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AUDIO MODULE A4.
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TM-11-5820-815-14 Service and Circuit Diagrams RADIO SET AN/GRC-171 Manual
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LOW-PASS FILTER (U1B)


TO 31R2-2GRC171-2
TM 11-5820-815-14
NAVELEX 0967-LP-544-5010
4-138. The receive audio circuit receives detected audio
inhibited by the keyer control circuit.
(receive af input) from receiver rf module A3.  The
receive af signal is amplified and passed through the
4-143.  The following describes individual circuit func-
bandpass filter to the receive data audio output.  The
tions of audio module A4. Refer to figure FO-21.
bandpass filter shapes the response of the signal so that
its amplitude does not vary by more than +1 to -3 dB
4-144.  INPUT TRANSFORMER AND AUDIO LEVEL
from 16 Hz to 25 kHz.  The receive af signal is also
STRAPPING.  Transformer T1 (figure FO-21) termi-
applied to the compression amplifier. The compression
nates 150- or 600-ohm main audio input lines.  Con-
amplifier maintains a constant output level for input audio
nected in series (P1-C to P1-D), the primary windings of
levels that vary as a function of the percent modulation of
T1 provides 600-ohm termination; individually, the
the receive rf signal. The constant level output signal
primary windings provide 150-ohm termination.  Voice
from the compression amplifier is passed through a high-
audio applied to the main audio input is coupled through
pass filter followed by a low-pass filter to shape the audio
transformer T1 to the audio level strapping circuit where
response of the signal so that the signal amplitude does
voltage divider R2-R3-R4 attenuates the -15- to +10-dB
not vary by more than +1 or -2 dB from 300 to 3000 Hz.
mW audio signal by about 20 dB (normal strapping).
The audio level from the low-pass filter is adjusted by the
The attenuated audio signal (-35 to -10 dB mW) is
headset volume control and applied to the power
applied to the variable attenuator (R6-Q1) of the
amplifier to drive the headset. Also, the audio level from
compression amplifier circuit. For -35- to -15-dB mW
the low-pass filter is adjusted by the receive audio level
voice audio input signals, the optional -35-dB mW audio
control and applied to a second power amplifier to set
level strapping (strap E2 to E3 removed and strap E1 to
the audio level at the main audio output. Both outputs
E2 added) applies the output of T1 directly to the variable
are capable of 100 milliwatts of output power.
attenuator.
4-139. During transmit mode, transmit audio is detected
4-145. The microphone input is coupled directly to the
at the antenna (rf filter module A7) and fed back as
variable attenuator through coupling capacitor C1 and
sidetone audio. This audio is applied to the amplifier in
resistor R7. A -12 V dc applied through resistors R10
the compression amplifier circuit so that sidetone audio
and R11 provide dc current to the carbon microphone.
can be heard at the receive audio output (headset).
Resistor R10 provides a load to the microphone.
4-140. The squelch control and squelch gate provide
4-146.
COMPRESSION  AMPLIFIER  (TRANSMIT
receiver muting. AGC voltage that is related to rf signal
AUDIO).  The transmit audio compression amplifier
strength is compared by the squelch control to the
(figure FO-21) consists of the variable attenuator circuit
squelch reference input (squelch potentiometer) that sets
(Q1), amplifier circuit (U1A), and detector circuit (U2A,
the squelch threshold. When the rf input signal to the
U2B, Q2, Q3).  Functionally, the circuit maintains a
radio set decreases below the squelch threshold, as set
nearly constant audio output signal (about 180 millivolts
by the squelch potentiometer, the squelch gate mutes
rms at test point TP16) for audio input signals (test point
the receiver.  The squelch on/off line disables the
TP6) that range from about 14 (-35 dB mW) to about 245
squelch gate whenever the squelch is turned off.
millivolts (-15 dB mW).  Compression is achieved by
controlling the attenuation ratio of variable attenuator Q1
4-141. Keying information received at the main audio
which in turn controls the level of audio applied to
input is applied to the ptt keyer.  The ptt keyer has
amplifier U1A. Positive or negative audio peaks from the
strapping options that allow selection of normal ground
output of amplifier U1A that exceed the threshold of the
ptt keying or optional 6-, 26-, or 48-volt keying.  The
detector (about 250 millivolts peak or 180 millivolts rms)
remote ptt output of the keyer is connected to the
are converted into a dc control voltage and applied to the
REMOTE/LOCAL switch (front panel of chassis A10)
gate of variable attenuator Q1 to control the attenuation
which applies the remote ptt keying signal to the ptt input
ratio of the variable attenuator. As the audio input signal
of the keying control circuit whenever the radio set is
increases, the detector reduces the control voltage
operating in the remote mode.
applied to the variable attenuator to increase its
attenuation ratio. This reduces and maintains the audio
4-142. The keying control circuit receives ptt keying and
level at the input to amplifier U1A at the level necessary
radio set fault information and provides at its output two
to hold the output of the amplifier just at the detector
transmitter key lines and a fault switch circuit. The fault
threshold.  The compression amplifier attack time is
switch circuit turns on the ready lamp (radio control for
about 4 milliseconds; release time is about 150
remote operation or front panel of chassis A10 for local
milliseconds.
operation). When a synthesizer fault (ptt fault input) or rf
filter servo fault (rf filter fault input) is detected, the ready
4-147.  Variable Attenuator.  The variable attenuator
lamp is turned off and the key 2 key line is
consists of resistor R6 and FET Q1 that together function
as a variable voltage divider.  The drain to source
resistance  of  FET  Q1  varies  from  infinity  to
4-28


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