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RECEIVER OPERATION.
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TM-11-5820-815-14 Service and Circuit Diagrams RADIO SET AN/GRC-171 Manual
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POWER DISTRIBUTION.


TO 31R2-2GRC171-2
TM 11-5820-815-14
NAVELEX 0967-LP-544-5010
4-24.  During receive mode, the incoming rf signal from
transmitter.  The receive signal is also applied to the
the antenna is applied through the directional coupler to
compression amplifier.
The compression amplifier
the rf filter of rf filter module A7.
To eliminate
maintains a constant output level for variations in percent
interference from collocated equipment, the rf filter is
modulation of the receive signal.  The output of the
tuned (as described in paragraph 4-19) to the radio
compression amplifier is passed through a high-pass
control frequency.
This prevents large out-of-band
filter and a low-pass filter that together shape the audio
signals from being applied to receiver rf module A3. The
response of the receive audio signal. The output of the
output from the rf filter is applied to the transmit/receive
low-pass filter is then applied to the receive audio (RCV
switch (actually a diode type switch) that also acts as an
AUDIO) control and to the headset volume (VOL) control
rf attenuator to provide front-end signal attenuation. The
located on the front panel of the receiver-transmitter.
amount of attenuation is controlled by the rf attenuation
The RCV AUDIO control is adjusted to set the receiver-
(AGC) signal from the AGC amplifier of receiver rf
transmitter audio output level, and the VOL control is
module A3 and is dependent upon rf signal strength.
adjusted to set the headset audio level. The output of
When the trans- mitter is un-keyed, the slow release of
the RCV AUDIO control is applied to the 100-milliwatt
key 1 causes the receive rf output of the rf filter module
power amplifier of audio module A4. The output of the
transmit/ receive switch to be routed to the uhf mixer of
power amplifier is transformed into two 150-ohm lines by
receiver rf module A3. Also, when the transmitter is un-
transformer A4T2 and then applied through chassis A10
keyed, the fast release of key 2 causes the receive
emi filter to the audio output of the receiver-transmitter.
injection output of the frequency synthesizer module
The output of the VOL control is applied to a second
transmitter receive switch to be routed to the uhf mixer.
100-milliwatt power amplifier in audio module A4. The
The receive injection signal is generated by the same
output of this amplifier is transformed into a 600-ohm
phase-locked loop that generates the transmit rf signal.
output by transformer A4T3 and then applied to the
However, when the transmitter is un-keyed, the key 1
headset jack on the front panel of the receiver-
key line and the 30-MHz decode circuit of frequency
transmitter through chassis A10 emi filter to headset
synthesizer module A2 cause the phase- locked loop to
audio out (J22-k).
shift down in frequency by 30 MHz. This results in a
receive injection signal that is 30 MHz less than the
4-26.  During the transmit mode, sidetone audio is
selected radio control frequency. The slow release of
taken from the output of the forward power amplifier in rf
key 1 assures that the switch to receive mode has been
filter module A7 and applied to the SIDETONE control on
made before the phase- locked loop is shifted in
the front panel of the receiver- transmitter. The output of
frequency.
the SIDETONE control is applied to the compression
amplifier to provide sidetone audio in the output audio
4-25.  In the receiver rf module, the uhf mixer mixes
signal or in the headset during transmit mode.
the receive rf signal (225 to 399.975 MHz) with the
receive injection signal (195 to 369.975 MHz) to produce
4-27.  For receive squelch, the squelch sample signal
the 30-MHz if signal that is applied to the 30-MHz if
developed from AGC voltage is applied to the squelch
amplifier. The 30-MHz if signal is split into two signal
control circuit of audio module A4. Also applied to the
paths; one to the second mixer and one to the noise
squelch control circuit is the squelch reference input from
channel.  The 30-MHz if signal that is applied to the
the SQUELCH control on the front panel of the receiver-
second mixer is mixed with the 19.3-MHz output signal
transmitter.  The squelch control circuit compares the
from the injection amplifier to produce the 10.7-MHz if
squelch sample signal, which is a function of the level of
signal.  Impulse type noise that is received at the
the incoming rf signal, to the squelch reference input.
antenna is routed through the noise channel where it is
The output of the squelch control circuit operates the
used to disable the 19.3-MHz oscillator input to the
squelch gate to mute the receive audio whenever the
second mixer. This action mutes the receive so that the
incoming rf signal is below the squelch threshold (as set
noise pulse does not appear at the receiver audio output.
by the SQUELCH control).  When operating remotely
The output of the second mixer is applied to the 10.7-
through the front panel REMOTE/LOCAL switch, the
MHz if amplifier where the 10.7-MHz if signal is amplified
squelch can be disabled by the SQLCH ON/OFF switch
and applied to the detector and to the if jack of the
of C-7999/ GRC-171.  When operating locally, the
receiver- transmitter.  The detector demodulates the
squelch can be disabled by the front panel SQUELCH
amplitude modulated signal to produce AGC voltage and
ON/OFF switch.
the receive audio signal (or data signal) to audio module
A4. The AGC voltage is amplified by the AGC amplifiers
4-28.  The output from the squelch control circuit of
and applied to the 10.7-MHz if amplifiers, 30- MHz if
audio module A4 is also applied to keyer module A9.
amplifiers, and to rf filter module A7 to control the gain of
When the squelch is turned on and the incoming rf signal
the receiver.  The receive signal is applied to the
is above squelch threshold, the squelch com- parator
bandpass filter.
The bandpass filter shapes the
and relay driver circuit of keyer module A9 causes
frequency response of the receive data signal and
squelch  relay A9K1  to  energize.
The  squelch
applies it to the data output of the receiver-
4-4


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