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POWER AMPLIFIER MODULE A8.
Up
TM-11-5820-815-14 Service and Circuit Diagrams RADIO SET AN/GRC-171 Manual
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RF AMPLIFIERS A8A5 AND A8A6


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
lines
4-44


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