RF PREAMPLIFIER/MODULATOR A8A2

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

4-249. ALC/modulator A8A1 includes a monitor circuit

4-253. RF PREAMPLIFIER/MODULATOR A8A2. Refer

that reduces the power amplifier rf output to about 4

to figure FO-27. The transmit rf input signal is coupled to

watts if a high heat-sink temperature, high vswr, or high

the stage 1 class A rf amplifier, Q1, through the resistive

rf filter insertion loss is detected. The monitor circuit

tee attenuator composed of resistors R1, R2, and R3

consists of antenna vswr comparator, pa/antenna power

and through the impedance matching network composed

comparator, and the overtemperature detector. The

of inductors L1 and L2; capacitors C1, C2, and C3; and

antenna vswr comparator compares antenna reflected

resistor R4. Resistors R1, R2, and R3 are test selected

power to antenna forward power to activate the pa power

to provide proper input impedance (50 ohms) and input

turndown switch (via the OR gate) if the antenna vswr

signal attenuation so that the rf output from A2 is about

exceeds about 3 to 1.

The pa/antenna power

0.3-watt carrier power for a +14-dB mW transmit rf input

comparator compares pa forward power to antenna

signal. Transistor Q1 is biased to about 30-milliampere

forward power to activate the pa power turndown switch

collector current by base current supplied from bias

if the pa to antenna power ratio exceeds about 2 dB.

transistor Q4. Transistor Q4 controls base current to Q1

The overtemperature detector activates the pa power

by comparing a reference voltage developed across

turndown switch if the heat-sink temperature at the rf

resistor R6 and diode CR1 of voltage divider R6-CR1-

amplifier transistors exceeds 100 degrees Celsius.

FL2-R7 to the voltage developed across resistor R8

When activated, the pa power turndown switch reduces

which is a function of base current and collector current

the pa power output reference voltage. The reference

of Q1. Diode CR1 provides temperature compensation

voltage is compared to the pa and antenna forward

for Q1.

power signals to develop the ALC voltage. When the

reference voltage decreases, the ALC voltage increases.

4-254. The amplified output of Q1 is coupled to the

This causes the pin diode ALC attenuator to attenuate

stage 2 high-level modulated rf amplifier, Q2, through the

the transmit rf signal by about 7 dB. The one-shot

impedance matching network composed of inductors L3

stretches momentary vswr faults into a greater than 300-

and L4 and capacitors C4, C5, and C6. Transistor Q2 is

millisecond activating signal to the pa power turn-down

biased by base current supplied from bias network R10-

switch. For momentary faults, this allows the ALC

CR2-FL5 and applied to the base of Q2 through resistor

control loop to stabilize at low power before returning to

R9. Resistor R10 is test selected to produce a reference

normal power.

voltage across diode CR2 that biases Q2 to about 13-

milliampere collector current.

Diode CR2 provides

4-250. The transmitter keying control (key 2) is applied

temperature compensation for Q2.

to the pa turndown switch, receive mode shutdown

switch, and ALC precharge amplifier of ALC/modulator

4-255. The amplified output of Q2 is coupled to the

A1. When the transmitter is unkeyed, the receive mode

stage 3 high-level modulated rf amplifier, Q3, through the

shutdown switch causes the ALC amplifiers to apply

impedance matching network composed of induc- tors

maximum ALC voltage to the pin diode ALC attenuator.

L5, L6, and L7; capacitors C7, C9, C10, and C11; and

This provides maximum rf signal attenuation during

resistor R11. Transistor Q3 is biased by base current

receive mode. In addition, the power amplifier turndown

supplied from bias network R13-CR3-C12 and applied to

switch removes bias (+5 V dc switched) voltage from the

the base of Q3 through resistor R12. Resistor R13 is

predriver amplifier causing the power output of the rf

test selected to produce a reference voltage across

predriver/ALC attenuator, A8A3, to go to 0 watt. This

diode CR3 that biases Q3 to about 12-milliampere

turns off the power amplifier during receive mode.

collector current. Diode CR2 provides temperature

compensation for Q3.

4-251. When the transmitter is unkeyed, the ALC

precharge amplifier applies a voltage to the ALC

4-256. The amplified output of Q3 is coupled to the

amplifiers that represents an antenna forward power

output of A2 through the impedance matching network

slightly above the normal power output from the trans-

composed of inductors L8, L9, and L10; capacitor C13;

mitter. The time constant control circuit allows this to

and resistor R15.

occur in minimum time. The ALC precharge voltage

preconditions the ALC circuit so that when the trans-

4-257. Refer to paragraph 4-270 for a discussion of the

mitter is again keyed, the ALC circuit will be turning the

modulation of transistors A8A2Q2 and A8A2Q3.

power output down at the same time as the power output

is coming up. The combination of this circuit action

4-258. RF PREDRIVER/ALC ATTENUATOR A8A3.

results in the transmitter going to full power output with

Refer to figure FO-28. Capacitor C1 couples the input rf

no overshoot and in minimum key-on time (about 120

signal to the pin diode ALC attenuator where the signal is

milliseconds).

attenuated in accordance with the level of applied ALC

voltage. To understand the operation of the pin diode

4-252. The following is a detailed description of the

ALC attenuator, consider the operation of hybrid couplers

individual subassemblies of the rf power amplifier

in general. Hybrid couplers used in this equipment

module.

consist of four terminal devices containing two coupled,

quarter

wavelength

lines

4-44