that uses switch S1 is read with

a digital voltmeter (DVM) that is

connected from E1 to E2 on the board.

This voltage reading converts to the

idling current, with no RF drive applied,

or the operating current, with RF drive

applied, of the transistor.

4.8.12 (A8) Amplifier Control

Board (1265-1414; Appendix D)

The amplifier control board provides

LED fault and enable indications on the

front panel of the tray and also

performs the following functions:

automatic gain control (AGC);

overdrive cutback, when the drive level

reaches the amount needed to attain

110% output power; and

overtemperature, VSWR, and overdrive

faults. The board also provides

connections to the front panel meter for

monitoring the AGC, % Reflected

Power, % Output Power, and the power

supply voltage.

A –12-VDC enable is applied to the

board at J4-1 from the transmitter

control board in the UHF exciter tray.

The enable causes the J-FET Q5 to be

biased off, making the drain go high;

the high is applied to Q4, which is

biased on. The drain of Q4 goes low

and lights the green Enable LED DS4

on the front panel. The high at the

drain of Q5 is also applied through CR5

to J4, pin 7, which is a high enable, to

the +26-VDC switching power supply,

switching it on.

If there is an overtemperature fault,

which is a low applied to J4, pin 3, the

low connects through CR4 and

overrides the high enable, switching off

the switching power supply. As long as

the fault is present, the switching

power supply is off and the red

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-41

Overtemperature LED DS3 on the front

panel is lit.

4.8.12.1 Phase Control

The phase control pot on the front of

the tray connects to J8-1 (+26 VDC),

J8-3 (phase I/P), and J8-4 (RTN) on

the board. The phase-control output

connects from J7 on the board to the

input of the phase-control circuit on the

variable gain/phase board (1265-

1425).

4.8.12.2 Automatic and Manual Gain

Control Circuits

The amplifier control board contains the

AGC function for the UHF amplifier tray

in which it is mounted. An AGC

reference-level input from the UHF

exciter tray is applied to J3, pin 1, and

is amplified by U3B. The output of U3B

is connected to J5, which is wired to

the front panel gain pot that sets the

output power level of the tray when the

AGC is in the Auto position. The voltage

at the arm of the front panel gain pot is

amplified by U2D and is compared to a

sample of the output power of the tray

in U2A. The error voltage from U2A is

sent through Auto/Manual switch S1 to

J10, which connects to the pin-diode

attenuator circuit on the variable

gain/phase board. A sample of the AGC

voltage level is connected to position 1

on the front panel meter switch. The

tray can also be operated in manual

gain by switching S1 to the Manual

position and adjusting R16 for the

desired output power level.

FETs Q1 and Q3 delay and slowly

reapply the AGC voltage to the variable

gain/phase board when the system is

switched on or when the board is

switched from Auto to Manual (or back)

to prevent the overdriving of the tray.

4.8.12.3 Overdrive Circuit

A sample of the output of the single

stage amplifier assembly, class A, from

the coupler board assembly connects to

J11 on the amplifier control board. The

sample is peak detected by CR7 and

U3A and the output is connected to

U3D. If the input drive level increases

above the overdrive threshold

reference set by R71, which is the drive

level needed to produce 110% output

power, the output of U3D goes high

and is split three ways. One of the

highs is connected through R38 and

CR3 to U3C, causing its output to go

high and lighting the red Overdrive LED

DS2 on the front panel. Another of the

highs is connected through R74 and

R75 to Q6, which is biased on, causing

its output to go low. The low is

connected through J10 to the variable

gain/phase board and its output power

is cut back.

The final high from U3D is connected

through CR2 and R37 to U1D, which is

biased on, causing its output to go

high. The high is connected to U2A,

whose output decreases and its output

power is cut back. If this path is not

present, the AGC, because the forward

power decreases, will try to drive the

variable gain/phase board harder,

creating a positive feedback loop that

could damage the amplifier tray.

4.8.12.4 Metering Circuits

The +26 VDC that is connected to the

board from the switching power supply

is applied to jack J6, pin 1, of the board

and connected through R63, R96, R66,

R65, and R76 to the front panel meter

for monitoring. R65 can be adjusted to

calibrate the voltage reading to +26

VDC on the front panel meter, bottom

scale. This calibration was completed at

the factory and should not need to be

adjusted at this time.

A forward power sample of the output

of the tray is applied to jack J1-1 and

J1-2 of the board from the dual peak

detector board, single supply. The

forward power sample is connected

through R1 and R2 to U1A, a buffer

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-42

amplifier. The output of U1A is split,

with one part going to the AGC circuits,

another sample connected to J1-5 for

remote metering, and the final sample

applied to the meter at position 3 on

S2, the front panel meter switch. R2

can be adjusted to calibrate the %

Forward Power indication on the front

panel meter.

A sample of the reflected power output

of the tray is applied to jack J2-1 and

J2-2 of the board from the dual peak

detector board, single supply. The

reflected power sample is connected

through R21 and R22 to U1B, a buffer

amplifier. The output of U1B is split,

with one part going to the VSWR

threshold circuit, another sample

connected to J2-5 for remote metering,

and the final sample applied to the

meter at position 2 on S2, the front

panel meter switch. R22 can be

adjusted to calibrate the % Reflected

Power indication on the front panel

meter.

A sample of the AGC voltage level at

R20 on the board is connected through

a divider network consisting of R87,

R88, R19, and R18 to the meter at

position 1 of S2, the Front Panel Meter

switch.

4.8.12.5 Operational Voltages

The voltage input to the board is +26.5

VDC from the switching power supply.

The +26.5 VDC connects to the board

at J6-1 and is wired to U4, which is a 3-

terminal regulator IC that takes the

+26 VDC input and produces the +12

VDC needed for the operation of the

board.

The +12 VDC from U4 is connected

through CR6 to U5, a +5-VDC

regulator, that takes the +12 VDC

input from U4, or the transmitter

control board in the UHF exciter tray,

through J6, pin 3, and produces a +5-

VDC output that is applied to the rest of

the board. This +12 VDC is connected

to U5, a regulator IC, which produces a

+5 VDC output that is applied to the

Enable and Overtemperature LEDs that

operate even when the +26.5 VDC

input to the board from the switching

power supply is removed.

4.8.13 (A2) Variable Gain/Phase

Board Enclosure (1265-1426;

Appendix D)

The variable gain/phase enclosure

assembly provides EMI and RFI

protection for the variable gain/phase

board (1265-1425) that is mounted

inside of the assembly.

The RF input to the assembly is at SMA

jack J1 and the RF output is at SMA

jack J2. There are two control inputs

that connect to the assembly: the

attenuator bias input and the phase

control input. The attenuator bias input

from the amplifier control board

connects to FL4 and FL5 on the

assembly and is wired to J5 on the

board. The phase control input from the

amplifier control board connects to FL2

and E1 on the assembly and is wired to

J4 on the board. The input fault to the

amplifier control board connects to FL3

on the assembly and is wired to J6-1 on

the board. E1 on the assembly

connects to J3-4 on the board.

The +26 VDC needed to operate the

board connects to FL1 on the assembly

which is wired to J3-3 on the board.

4.8.14 (A2-A1) Variable Gain/

Phase Board (1265-1425;

Appendix D)

The variable gain/phase board provides

the circuits that adjust the phase and

the gain of the RF signal for the

amplifier tray in which it is mounted.

The RF input signal at J1 is split, with

one output connected to a detector

circuit consisting of C8, CR4, and U3A.

This detected level is then applied to

comparator U3D, which provides a high

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-43

output when the input signal level

drops below a threshold set by R16,

R17, and CR5. This high is applied to

the red Input Fault LED DS1, which

lights to indicate an input fault. DS1

can be seen through the hole in the lid

on the variable gain/phase assembly.

The high is also connected to the gate

of Q1, which biases it on and causes its

drain to go low. The low is applied to

the pin-diode attenuator circuit

consisting of CR1, CR2, and CR3. The

low to CR3 decreases the current

through it and increases its resistance,

decreasing or completely shutting off

the RF that flows through it.

The other output of the RF input signal

from J1 is connected through C1 to a

voltage-controlled, pin-diode attenuator

circuit consisting of diodes CR1, CR2,

and CR3. The diodes are pin diodes in a

pi-type configuration whose resistance

varies inversely with the DC current

flow through them.

As the AGC voltage, attenuator bias,

applied to J5 increases, CR3 is forward

biased even more. This increases the

current flow through it by decreasing

its resistance; the RF signal that flows

through it increases in level. CR1 and

CR2 have less current through them;

this raises their resistance, causing the

RF signal that is applied to them to

decrease in level.

The three diodes form a pi-type

attenuator whose attenuation

decreases with the increasing AGC

voltage. U4 provides amplification,

approximately 8 dB, of the RF signal

before it is connected to the phase-

shifter circuit. The phase-shifter circuit

consists of L1, C16, C17, CR7, and

CR8. L1 is a 90°, 2-way splitter. The

signal at pin 1 of L1 is split and applied

to pins 2 and 4. The signal reflects off

CR7 and CR8 and is passed to pin 3.

The phase shift between pins 1 and 3

changes with the voltage applied across

CR7 and CR8. This voltage is controlled

by an external phase-adjust pot that

connects to J4. The +26 VDC from the

external switching power supply is used

as the reference that is applied to the

phase-control pot. The IC U2 provides

approximately 10 dB of gain at the

output of the phase-shifter circuit that

connects to two class A amplifier

stages, Q2 and Q3, with a total gain of

approximately 20 dB.

The first amplifier stage, Q2, is biased

at a collector current of approximately

100 mA. This current is set by R29,

R30, VR1, and Q2. VR1 forces the

voltage at the collector to stay at 8.9

VDC. This biases on Q2 and draws

enough current through R29 and R30

to keep the collector voltage at 8.9

VDC. The amplified output connects to

the second amplifier Q3. The bias

circuit for Q3 works in a manner similar

to the bias circuit for Q2. VR2 and VR3

maintain a collector voltage of 21 VDC,

while R36 and R37 limit the collector

current to 650 mA.

The output connects to J2 on the

board. A sample of the output is

detected by CR10 and connected to

TP4. A DVM can be connected to TP4 to

give a voltage indication of the RF

output level.

The +26 VDC connects to the board at

J3-3 and is split, with one half

connected to the two, class A amplifier

circuits. The other half of the +26-VDC

input is filtered, isolated by L4 and C13,

and connected to U1. U1 is a +12-VDC

regulator IC that produces the +12

VDC needed to operate the ICs on the

board.

4.8.15 (A5-A6) 4-Way Combiner

Assembly (1265-1428 [Channels

14 to 26], 1265-1429 [Channels

27 to 43], and 1265-1430

[Channels 44 to 69]; Appendix D)

The 4-way combiner assembly contains

a 4-way combiner board, low band. The

4-way combiner board is made up of

three, 2-way Wilkinson stripline

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-44

combiners. Two of the RF inputs to the

board are soldered directly to the

inputs for the 2-way combiner that

contains R5. R5 is a balancing resistor

in which any RF due to mismatching in

the combiner will be dissipated. The

other two RF inputs are soldered

directly to the 2-way combiner that

contains R6. R6 is a balancing resistor

in which any RF due to mismatching in

the combiner will be dissipated. The

outputs of the two, input 2-way

combiners connect to the third 2-way

combiner that contains R7. R7 is a

balancing resistor in which any RF due

to mismatching in the combiner will be

dissipated. The output of the third

combiner is connected to the output of

the 4-way combiner board that is

cabled to an external circulator.

A directional coupler is built into the RF

output circuit on the 4-way combiner

board. The coupler provides a forward

power sample at J1 that is -40 dB down

from the level of the RF output signal.

A reject power sample from the

circulator is fed to reject sample input

jack J2 on the 4-way combiner board. A

directional coupler is built into the

reject sample circuit that provides J3

with a signal that is -40 dB down from

the level of the reject power sample.

NOTE: Your Transmitter will contain

either a 1278-1200 Single Phase or a

1278-1100 Three Phase AC Distribution

Assembly.

4.9 (A10) AC Distribution

Assembly, Single Phase (1278-

1200; Appendix C)

The AC Distribution Assembly, Single

Phase, Amplifier Assembly (1278-1200)

distributes the Main AC Input, 208/240

VAC @ 100 Amps, to the individual

Trays in the Amplifier Cabinet which

make up the Amplifier Array.

The 208/240 VAC Single Phase Main AC

Input to the Amplifier Cabinet connects

to the Terminal Block TB1, Line 1 to

TB1-1A, Line 2 to TB1-3A and Safety

Ground to TB1-4A.

The 208/240 VAC Main AC Input is

wired from the Terminal Block TB1-1B

(Line 1) to the 100 Amp Circuit Breaker

CB1-1 and TB1-3B (Line 2) to the

Circuit Breaker CB1-5. The switched

Main AC Input is wired from the main

circuit breaker to the up to six 20 Amp

Circuit Breakers, CB2-CB7, that

distribute the AC to the up to six UHF

Amplifier Trays in the Amplifier Array

and two 3A circuit breakers CB8 and

CB9 which connect the AC to the Fan

(A12-A3) mounted on the Reject Load

Assembly. The number of 20 Amp

circuit breakers is determined by the

number of Amplifier Trays which is

determined by the output power of the

Transmitter in which the Amplifier

Assemblies are used. Four circuit

breakers, CB2-CB5, are needed for a 4

kW Transmitter, five circuit breakers,

CB2-CB6, are needed for a 5 kW

Transmitter and six circuit breakers,

CB2-CB7, are needed for a 6 kW

Transmitter. Three MOVs, for surge

and transient protection, are connected

to the AC Lines from the Circuit

Breaker. VR4 and VR6 connect from

the AC Lines to Ground and VR3

connects across the AC Lines.

CB2 is a 20 Amp Circuit Breaker which

protects the AC connected through J1

on the AC Distribution Assembly to J4

on the (A1) UHF Amplifier Tray. CB3 is

a 20 Amp Circuit Breaker which

protects the AC connected through J2

on the AC Distribution Assembly to J4

on the (A2) UHF Amplifier Tray. CB4 is

a 20 Amp Circuit Breaker which

protects the AC connected through J3

on the AC Distribution Assembly to J4

on the (A3) UHF Amplifier Tray. CB5 is

a 20 Amp Circuit Breaker which

protects the AC connected through J4

on the AC Distribution Assembly to J4

on the (A4) UHF Amplifier Tray. CB6

Digital UHF Transmitter Chapter 4, Circuit Descriptions

DT835A, Rev. 1 4-45

and CB7 are 20 Amp Circuit Breakers

that are added to the AC Distribution

Assembly when the two extra UHF

Amplifier Trays are needed for the 5

kW and 6 kW Transmitters. CB6 is a

20 Amp Circuit Breaker which protects

the AC connected through J5 on the AC

Distribution Assembly to J4 on the (A5)

UHF Amplifier Tray. CB7 is a 20 Amp

Circuit Breaker which protects the AC

connected through J6 on the AC

Distribution Assembly to J4 on the (A6)

UHF Amplifier Tray.

4.10 (A10) AC Distribution

Assembly, Three Phase (1278-

1100; Appendix C)

The AC Distribution Assembly, 3 Phase

AC Input distributes the Main AC Input,

208/240 VAC 3 Phase @ 55 Amps, that

connects to the individual Trays in the

Amplifier Cabinet which make up the

Amplifier Array.

The 208/240 VAC 3 Phase Main AC

Input to the Amplifier Cabinet connects

to the Terminal Block TB1, Line 1 to

TB1-1A, Line 2 to TB1-2A, Line 3 to

TB1-3A and Safety Ground to TB1-4A.

The 208/240 VAC Main AC Input is

wired from the Terminal Block TB1-1B

(Line 1) to the 55 Amp Circuit Breaker

CB1-1, TB1-2B (Line 2) to the Circuit

Breaker CB1-3, TB1-3B (Line 3) to the

Circuit Breaker CB1-5 and TB1-4B

(Safety Ground) to Chassis Ground #1.

The switched Main AC Input is wired

from the Circuit Breaker to the up to six

20 Amp Circuit Breakers, CB2-CB7,

that distribute the AC to the up to six

UHF Amplifier Trays in the Amplifier

Array. The number of circuit breakers

is determined by the number of

Amplifier Trays which is determined by

the output power of the Transmitter in

which the Amplifier Assemblies are

used. Six MOVs, for surge and

transient protection, are connected to

the AC Lines from the Circuit Breaker,

VR4, VR5 and VR6 from the AC Lines to

Ground and VR1, VR2 and VR3 across

the AC Lines.

CB2 is a 20 Amp Circuit Breaker which

protects the AC connected through J1

on the AC Distribution Assembly to J4

on the (A1) UHF Amplifier Tray. CB3 is

a 20 Amp Circuit Breaker which

protects the AC connected through J2

on the AC Distribution Assembly to J4

on the (A2) UHF Amplifier Tray. CB4 is

a 20 Amp Circuit Breaker which

protects the AC connected through J3

on the AC Distribution Assembly to J4

on the (A3) UHF Amplifier Tray. CB5 is

a 20 Amp Circuit Breaker which

protects the AC connected through J4

on the AC Distribution Assembly to J4

on the (A4) UHF Amplifier Tray. CB6

and CB7 are 20 Amp Circuit Breakers

that are added to the AC Distribution

Assembly when the two extra UHF

Amplifier Trays are needed. CB6 is a

20 Amp Circuit Breaker which protects

the AC connected through J5 on the AC

Distribution Assembly to J4 on the (A5)

UHF Amplifier Tray. CB7 is a 20 Amp

Circuit Breaker which protects the AC

connected through J6 on the AC

Distribution Assembly to J4 on the (A6)

UHF Amplifier Tray.

4.11 (A12) Combiner Reject Load

Assembly (1278-1312)

NOTE: The Combiner Reject Load

Assembly is only used in the 2.5 and 3

kW Amplifier Arrays.

The Reject Load Assembly provides

connections to 6 loads to which the

reject outputs of the 6 Way Combiner

Board are cabled. The Assembly

consists of (A2) a Combiner Reject

Load Board (1278-1311), mounted on

a heatsink, two Thermal Switches (A1

& A4) and two Fans (A3 & A5).

If a miss-match due to a malfunction or

UBS Axcera DT835A-6 3000-Watt UHF DTV Transmitter User Manual title page DT835A

UBS-Axcera 3000-Watt UHF DTV Transmitter title page DT835A

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