POWER AMPLIFIER MODULE A8.

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

voltage in accordance with audio content of the transmit

dc component that represents the average forward

audio signal which, in turn, varies the gain of the high-

power level.

Capacitor C33 couples the audio

level modulated rf amplifiers to amplitude modulate the

component to peak detector circuit CR15-C34 where the

transmit rf signal.

Modulation feedback from the

negative peak voltage of the modulation envelope is

modulation transistor and envelope feedback taken from

developed across capacitor C34. The negative voltage

the power amplifier forward power signal are fed back as

is applied to the input of U2B where it is amplified by a

negative feedback to control ac gain of the modulator

voltage gain of about 2.4 V/V (inverting). The output of

and to reduce distortion in the transmitted rf signal.

U2B is applied to a voltage divider consisting of resistor

R60 and FET Q5 of the variable attenuator.

4-245.

The output rf signal from rf preamplifier/

modulator A8A2 is applied to the pin diode ALC

4-239.

FET Q5 operates in the voltage-controlled

attenuator of rf predriver/ALC attenuator A8A3. Under

resistor mode.

Negative voltage developed across

the control of ALC voltage, the pin diode ALC attenuator

potentiometer R59 and applied to the gate of Q5 through

varies circuit gain to maintain a constant forward power

R64 biases Q5 to a high resistance. As forward power

output from the receiver-transmitter. ALC voltage is

increases, the positive dc output voltage from U2A is

developed from the ALC circuit which consists of an

applied to the gate of Q5 through resistor R58.

inner (secondary) control loop and outer (primary) control

Capacitor C35 filters out the audio component. The

loop. In the secondary control loop, power amplifier

positive dc voltage makes the gate voltage less negative

forward power from reflectometer A8A7 is processed by

to reduce the drain to source resistance of Q5 which

the ALC amplifiers. In the primary control loop, antenna

increases the attenuation of the output signal from U2B.

forward power from the rf filter module is processed.

For a constant modulation percentage, this circuit action

Under normal circuit conditions, the primary control loop

compensates for the increase in peak-to-peak amplitude

controls the power output from the receiver- transmitter.

of the modulation envelope as the power level increases.

Variations in the antenna forward power signal caused

The result is a nearly constant voltage applied across

by changes in loading will cause the ALC voltage to

R61 and the meter circuit of chassis A10 to indicate the

affect the pin diode ALC attenuator in such a manner as

percent modulation.

to counteract the variations to maintain a constant power

output. The rf output signal of the pin diode ALC

4-240. Potentiometer R59 allows calibration of the

attenuator is amplified by the two linear rf amplifiers and

percent modulation monitor.

applied to rf driver A8A4.

4-241. POWER AMPLIFIER MODULE A8.

4-246.

The output signal from rf predriver/ALC

attenuator A8A3 Is applied to the gain slope pad of rf

4-242. Power amplifier module A8 consists of seven

driver A8A4. The gain slope pad provides frequency-

subassemblies A8A1 through A8A7 and a chassis

versus-gain compensation for the rf power amplifier

(finned heat sink). Each subassembly has an individual

module. The output of the gain slope pad is amplified by

schematic diagram (figures FO-26 through FO- 31) and

the rf amplifier (no 6) to provide about 12-watt output

the chassis (A8) an interconnect diagram (figure FO-25)

carrier level.

that ties all the subassemblies together. A block diagram

of the overall rf power amplifier module is shown in figure

4-247. The output signal from rf driver A8A4 is applied

FO-10.

to the 3-dB, 90-degree hybrid coupling circuit (A8HY1)

where the signal is split into two signals. Each signal is

4-243. GENERAL. Refer to figure FO-10. The transmit

then applied to a 3-dB, 90-degree hybrid coupler in each

rf signal (+12 to +16 dB mW) from the synthesizer

of the amplifiers, A8A5 and A8A6, where each signal is

module to be transmitted is applied to the resistive tee

again split into two signals. Through the two tiers of

attenuator of rf preamplifier/modulator ABA2.

The

hybrid couplers, the rf signal is applied to four broadband

resistive tee attenuator compensates for gain variations

rf amplifiers that are isolated from one another and

in the three rf amplifiers of A8A2. The amount of

operate in parallel to share one-fourth the load. The rf

attenuation is determined through a test select process

signals are amplified by the four rf amplifiers and

to provide about 0.3- watt carrier level output from A8A2

combined into one rf signal by two tiers of 3-dB, 90-

for +14-dB mW input.

degree hybrid couplers to produce about 30-watt carrier

level at the output of hybrid A8HY2.

4-244. The transmit audio signal from the audio module

to be transmitted is applied to the modulator of

4-248. The output signal from hybrid A8HY2 is applied

ALC/modulator A8A1. The modulator amplifies the

to reflectometer A8A7 where the level of power amplifier

transmit audio signal to provide a modulation base drive

forward power and reflected power is detected and

signal to the modulation transistor that controls power

applied to ALC/modulator A8A1. The rf output signal

supply voltage applied to the high-level modulated rf

from the reflectometer is applied to the rf filter module.

amplifiers. The modulation transistor varies the supply

4-43