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TRANSMIT/RECEIVE SWITCH AND CONTROL CIRCUIT
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TM-11-5820-815-14 Service and Circuit Diagrams RADIO SET AN/GRC-171 Manual
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POWER AMPLIFIER MODULE A8.


TO 31R2-2GRC171-2
TM 11-5820-815-14
NAVELEX 0967-LP-544-5010
The level of current flow is a function of the current
of variable capacitor C22, capacitors C24 and C26,
shaping network which allows current to decrease at an
resistor R31, and detector diode CR12. The reflected
exponential rate from about 100 to 0 milliampere as the
power sensor consists of variable capacitor C21,
voltage drop across it varies from 5 to 0 V dc. As the
capacitors C23 and C25, resistor R30, and detector
AGC voltage increases above 2.5 V dc, the output of
diode CR11. Both sensors develop 3.5 to 4 V dc for 20
U1B decreases which, in turn, begins to turn receive
watts of forward or reflected power.
attenuator Q4 off.  This increases the voltage drop
across Q4 and reduces the voltage drop across the
4-236.  FORWARD POWER AMPLIFIER.  Forward
current shaping network. The result of this circuit action
power amplifier U2A (figure FO-24) amplifies the forward
causes the voltage applied to the noninverting input to
power output of directional coupler A7A2 to provide at its
U1B to increase and track the AGC voltage and at the
output approximately 4 volts per 20 watts (2 volts per 5
same time reduces the voltage drop across the current
watts) of forward power. The circuit functions as follows.
shaping network to reduce current flow through CR2
Forward power output from the directional coupler of
(increases series rf attenuation). When the AGC voltage
approximately 2 volts per 20 watts (1 volt per 5 watts) is
reaches about 4.8 V dc (maximum voltage attainable at
applied to the noninverting input of U2A through resistors
the noninverting input to U1B), the output of U1B goes
R34 and temperature-compensation network R32-R33.
negative and turns Q4 off completely. This reduces the
A negative voltage from potentiometer R41 of voltage
current flow through CR2 to about zero. Receive series
divider R44-R43-R41 is also applied to the noninverting
diode CR2 now provides maximum attenuation to the
input of U2A through resistor R39 and temperature-
receive rf signal.
compensation diode CR14.
This negative voltage
provides forward-biased voltage to direction coupler
4-232. When AGC voltage exceeds about 4.8 V dc, the
detector diode CR12 to bias it at the point of conduction
output of shunt diode control amplifier U1A decreases
(approximately 100 microamperes). To compensate for
below +4.2 V dc and begins to forward bias receive
the
negative
bias
voltage,
negative
voltage
shunt diode CR3 to reduce its rf resistance. As AGC
(approximately -0.4 V dc) is developed across resistor
voltage increases to about 8 V dc, the output voltage of
R47 of voltage divider R45-R46-R47 and applied to the
U1A decreases which, in turn, increases the voltage
inverting input of U2A. Potentiometer R41 is adjusted to
across the current shaping network. This allows current
give zero output voltage from U2A when forward power
through CR3 to increase as the AGC voltage increases.
is 0 watt. Variable resistor R49 controls the gain of U2A
As current through CR3 increases, the rf resistance to
and is adjusted to give 4-volt output voltage for 20 watts
ground (through C7) decreases to attenuate the rf signal.
of forward power.  Capacitor C32 in conjunction with
When the transmitter is keyed, the rf attenuation voltage
resistor R45 and capacitor C27 in conjunction with
goes to about 11 V dc. This causes maximum signal
resistor R44 decouple the bias voltages applied to the
attenuation (receiver muting) to the receive rf module.
forward power amplifier from the -12-V dc line.
4-233. RF FILTER. Rf filter A7A1 (figure FO-24) is a 4-
4-237. REFLECTED POWER AMPLIFIER. Reflected
pole, tunable bandpass filter. Under control of d/a servo
power amplifier U3 (figure FO-24) amplifies the reflected
amplifier module A1, servo motor B1 positions the filter
power output of directional coupler A7A2 to provide at its
to the center frequency as determined by the radio set
output approximately 4 volts per 5 watts of reflected
control.  Position feedback potentiometer R20 feeds
power. The circuit functions the same as forward power
position information back to d/a servo amplifier module
amplifies U2A except variable resistor R53 is adjusted to
A1.
Balance potentiometers R21 and R22 allow
give 4-volt output voltage for 5 watts of reflected power.
adjustment of the voltage across R20 so that the position
Variable resistor R40 is adjusted to give a slight negative
feedback voltage tracks the filter position with respect to
output voltage (about -0.120 V dc) for 0 watt of reflected
center frequency.
power. This negative voltage prevents the pa/antenna
power comparator circuits in power amplifier module A8
4-234. The filter provides a bandwidth of 1.5 MHz from
from tripping at small forward power levels or during
center frequency and maximum insertion loss of about 2
modulation troughs.
dB within this bandwidth.  At 7 MHz from center
frequency, the filter provides more than 40 dB of
4-238.  PERCENT MODULATION MONITOR.  The
attenuation; at 15 MHz from center frequency, the filter
percent modulation monitor (figure FO-24), consisting of
provides more than 70 dB of attenuation; and at 30
amplifier U2B and variable attenuator Q5, processes the
MHz from center frequency, the filter provides more than
forward power output signal from U2A to develop a dc
80 dB of attenuation.
percent modulation output voltage (P4-14) that is
proportional to the percent modulation of the transmitted
4-235.  DIRECTIONAL COUPLER.  The directional
signal. The forward power output signal is composed of
coupler (A7A2 of figure FO-24) senses forward and
an audio component that represents the modulation
reflected power.  The forward power sensor consists
envelope
superimposed
on
a
positive
4-42


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