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NOISE CHANNEL AGC AMPLIFIER
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TM-11-5820-815-14 Service and Circuit Diagrams RADIO SET AN/GRC-171 Manual
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AGC CIRCUIT


TO 31R2-2GRC171-2
TM 11-5820-815-14
NAVELEX 0967-LP-544-5010
4-122.  10.7-MHz IF AMPLIFIER.  The 10.7-MHz if
The secondary of transformer T3 transforms the 30-MHz
amplifier (U4, U5, and Q16 of figure FO-20) consists of
if signal into two balanced, 180-degree-phase-related
two gain-controlled integrated circuit if amplifiers (U4 and
signals and applies them to the gates of FET's Q10 and
U5)  and  a  temperature-compensated  transistor  if
Q11.  Variable capacitor C61 is adjustable to allow
amplifier (Q16) that together provide about 88 dB of
balancing of the mixer. The 19.3- MHz injection signal
open loop gain. Capacitor C75 couples the 10.7-MHz if
is applied to the sources of FET's Q10 and Q11. The dc
signal to if amplifier U4. AGC voltage developed by the
level from the injection amplifier provides bias voltage to
AGC circuit is applied through resistor R62 to control the
FET's Q10 and Q11 so that when the 19.3-MHz injection
gain of U4. Inductor L18 and swamping resistor R65
signal is applied, the FET's operate in the square law
broadband tune the output of U4 to 10.7 MHz. Resistor
region for mixing action and conversion gain.  The
R65 is test selected to adjust the 10.7-MHz if amplifier
second mixer provides about 12 dB of gain to the if
gain to produce an AGC voltage that sets the squelch
signal. The 10.7-MHz difference frequency is devel-oped
threshold at about 3 microvolts when the squelch control
across the secondary of transformer T3 and is coupled
is set for maximum sensitivity.
Capacitor C92,
to the 10.7-MHz crystal filters (FL1). Capacitors C62 and
temperature-compensation network RT2-R66-R67, and
C63 tune the primary of T4 to 10.7 MHz.
capacitor C93 couple the output of if amplifier U4 to the
input of if amplifier U5. AGC voltage applied through
4-121.  10.7-MHz CRYSTAL FILTER.  The 10.7-MHz
resistor R63 controls the gain of U5. The output of U5 is
crystal filter (FL1 of figure FO-10) determines the
broadband tuned to 10.7 MHz by inductor L20 and
receiver selectivity.  To be compatible with 50-kHz
coupled through capacitor C96 to if amplifier Q6.
channel spacing, the filter provides an if bandwidth of
Capacitor C87 and inductor L21 tune the output of Q16
15 kHz from center frequency with maximum attenua-
to 10.7 MHz. Capacitor C85 couples the output of Q16
tion of 2 dB within this bandwidth.  At 18 kHz from
to voltage divider R72-R73 which provides isolation for
center frequency, the filter provides a maximum of 6 dB
the 10.7-MHz if output. The signal level at the 10.7-MHz
of attenuation. At 40 kHz from center frequency, the
if output is about 10 millivolts when loaded externally by
filter provides a minimum of 80 dB of attenuation.
a 50-ohm termination. Capacitor C86 couples the 10.7-
Variable capacitor C64 and test select capacitor C109
MHz  if  signal  to  the  detector.
Temp-erature-
tune the filter input while variable capacitor C74 tunes
compensation network RT3-R71-C84 in the emitter
the filter output for minimum ripple through the passband
circuit of Q16 and temperature-compensation network
of the filter. Resistors R60 and R61 provide the proper
RT2-R67-R66 between if amplifiers U4 and U5 improve
load to terminate the filter.
closed-loop gain characteristics by making the if gain
less dependent upon temperature. Low-pass filters L17-
NOTE
C77, L19-C94, and C97-R70 decouple each stage of the
10.7-MHz if amplifier from the 12-Vdc line.
To receive wide bandwidth data, the 10.7-
MHz crystal filter can be replaced with a
4-123. DETECTOR. The detector (CR9 of figure FO-
wide bandwidth filter having an if bandwidth
of 25 kHz from center frequency with
20) demodulates the 10.7-MHz if signal and develops the
maximum attenuation of 1 dB. At 30 kHz
detected signal across RC filter network R75-C89. The
detected signal contains the audio signal superimposed
from center frequency the wide bandwidth
on a dc level that is proportional to the carrier level. Bias
filter provides a maximum of 6 dB of
attenuation, and at 60 kHz from center
network R74-CR10-C91 applies a dc voltage equivalent
to one diode drop through inductor L29 to the anode end
frequency the filter provides a minimum of
of detector diode CR9. This biases detector diode CR9
80 dB of attenuation. When installed in the
at the point of conduction to make the AGC voltage
radio set, a special ground pin on the wide-
independent of the diode drop across detector diode
band filter disables the noise blanker circuit.
CR9.
NOTE
4-124. AF AMPLIFIER. The af amplifier (U6A of figure
FO-20) amplifies the detected audio signal and the
To be compatible with 25-kHz channel
carrier derived dc voltage by a voltage gain of about 5.6
spacing, the 10.7-MHz crystal filter can be
volts per volt (V/V). Capacitor C90 couples the amplified
replaced with a narrow bandwidth filter
audio signal through voltage divider R78-R79 to the
having an if bandwidth of 10 kHz from
receiver af output of receiver rf module A3. The audio
center frequency with maximum attenuation
voltage developed across R79 is about 225 millivolts for
of 6 dB. At 25 kHz from center frequency
a 30-percent modulated receive rf input signal.  The
the filter provides a minimum of 80 dB of
amplified dc signal is direct coupled to integrator U6B in
attenuation
the AGC circuit where it is used to develop AGC voltage
to control receiver gain
4-25


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