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AGC CIRCUIT
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TM-11-5820-815-14 Service and Circuit Diagrams RADIO SET AN/GRC-171 Manual
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INPUT TRANSFORMER AND AUDIO LEVEL STRAPPING


TO 31R2-2GRC171-2
TM 11-5820-815-14
NAVELEX 0967-LP-544-5010
rf filter module. The series attenuator begins to provide
transistor Q19 off.  This turns mute switch Q18 on.
attenuation to the rf signal when rf attenuator (AGC)
When on, Q18 applies about 9 V dc to the 10.7-MHz
voltage exceeds about 3.4 V dc and provides maximum
AGC line which, in turn, causes AGC amplifiers U7B and
attenuation after the voltage exceeds approximately 5 V
Q17 to apply about 10 V dc to the 30-MHz AGC line and
dc. Refer to the circuit description of rf filter module A7
about 11 V dc to the rf attenuator (AGC) line. This high
(paragraph 4-221) for circuit operation of the series
AGC voltage mutes the receiver by turning the receiver
attenuator.  The gain reduction by the 10.7-MHz if
gain down to provide maximum attenuation to the rf
amplifiers and the gain reduction by the series attenuator
signal.
overlap so there is a smooth transition at the crossover
point.
4-135. AUDIO MODULE A4.
4-132. For AGC voltage above -2 volts, the third stage of
4-136. Refer to the block diagram of figure FO-7 and
gain reduction is controlled by rf attenuator (AGC)
schematic diagram of figure FO-21 while reading the
voltage applied to the shunt attenuator in the rf filter
following circuit description. Unless otherwise specified,
module and by 30-MHz AGC voltage applied to shunt
reference designators apply to components of audio
attenuators (CR1 and CR6) in the receiver rf module.
module A4.
Refer to the circuit description of rf filter module A7
(paragraph 4-221) for circuit operation of the shunt
4-137. GENERAL. Audio module A4 is composed of a
attenuator in that module. In receiver rf module A3, the
transmit audio circuit, receive audio circuit with squelch,
shunt attenuator (CR1) at the output of the uhf mixer
a ptt keyer circuit, and a keyer control circuit.  The
(U1) functions as follows:  When forward biased, pin
transmit audio circuit receives voice audio from the main
diode CR1 acts as an rf attenuator by shunting part of
audio input and microphone input or wide bandwidth data
the 30-MHz if signal to ground through capacitor C21.
from the data audio input. Voice audio that can range in
The amount of attenuation is controlled by the amount of
level from -15 to +10 dB mW is coupled through the input
forward bias applied through the parallel combination of
transformer to the audio level strapping circuit.  For
diode CR2 and resistor R25. For 30-MHz AGC voltage
normal strapping, the voice audio is attenuated by 20 dB
below about 3.8 V dc, negative voltage developed across
and applied to the compression amplifier.  For voice
resistor R26 of voltage divider R24-R26 reverse biases
audio levels that range from -35 to -15 dB mW, optional
CR1 so that it offers no attenuation to the 30-MHz if
audio level strapping couples the signal directly to the
signal. As the 30-MHz AGC voltage increases above 3.8
compression amplifier without attenuation. To prevent
V dc, pin diode CR1 becomes forward biased to begin
overmodulation of the transmitter, the compression
attenuating the 30-MHz if signal. Maximum attenuation
amplifier maintains a constant output level for input
(approximately 25 dB of receiver gain reduction) is
levels that vary from -35 to +10 dB mW depending on
attained when the AGC voltage exceeds about 4.4 V dc.
the strapping option used. In addition, the compression
In a similar manner, pin diode CR6 attenuates the 30-
amplifier  amplifies  the  microphone  input  to  hold
MHz if signal at the output of the second if amplifier
microphone audio at the same constant output level.
(Q12).
The constant level output signal from the compression
amplifier is passed through a low-pass filter followed by a
4-133.  The 30-MHz AGC voltage is developed as
high-pass filter to shape the audio response so that the
follows:  When the AGC voltage is -1 V dc, AGC
modulation amplitude does not vary by more than +1 to -
amplifier Q17 amplifies the output of U7B to produce a
2 dB from 300 to 6000 Hz. The output of the high-pass
30-MHz AGC voltage of about 3.4 V dc.  This is just
filter passes through a second high-pass filter to the
below the turn-on point of the shunt attenuators.  As
percent modulation control.  The percent modulation
AGC voltage increases above -1 V dc, pin diodes CR1
control sets the modulation audio level to modulate the
and CR2 of the shunt attenuators begin to turn on to
transmitter at the desired modulation percentage. The
provide receiver gain reduction. Once the receiver input
output of the percent modulation control is passed
signal increases to the point where the AGC voltage
through a clipper to remove audio peaks that could
exceeds about -2 V dc, the 30-MHz AGC voltage
overmodulate the transmitter. The output of the clipper
exceeds 4.7 V dc and the shunt attenuators (including
passes through a second low-pass filter to provide
the shunt attenuator of rf filter module A7) take over to
transmit audio to the modulator circuit of power amplifier
provide receiver gain reduction. The gain reduction of
module A8. The second high-pass and low-pass filters
the series attenuator overlaps the gain reduction of the
have no effect (other than signal amplification) on the
shunt attenuators so there is a smooth transition of the
audio response of the voice audio since they have cutoff
crossover point.
frequencies of 16 Hz and 25 kHz respectively. These
filters shape the frequency response of the wide
4-134.  RECEIVER MUTE.  The receiver mute circuit
bandwidth data so that the modulation amplitude does
(Q19 and Q18 of figure FO-20) turns down the receiver
not vary by more than -1 to -3 dB from 16 Hz to 25 kHz.
gain to mute the receiver whenever the transmitter is
The modulation level of the wide bandwidth data is set by
keyed.  When the transmitter is keyed, the ground
a potentiometer at the data audio input.
applied through resistor R98 turns mute
4-27


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