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Figure 4-9. Power Amplifier Modulator, Simplified Schematic Diagram
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TM-11-5820-815-14 Service and Circuit Diagrams RADIO SET AN/GRC-171 Manual
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Antenna VSWR and PA/Antenna Power Temperature Monitor


TO 31R2-2GRC171-2
TM 11-5820-815-14
NAVELEX 0967-LP-544-5010
4-274.  The pa power output reference source, U6A,
be varied to set the receiver-transmitter power output to
amplifies (by unity gain) a portion of the reference
the desired rf level. To maintain a constant power output
voltage from U1 applied across voltage divider R56-R57-
from the receiver-transmitter, variations in power output
R58 to produce the pa power output reference voltage.
appear as variations in the forward power signal. These
This reference voltage, as set by potentio-meter R57,
variations, when compared to the pa power output
determines  the  power  output  from  the  receiver-
reference voltage, cause the ALC voltage to change in
transmitter.
such a direction to counteract the power output
variations.
4-275.  In the primary ALC control loop, when the
transmitter is keyed (logic 0 key 2 voltage), the ALC
4-280. If a vswr, pa/antenna power, or temperature fault
differential amplifier,  U6B, compares  the antenna
is detected, the ALC voltage is increased to reduce the
forward power signal applied to its noninverting input
power output to approximately 4 watts.
This is
through low-pass filter C13-R41 to the pa power output
accomplished by the following circuit action:  When a
reference voltage applied to its inverting input through
fault is detected, the pa power turndown switch, U2A,
resistor R62. Low-pass filter C13-R41 removes modula-
connects resistor R59 to ground in parallel with
tion from the antenna forward power signal. The ampli-
potentiometer R57 and resistor R58. This decreases the
fied difference signal at the output of U6B is applied to
voltage applied to the pa power output reference source
the noninverting input of U4A through resistor R64.
(U6A) by about 7 dB which, in turn, causes the output of
U6A to decrease by the same amount. The decrease in
4-276. In the circuit of U6B, diode CR7 prevents the
pa power output reference voltage causes an increase in
output from going negative by more than about -0.6 V dc.
the differential input voltage at the input of U4A. This
Capacitor C12 in parallel with resistor R63 in conjunction
increase in voltage, when amplified by the ALC
with U6 forms an active low-pass filter to remove
differential amplifier, U4A, and ALC output amplifier, Q3-
modulation. Diode CR8 provides temperature stability.
Q4, causes the ALC voltage to go to a high positive
During transmit mode, the ALC precharge amplifier,
value to reduce the power output of the receiver-
U4B-Q5, and time constant control Q1-Q6-Q7, have no
transmitter to 0 watt. As a result of this, the antenna
effect on the output of U6B, since the logic 0 key 2
forward power and pa forward power go to zero, but
voltage causes Q5 to be turned off and FET's Q6 and Q7
because of the antenna forward power voltage stored in
to be biased at maximum resistance.
capacitor C13 and the lag in the output of U6B, the
reduction in reference voltage applied to U6B does not
4-277.  In the secondary ALC control loop, the pa
cause an instantaneous increase in its output voltage.
forward power amplifier, U3A, amplifies the pa forward
The resulting voltage at the output of U6B, when applied
power signal and applies the result to the noninverting
to U4A, holds the ALC voltage at a high positive value
input of U4A through low-pass filter C10-R44, voltage
until the charge across capacitor C13 discharges. As the
divider R45-R53, buffer U7, and isolation diode CR12.
charge across C13 decreases, the output of U6B
Resistor R67 in the circuit of U3A is test selected to
decreases which, in turn, causes the ALC voltage to
adjust the gain of U3A to compensate for level variations
decrease. After a period of time, power again appears at
in pa forward power that exist between different
the antenna, but because of the reduction in the pa
reflectometer, A8A7, subassemblies.  Low-pass  filter
power output reference voltage, the new power output
C10-R44  removes modulation from the pa forward
level is approximately 4 watts.
power signal. Under normal circuit conditions, the output
voltage from U6B is greater than the output of U7,
4-281. When the fault is cleared, the pa power turn-
therefore, making it the primary controlling signal. Diode
down switch, U2A, removes resistor R59 from ground
CR12 isolates the two control loops so that they do not
and allows capacitor C11 to charge.  The RC time
interfere with each other.
constant of R59 and C11 determines the rise time of the
pa power output reference voltage which, in turn,
4-278.  ALC differential amplifier U4A compares the
determines the power up time of the power amplifier
voltage from either U6B or U7 applied to its noninverting
module to the normal power output level.
input to the pa power output reference voltage applied
through resistor R74 to its inverting input. The amplified
4-282.  When the receiver-transmitter is unkeyed, the
difference voltage from U4A drives the ALC output
logic 1 key 2 voltage (P1-9) causes the following circuit
amplifier, Q3-Q4, to produce the ALC voltage.
actions to occur to turn off the rf power amplifier and to
precondition the ALC circuit.
4-279.  As a result of the above circuit action, ALC
voltage applied to the pin diode ALC attenuator is a
4-283. Logic 1 key 2 voltage applied to U2E of the pa
function of both the pa power output reference voltage
turndown switch causes transistor Q2 to turn off. When
and either the antenna forward power or pa forward
Q2 turns off, +5 V dc is removed from the +5 V dc
power. By adjusting potentiometer R57, the ALC voltage
switched output (P1-2) and from the antenna vswr and
(via the pa power output reference voltage) can
pa/antenna
power
(U5A,
U5B).
4-48


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