M/A-Com 19D902797G5 Maintenance Manual

Maintenance Manual

LBI-39127H

851-870 MHz, 110 WATT POWER AMPLIFIER

19D902797G5, Revision A only

TABLE OF CONTENTS

Page

DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Front Page

SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

CIRCUIT ANALYSIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

POWER AMPLIFIER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Small Signal Gain Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Driver Module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Final Power Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Power Sense and Isolation Stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

POWER CONTROL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Theory of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Signal Inte rface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

BLOCK DIAGRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

TROUBLESHOOTING GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

POWER AMPLIFIER READINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

IC DA TA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

ASSEMBLY DIAGRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

PARTS LIST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

OUTLINE DIAGRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

ASSEMBLY DIAGRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

SCHEMATIC DIAGRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

PARTS LIST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Back Cover

PRODUCTION CHANGES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Back Cover

SYMBOL PART NO. DESCRIPTION

4 SBS 123 01/10 Spring nut.
5 19A702339P510 Screw, thread forming,

flat head.

6 19A701312P5 Washer, plain steel, 3.5

mm.

7 19A702381P510 Screw, thread forming,

pan head.
9 19A700136P19 Sleeve
10 105 8567/1 Guard, fan.
11 19A701863P12 Clamp, loop.
13 19A700033P6 Washer, lock, ext tooth.
14 19A700034P5 Nut, hex, steel.
B1

and
B2

BKV 301 216/02 DC fan.

W1 344A3337P4 Cable.
WT1

and
WT2

7142645P1 Conductor, splice.

PRODUCTION CHANGES

Changes in the equipment to improve or to simplify circuits are identi­fied by a "Revision Letter", which is stamped after the model number
of the unit. The revision stamped on the unit includes all previous
revisions. Refer to the Parts List for descriptions of parts affected by
these revisions.

REV. A -

POWER AMPLIFIER 19D902797G5

Incorporated into production radios.

PA FAN PLATE ASSEMBLY

188D6127G1

DESCRIPTION

The 800 MHz MASTR III Power Amplifier Assem-bly is
a wide band RF power amplifier operating over the 851-870
MHz range without tuning. Its main function is to amplify the
10mW FM signal from the Transmitter Synthesizer to the
rated RF output at the antenna port. The output of the Power
Amplifier Assembly is adjust-able from rated power to 10dB
below rated power at the PA output J104.

The assembly consists of a printed wiring board (A1) and
associated components, including a power module and an RF

power transistor, mounted to the heat sink assembly. The
printed wiring board (A1) contains both the power amplifier
circuitry and the power control cir-cuitry. The heatsink as­sembly includes a copper heat spreader for the power transis­tor.

Unfiltered supply voltage, A+, for the power ampli-fier
circuits enters the assembly via feedthrough capaci-tor, C1.
Power cable W4 routes the A+ from C1 to J103 on the PWB.
Filtered A+ voltage for the power control circuit enters the
assembly via control cable W13 which connects to the PWB
at J201.

M/A-COM Wireless Systems

3315 Old Forest Road
Lynchburg, Virginia 24501
(Outside USA, 434-385-2400) Toll Free 800-528-7711
www.macom-wireless.com Printed in U.S.A.

The Power Control circuitry sets the output power level by
adjusting the PA Power Set level. It keeps the output power
constant despite variations in input power, power amplif ier gain,
or temperature through the use of a feedback control loop in the
PA a ssem bly.

CIRCUIT ANALYSIS

POWER AMPLIFIER

The power amplifier section of the PA Board consists of a
Small Signal Gain Stage, a Driver Module, a Final Power Stage,
and Power Sense and Isolation Stages. All these gain stages
have an input and output impedance of 50 ohms. Figure 1 is a
block diagram showing the signal flow within the Power Am­plifier Assembly.

Small Signal Gain Stage (U101)

This stage uses a broadband silicon monolithic microwave
integrated circuit (MMIC) amplifier. The signal from transmit­ter synthesizer, typically 10 dBm (10 mW), is input through a
10 dB resistive pad (R101, R102, and R103). The stage ampli­fies the resulting 0 dBm (1 mW) signal to the necessary level to
achieve desired PA output power (typical +5 dBm for 110W
out).

Bias for the MMIC is supplied by an 8V regulator (U4).
This voltage is DC coupled to pin 6 and is supplied through a
dropping resistor, R43, for pin 2. Power control for the Power
Amplifier Assembly is performed by controlling the RF level
out of the MMIC by varying the control voltage at pin 5.

Driver Module (U2)

The Driver Module is a 35 dB gain, 4-stage, linear hybrid
amplifier. The first two stages of the module are identical FET
amplifiers, class A biased to provide maximum gain. Stages 3
and 4 are bipolar transistors, class AB biased to provide maxi­mum saturated power and efficiency. The quiescent current of
these stages is set by variable resistors R1, R2, R4, and R3
respectively. Typical bias current for each stage is 100, 90, 90
& 90mA respectively.

The voltage for the bias circuitry is supplied by U7, a voltage
regulator. A transistor switch, Q3, is used to apply this voltage
when the unit is keyed. The nominal output power of this stage
is 42.4 dBm (17.5W).

Final Power Stage (Q5)

The Final Power Stage of the unit consists of a push-pull
pair of silicon bipolar power transistors mounted on a single
flange, Q5, and its associated matching and bias circuitry. A
quarter-wave transmission line transformer feeds a pair of 25
ohm lines in parallel, one of which is 180° longer than the
other, to create a microstrip balun. The resulting balanced
circuit is transformed by reactive components to match the
input impedance of the final device. Similar circuitry is used
on the output to match the device’s output impedance. The
quiescent current of transistor Q5 is set by variable resistor
R48 for Class AB operation. Typical bias current is 400 mA
for this stage.

The result is a typical 8.4 dB gain stage capable of
producing a nominal 50.8 dBm (120W) of RF power.

Power Sense and Isolation Stages

Integrated into the output microstrip balun, an eighth­wave directional coupler is used to sense the magnitude of
forward power. D6, an active detector, is used to convert this
level to a DC voltage proportional to forward power, and this
voltage is sent to the power control circuitry.

The amplified signal from the Final Power Stage is fed to
U6, a circulator, to provide 20 dB typical isolation from load
mismatches. The device insertion loss of 0.2 dB maximum,
along with the microstrip trace losses, provide 50.4 dBm
(110W) of RF power at the PA output, J104.

The third port of the circulator is terminated by a 20 dB
power attenuator, AT1, followed by a resistive pad, R35, R36,
R39. Any signal reflected back into the Power Amplifier
Assembly is directed by the circulator through the pads, and
is converted to a DC voltage by D7. This voltage is sent to
the power control circuitry.

POWER CONTROL

The Power Control circuitry performs four basic func­tions. It keys and unkeys the PA, sets the PA output power,
protects the PA against adverse conditions, and provides a
voltage proportional to output power at the interface cable.

Keying And Unkeying The PA

To key the PA, the digital controller places 5 volts on the
PA key line, J201-2. Zero volts on the PA key line causes the
PA to unkey. If the control cable (W13) is disconnected, with
nothing actively driving the PA key line, the PA will remain
unkeyed.

Copyright © 1994-2002, M/A-COM Private Radio Systems, Inc. All rights res e rv ed.

Figure 1 - Block Diagram

ITEM SPECIFICATION

FREQUENCY 851-870 MHz
OUTPUT POWER (RF) 11 to 110 W @ J104
INPUT POWER (RF) 10 mW min. into

<2:1 VSWR

TEMPERATURE RANGE

-30°C to +60°C (Ambient air)
SUPPLY V OLTA GE 26.0V
CURRENT 14A max. (11A typical @ 110W, 26.0 V)
DUTY CYCLE Continuous
STABILITY Stable into 3:1 VSWR; all temp., voltage,

FREQ. 11-110W

RUGGEDNESS AT HIGH VSWR No damage into open or shorted load

Table 1 - General Specifications

This manual is published by

M/A-COM Private Radio Systems, Inc.

, without any warranty. Improvements and changes to this manual necessitated by typographical errors, inaccuracies of current information, or

improvements to programs and/or equipment, may be made by

M/A-COM Private Radio Systems, Inc.

, at any time and without notice. Such changes will be incorporated into new editions of this manual. No part of this

manual may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying and recording, for any purpose, without the express written permission of

M/A-COM Private

Radio Systems, Inc.

These currents are factory set. Field adjustment is not
recommended, and may result in device failure.

CAUTION

These currents are factory set. Field adjustment is not
recommended, and may result in device failure.

CAUTION

LBI-39127H

1

PA Output Power Set

PA output power is set according to the level of the Power
Set line. Four (4) volts on this line will produce minimum
power. As the voltage increases toward eight (8) volts, the
power will increase to the maximum rated output. The PA
output power is initially set for an output of 110 watts at J104.
This is done by adjusting R204 while injecting a 10 mW
signal at J1 and applying 8 volts to J201-3. After setting the
maximum power level, changing the output power is done by
varying the voltage applied on the Power Set line.

PA Protection

The power control also protects the PA against over tem­perature and high VSWR conditions.

An over temperature condition exists when the flange
temperature of the final output transistor reaches 80°C. At
this point the output power will drop below its set level. The
output power will continue to drop such that when the flange
temperature reaches 125°C the PA output drops at least 10 dB
below its set level.

Reflected power is limited to 25% of the set power . If the
output VSWR degrades to worse than 3:1 the forward power
will be reduced to limit the reflected power to 25% of the set
power. The Power Sensor line indicates when the PA is
operating in a cutback condition. If the PA is keyed and the
power control is cutting back, the Power Sensor line will drop
to zero (0) volts and the PA alarm light on the station will turn
on.

Power Monitor

A DC voltage proportional to forward power, provided by
the detection circuit of the Power Sense stage, is buffered and
delivered to the PWR Monitor line of the interface connector.

Theory Of Operation

Power control of the MASTR III Power Amplifier is
accomplished with a feedback control loop. The three possi­ble feedback signals are: representation of forward power,
temperature sensitive scaled representation of forward power,
or representation of reflected power. The three signals are
input to a diode summing junction which selects the largest
of the three for use as the feedback.

The microstrip directional coupler samples the output
power and produces a voltage, Vf, proportional to the forward
output power. The power control compares the forward volt­age, Vf, to a reference voltage at U3. The output of U3 adjusts
the control voltage at pin 5 of the MMIC of the Small Signal
Gain Stage. This varies the gain through the stage, and

controls the power output level of the Power Amplifier As­sembly.

During over temperature operation, a scaled repre­sentation of the forward power is maintained constant by
varying the control voltage line. Thermal resistor RT1 sens­ing an increase in temperature causes the output of U1.1 to
increase. If the output of U1.1 becomes larger than the other
feedback lines, the output of U3.2 will begin to decrease.
This causes the gain of U101 to decrease. Since the scaling
is a function of temperature the power is reduced as the
temperature increases.

Under VSWR cutback operation the reverse voltage, Vr,
representative of the reflected output power is held below a
threshold by reducing the control voltage as necessary. If Vr
increases at U1.2 beyond the preset threshold an increase at
U3.2 will result. This causes a subsequent reduction in the
control voltage to U1. Thus the power control circuit reduces
the output power in order to limit the reflected power to 25%
of the set power.

Signal Interface

The signal interface to the MASTR III Power Amplifier
is supported by a six position feedthrough connector, J201,
with the following pinout:

1 - PWR Sensor

2 - PA Key

3 - PA PWR Set

4 - PWR Monitor

5 - Ground

6 - Fil A+

Pwr Sensor

This line indicates when the PA is experiencing adverse
conditions. Under normal operation, while the PA is keyed,
this line will be proportional to forward power. Minimum
power (zero watts) corresponds to 2.5 volts while maximum
power corresponds to 4.5 volts. This voltage is not tempera­ture compensated and no effort is made to calibrate this signal
to an absolute power level. It is intended to provide a relative
indication of forward power and to discriminate between
normal and cutback operation.

Zero volts on this line, when the PA is keyed, indicates the
forward power is cutback. This power cutback may be due
to high reflected power or may be due to high PA tempera-

tures. This fault condition may indicate a problem with the PA
or may indicate a system problem external to the Power Ampli­fier. High VSWR may be due to a poor antenna and high
temperature may be due to a blocked cabinet vent. Zero volts
on this line, when the PA is keyed, does not indicate zero
forward power. Zero volts indicates the PA is protecting itself
due to adverse conditions. If the adverse condition, either high
VSWR or high temperature is eliminated, the power will return
to normal and the PWR SENSOR voltage will rise above 2.5
volts.

PA Key (Interface Connector Pin 2)

This line is used to key and unkey the PA, UNKEY=0 volt
and KEY=5 volts. The driver of this line must be capable of
supplying 5 volts at 1.0 mA. The appropriate key sequence
requires RF from the transmit synthesizer be input to the PA
before the KEY line is energized.

PA PWR Set (Interface Connector Pin 3)

This line is used to set the RF Power Output of the PA.
Minimum power output equals 4 volts and maximum power
output equals 8 volts. The driver of this line must be capable of
supplying 8 volts at 1.0 mA.

Fil A+ (Interfaces Connector Pin 6)

This line provides the filtered supply voltage for the Power
Control. The driver of this line must be capable of supplying

13.4 volts ±20% at 100 mA.

PWR Monitor

This line provides a DC voltage proportional to forward
power.

LBI-39127H

2