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Chapter 4. PRINCIPLES OF OPERATION - TM-11-5820-815-140047
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TM-11-5820-815-14 Service and Circuit Diagrams RADIO SET AN/GRC-171 Manual
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RECEIVER OPERATION.


TO 31R2-2GRC171-2
TM 11-5820-815-14
NAVELEX 0967-LP-544-5010
output audio level that will not overmodulate the
to the headset output and audio (or data) output of the
transmitter. The output of the compression amplifier is
receiver-transmitter. Impulse type noise is blanked from
passed through a low-pass and high-pass filter that
the audio output by a noise channel blanking circuit in
together shape the audio response of the transmit audio
the 30-MHz if circuit. Receive squelch is provided by a
signal. The input data signal to be transmitted is fed
squelch circuit in the audio circuit. Receiver selectivity is
around the compression amplifier and applied to a
determined by a crystal filter in the 10.7-MHz if circuit.
second high-pass filter. This filter in conjunction with the
Audio (or data) response is determined by active filters in
low-pass filter following the percent modulation (% MOD)
the audio circuit. The crystal filter (36-kHz bandwidth)
control shapes the frequency response of the transmit
supplied with the radio set provides for 50-kHz channel
data signal. The output of the high- pass filter is applied
spacing. An optional narrow bandwidth (20-kHz) or wide
to the % MOD control located on the front panel of the
bandwidth data (60-kHz) filter can be installed for 25-kHz
receiver-transmitter. The % MOD control is adjusted to
channel  spacing  or  100-kHz  channel  spacing
set the audio level to give the desired percent of
respectively. Noise channel blanking is disabled when
modulation of the transmit rf signal. The audio signal
the wide bandwidth filter is installed.
from the % MOD control is passed through the
clipper/low-pass filter of audio module A4 to the
4-9. The transmitter section of the receiver- transmitter
modulator of power amplifier module A8.  The clipper
contains a broadband rf amplifier that amplifies the
removes audio peaks that could overmodulate the
transmit rf signal from the frequency synthesizer to
transmitter.
provide 16- to 24-watt (nominal 20- watt) carrier power at
the antenna.  An automatic load control (ALC) circuit
4-14
To key the transmitter, push-to-talk (ptt) keying
receives forward and reflected power signals to maintain
information from the receiver-transmitter front panel
control of the power output level. The transmit rf signal
REMOTE/LOCAL switch is applied to the keying con- trol
is amplitude modulated by input audio (or data) that is
circuit of audio module A4. The keying control circuit
amplified, filtered, and applied to the modulator of the
provides two transmitter key lines, one a fast attack-slow
power amplifier. Harmonics in the transmit rf signal are
release key line (key 1) and the other a slow attack-fast
attenuated by the rf filter.
release key line (key 2). For remote operation, keying
from five separate inputs can key the transmitter. They
4-10.  A meter circuit in the receiver-transmitter allows
are the audio loop, current loop, and voltage loop keying
measurement (from the front panel) of power supply
inputs, the remote ptt key-line input, and the keying
voltages, antenna forward and reflected powers, the
information supplied on the audio inputs (CT 1 and CT 2)
percent modulation of the transmit rf signal, and the
to the receiver-transmitter. The audio loop, current loop,
relative heat-sink temperature of the power amplifier.
and voltage keying signals are applied through the emi
filter to the keyer circuit of keyer module A9. The remote
4-11.
TRANSMITTER OPERATION.
key output of the keyer is applied to the remote side of
the REMOTE/LOCAL switch. The remote pit key line is
4-12.  Refer to the transmit signal flow diagram of
applied  directly  to  the  remote  side  of  the
figure FO-16.  The transmit section of the receiver-
REMOTE/LOCAL switch. Keying information sup- plied
transmitter consists of power amplifier module A8,
on the audio input lines is applied to the remote ptt keyer
transmit audio circuits of audio module A4, and the
of audio module A4. The remote ptt output of the remote
common rf signal path through rf filter module A7.
ptt keyer is applied to the remote side of the
Transmitter keying circuits are contained in audio module
REMOTE/LOCAL switch.  For local operation, the ptt
A4 and keyer module A9. Tuning for the rf filter of rf filter
lines from the MIC jacks and from the PTT/ CARRIER
module A7 is provided by d/a servo amplifier Al.
TEST switch on the front panel of the receiver-
transmitter are connected to the local side of the
4-13. For remote operation, three audio inputs can be
REMOTE/LOCAL switch.
applied to the transmitter. They are the micro- phone
input, audio input, and data input. All three inputs are
4-15.  Refer to figure FO-16, sheet 2. The rf signal to
applied to audio module A4 through the electromagnetic
be transmitted is generated by the phase-locked loop
interference (emi) filter on chassis A10 of the receiver-
(p11) of frequency synthesizer module A2.  The fre-
transmitter.  For local operation, microphone audio is
quency of the rf signal is determined by bcd frequency
applied to audio module A4 from jacks located on the
select information applied to the phase-locked loop from
front panel (chassis A10) of the receiver-transmitter.
the radio control. During transmit mode, the output of
The input audio is coupled through transformer T1 to the
the phase-locked loop is applied to power amplifier
compression amplifier. The microphone audio is applied
module  A8  through  the  frequency  synthesizer
directly to the compression amplifier. The compression
transmit/receive switch (actually a diode switch) that is
amplifier amplifies the audio signal and maintains a
closed by the slow attack-fast release transmitter key line
constant
(key 2).
4-16.  The transmit rf signal (carrier signal) from
frequency synthesizer module A2 is applied to the
4-2


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