M/A-Com 19D902797G1 Maintenance Manual
Maintenance Manual
LBI-38531G
136-174 MHz, 110 WATT POWE R AMPL IFIE R
19D902797G1
TABLE OF CONTENTS
Page
DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Front Cover
SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
CIRCUIT ANALYSIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
POWER AMPLIFIER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Exciter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Low Level Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
PA Block Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Dri ve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Pow er Amp lifi er Finals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
POWER C ONTROL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Theory of Operat ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Signal Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
TROUBLESHOOTING GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
IC DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
PARTS LIST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
PRODUCTION CHANGES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
POWER AMPL IFIER ASSEM BLY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
LOW PASS FILTER MODULE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
OUT LINE DIAGRAMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
POWER AMPLIFIER SCHEMATIC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
DESCRIPTION
The VHF Power Amplifier Assembly is a wide ba nd RF
power amplifier operating over the entire 136 to 174 MHz
range without tuning or band splitting. Its main function is
to amplify the 10 mW FM signal from the Transmitter
Synthesizer to the rated RF output of 110 watts at the antenna
port. The output of the Power Amplifie r Assembly is adjustable from 65 to 135 watts at the PA output J104. This
corresponds to a rated RF output of 55 to 110 watts at the
antenna.
The assembly consists of a printed wiring board (A1)
and associated components, including a power module and
three RF power transistors, mounted to the heat sink assembly. The printed wiring board (A1) contains both the power
amplifier circuitry (100 series components) and the power
control circuitry (200 series components).
Unfiltered supply voltage, A+, for the power amplifier
circuits enters the assembly via feedthrough capacitor, C1.
Power cable W4 routes the A+ from CI to J103 on the PWB.
Filtered A+ voltage for the power control circuit enters the
assembly via control cable W13 which co nnects to the PWB
at J201.
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
amplifier gain, or tempera tur e thro ugh the use of a fe edba ck
control loop in the PA assembly.
M/A-COM Wireless Systems
3315 Old Forest Road
Lynchburg, Virgini a 24 50 1
(Outside USA, 434-385-2400) Toll Free 800-528-7711
macom-wireless.com Printed in U.S.A.
www.
TABLE 1 - GENERAL SPECIFICATIONS
ITEM SPECIFICATION
FREQUENCY 136 MHz - 174 MHz
OUTPUT POWER 65 watts - 135 watts (into Low Pass Filter)
INPUT POWER (RF) 10 mW min. into 2:1 VSWR
TEMPERATURE RANGE 30°C TO + 60°C (Ambient air)
SUPPLY VOLTAGE 13.4 Vdc
CURRENT 29 Amps max. (25 A typical @ 135W, 13.4V)
DUTY CYCLE Continous
STABILITY Stable into 3:1 VSWR; all temp. ,voltage, freq. 65 watts -
135 watts
RUGGEDNESS AT HIGH VSWR No damage into open or short load.
LBI-38531G
CIRCUIT ANALYSIS
POWER AMPLIFIER
The power amplifier section of the PA Board consists of
an Exciter, a Low Level Amplifier, a Driver, and the Power
Amplifier Finals. All these gain stages have an input and output
impedance of 50 o hms. Figure 1 is a block diagram showin g
the signal flow within the Power Amplifier Assembly.
Exciter (UIOI)
The Exciter stage uses a broadband silicon monolithic
microwave integrated circuit (MMIC) amplifier. The signal
from transmitter synthesizer, typically 10 dBm (10 mW), is
input to the Exciter through a 10 dB resistive pad (R100 &
R102). The Exciter amplifies the resulting 0 dBm (1 mW)
signal to 20 dBm (100 mW). Following the Exciter is a 3 dB
resistive pad (R104 & R106). This attenuator reduces the
MMIC output power to 17 dBm (50 mW).
The MMIC requires a 5 volt supply source. The 8 volt
regulator (U100) provides the 5 volts to the MMIC via a
dropping resistor R103.
Low Level Amplifier (U1O2)
The Low Level Amplifier (LLA) stag e uses a 50 ohm thick
film RF Power Module to amplify and control of the output
power. Internally, the module is a two stage amplifier. The
power control circuitry controls the gain of the first stage by
varying the collector voltage of Q203. The second stage gain
remains constant with A + providing the DC supply voltage.
The signal f rom the Exc iter stage, typically 1 7 dBm (50
mW), is input into the LLA. Under maximum Power Set
conditions, the LLA amplifies the signal to a typical output
level of 40 dBm (10 W).
Driver (Q101)
The driver is a 6 dB RF amplifier. A ne twork consisting of
C114, C117 and C139 and L103 and L1 05 provides interstage
impedance matching between U102 and Q101. The signal from
the LLA, typically 40 dBm (10 W), is amplified to 46 dBm (40
W). Impedance matching between the driver output and the
input to U103 is provided by C145, C148, C152, C153, and
L108. The splitter , U103, is a quadra ture 90° hybrid coupler. It
divides the signal and applies equal power to the two Power
Amplifier Finals, Q102 and Q103.
Figure 1 - Block Diagram
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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.
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
Copyright© 1992-2002, M/A-COM Private Radio Systems, Inc. All rights reserved.
M/A-COM Private Radio Systems, Inc.
, at any time and without notice. Such changes will be incorporated into new editions of this manual.
M/A-COM Private Radio Systems, Inc.
, without any warranty. Improvements and changes to this manual
1
LBI-38531G
Power Amplifier Fina ls (Q102, Q103)
Each of the Power Amplifier Final de vices is capab le of
producing 5.5 dB of gain. The output signal from the Splitter
is impedance matched to each of the finals. Under optimum
conditions each final amplifies the 43 dBm (20 W) power
input signal to 48.45 dBm (70 W) output power. The outputs
are then impedance matched to the input of the Combiner,
U104. The Combiner is a quadrature 90° hybrid coupler
which combines (sums) the output power of the finals. This
produces an output power of approximately 51.3 dBm (135
W) which is coupled through C 140 to the directional coupler
(part of A1 PWB) and on to the antenna circuits. In addition,
the directional coupler samples both forward and reverse
power and sends this sample to the Power Control circuitry.
POWER CONTROL
The Power Control circuitry performs three basic functions. It keys and unkeys th e PA, sets the PA output power,
and protects the PA against adverse conditions.
Keying and Unkeying the PA
To key the PA, the digital controller places 5 volts on
the PA key line, J201-2. Zero vo lts on the PA key line ca uses
the PA to unkey. If the control cable (W13) is disconnected,
with nothing actively driving the PA key line, the PA will
remain unkeyed.
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 its maximum rated output.
The P A output power is initially set for an output of 135 watts
at J104. This is done by adjusting R217 while injecting a 10
mW signal at J1 and applying 8 volts to J201-3. Af ter setting
the maximum power level, changing the output power is
done by varying the voltage applied on the Power Set line.
drop to zero (0) volts and the PA alarm light on the station
will turn on.
Theory of Operation
Power control of the MASTR III Power Amplifier is
accomplished with a feedback c ontrol loop. Th e three p ossible feedback signals are: representation of forward power,
temperature sensitive scaled representation of forward
power, or representation of reflected power. These three
signals are input to a diode summing junction which selects
the largest of the three for use as the feedback.
The stripline directional coupler samples the output
power and prod uces a voltage, Vf, pr oportional to the forward output power . The power contr ol compares the forwa rd
voltage, Vf, to a reference voltage at U201D. The output of
U201D controls the current flow thru Q202 and the output
of Q203. The co llector output of Q203 adjusts the cont rol
voltage, Vct1. This control voltage is capable of adjusting
the total P A output power sinc e it provides the first stage DC
supply to the Low Level Amplifier, U102.
During over temperature operation, a scaled representation of the forward power is maintained constant by
varying the control voltage line. Thermal resistor R209
sensing an increase in temperature causes the output of
U201A to increase. If the output of U201A becomes larger
than the other feedback lines, the output of U201D will begin
to decrease. This in turn will cause the output of Q203 to
decrease reducing the supply voltage to U102. Since the
scaling is a function of temperature the power is reduced as
the temperature increases.
Under VSWR cutback operation the reverse voltage, V r ,
representative of the reflected output power is held below a
threshold by re du cing th e c ontro l vo ltag e a s ne ces sar y. If Vr
increases at U201 B be yo nd the pr ese t th resho ld an in cr ease
at U201D will result. This causes a subsequent reduction in
the control voltage to U102. Thus the power control circuit
reduces the output power in order to limit the reflec ted power
to 25% of the set power.
power (zero watts) corresponds to 2.5 volts while maximum
power corresponds to 4.5 volts. This voltage is not temper ature
compensated and no effort is made to calibra te this signal to an
absolute power level. It is intended to provide a relative indication of forward power and to discriminate between normal
and cutbac k 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 (VSWR) or may be due to high PA
temperatures. This fault co ndition may indicate a proble m with
the PA or may indicate a sy stem problem external to the Po wer
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 ca pable
of supplying 5 vo lt s at 1.0 mA. Th e ap pr op ria te ke y seque nc e
requires RF from the transmit synthesizer be input to the PA
before the KEY line is energized.
Power Set (Interface Connector pin 3)
This line is used to set the RF Power Output of the PA.
Minimum pow er output equals 4 volts an d maximum power
output equals 8 volts. The driver of this line must be capable
of supplying 8 volts at 1.0 mA.
13.8 VF (Interface Connector pin 6)
This line provides the filtere d supply voltage for the Power
Control. The dr iver of thi s line must be ca pable of suppl ying
13.8 volts ±20% at 100 mA.
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 r eaches 125°C t he PA output drop s 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 re flected power to 25% of
the set power . The Po wer Sensor line indica tes whe n 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
2
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 - POWER SENSE
2-PA Key
3 - POWER SET
4-NC
5 - Ground
6-13.8 VF
Power Sense
This line indicates when the PA is experie ncing adverse
conditions. Under normal operation, while the PA is keyed,
this line will be proportional to forward power. Minimum
VHF POWER AMPLIFIER VOLTAGE CHART
LBI-38531G
SYMPTOM AREAS TO CHECK INDICATIONS
1. No Power or low Power at
Antenna Port
2. No power at PA out put port and
PA ALARM is OFF
3. No power at PA out put port and
PA ALARM is ON.
TROUBLESHOOTING GUIDE
1. Measure the transmitter output power
before the duple xer or antenna switch
(for simplex mode).
2. Measuer the transmitter output power
before the low pass filter.
3. Measure the transmitter output power
before the optional isolator at the PA
output port.
1. Station is in receive mode.
1. No RF input to PA. Check connection
between PA and TX Synthesizer.
2. Check the logic or DC inputs to the PA
from the Interfac e Bo ar d thr ou gh J201.
a. J201-2 PA KEY 5 volts during transmit
The presence of power at this port is an
indication of a defective duplexer,
switch, or cables.
The presence of power at this port is an
indication of a defective filter or cables.
The presence of power at this port is an
indication of a defective isolator or
cables.
TX Synthesizer should deliver a
minimum of 10 mW (1 0 dBm) to the PA.
PARAMETER
(50 OHM, -30° TO +60° C)
SUPPLY VOLTAGE A+
CONTROL VOLTAGE Vct 1 0 - 12 V
FORWARD VOLTAGE Vf 3 - 7 V
REVERSE VOLTAGE Vr 2 - 6 V
POWER SENSE J201-1 2.5 - 4 V
PA KEY J201-2 5 V
POWER SET J201-3 4 - 8 V
13.8 VF J201-6
REFERENCE
SYMBOL
READINGS
(volts DC)
13.4 V ±20%
13.8 V ±20%
VHF POWER AMPLI FIER TYPICAL VOLTAGE READINGS
(50 ohm, room temperature, 13.4 Vdc supply voltage, and 110 watt output)
4. Low power at PA output port
and PA ALARM is OFF
5. Low power at PA output port and
PA ALARM is ON.
b. J201-3 POWER SET 4 volts to 8 volts (4 volts represents zero
RF power)
c. J201-6 13.8 VF
3. Defective PA Replace PA
1. Low RF input to PA from TX
Synthesizer.
2. Check the vol ta g e on J2 01 -3
(POWER SET).
3. Check the power su pp l y vo lt age on
the collector of Q101, Q102 and Q103.
4. One of the two final PA transistors
(Q102 or Q103 is defective.
1. Check for over temperature and/or a
high VSWR condition due to a mismatch at the output port.
13.8 Vdc ±20%
Power should be a minimum of 10 mW
(10 dBM).
For nominal output power, this voltage
should be above 6 volts.
Voltage should be nominal 13.4 Vdc.
Replace the defective transistor.
The power control circuit protects the
PA by cutting back the power. In case
of a mismatch, refer t o symptom 1.
REFERENCE
SYMBOL
Vct1 7 - 10 V 6 - 8 V 4 - 6 V 4 - 6 V
Vf 5 - 7 V 5 - 7 V 5 - 7 V 5 - 7 V
Vr 2 - 3 V 2 - 3 V 2 - 3 V 2 - 3 V
J201-1 2.5 - 4 V 2.5 - 4 V 2.5 - 4 V 2.5 - 4 V
J201-3 6 - 8 V 6 - 8 V 6 - 8 V 6 - 8 V
J201-6 13.4 V 13.4 V 13.4 V 13.4 V
@ 136 MHz
(volts DC)
@ 150 MHz
(volts DC)
@ 162 MHz
(volts DC)
@ 174 MHz
(volts DC)
RATED POWER FOR MASTR III VHF BASE STATION
STANDARD WITH
DUPLEXER
110W 75W 95W 70W
WITH
ISOLATOR
WITH DUPLEXER
AND ISOLATOR
3
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