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INPUT TRANSFORMER AND AUDIO LEVEL STRAPPING
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TM-11-5820-815-14 Service and Circuit Diagrams RADIO SET AN/GRC-171 Manual
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CLIPPER AND LOW-PASS FILTER (U7A)


TO 31R2-2GRC171-2
TM 11-5820-815-14
NAVELEX 0967-LP-544-5010
approximately 100 ohms as the detector-developed
amplifier.  When audio is first applied or the level of
control voltage applied to its gate varies from -12 V dc to
audio is increased, the fast discharge of capacitor C9
0 volt. Through voltage divider action, the audio signal
through resistor R23 sets the attack time at about 4
applied to the noninverting input of amplifier U1A
milliseconds.  When audio is removed or the level is
decreases as the detector output voltage decreases
decreased, the slow charge of C9 through R24 sets the
toward zero.
release time at about 150 milliseconds.
4-148. Amplifier. Amplifier U1A provides a voltage gain
4-151. LOW-PASS FILTER (U1B). The low-pass filter
of about 57 V/V. Together, the variable attenuator and
(U1B of figure FO-21) shapes the high-frequency
amplifier provide a variable compression amplifier gain
response of the output signal from the transmit audio
that varies from about 57 to less than 1. The minimum
compression amplifier. The low-pass filter is a 3-pole,
gain depends upon the minimum turn-on resistance of
active filter with unity gain and cutoff frequency of 6 kHz.
FET Q1.
The three poles are determined by resistor-capacitor
networks R12-C10, R13-C11, and R14-C12.  In the
4-149. Detector. The detector circuit (U2A, U2B, Q2,
passband, the output varies less than +1 or -2 dB when
and Q3) functions as a peak waveform detector to
referenced to the output at 1000 Hz. In the stopband,
reduce compression amplifier gain whenever positive or
the output is down greater than -10 dB at 10 kHz.
negative audio peaks at the output of amplifier U1A try to
exceed the detector threshold (approximately 0.25 volt).
4-152. HIGH-PASS FILTER (U7B). The high-pass filter
Resistor R22 of voltage divider R21-R22 sets the
(U7B of figure FO-21) shapes the low-frequency
detector threshold by developing a bias voltage of about
response of the output signal from the compression
-0.23 V dc that is applied to the inverting inputs of
amplifier that is passed through the low-pass filter (U1B).
amplifiers U2A and U2B. Each amplifier amplifies the
The high-pass filter is a 3-pole, active filter with a voltage
bias voltage by a voltage gain of about 15.4 V/V to
gain of about 3.9 V/V and cutoff frequency of 300 Hz.
produce a positive output voltage of about 3.55 V dc.
The three poles are determined by resistor-capacitor
The high positive voltage biases transistors Q2 and Q3
networks R62-C28, R63-C29, and R64-C30.  In the
into cutoff. With Q2 and Q3 off, -12-V dc power supply
passband, the output varies less than +1 or -2 dB when
voltage applied through resistor R24 charges capacitor
referenced to the output at 1000 Hz. In the stopband,
C9 to about -12 V dc. This voltage, when applied to the
the output is down greater than -10 dB at 100 Hz.
gate of FET Q1, results in maximum FET resistance and
therefore maximum compression amplifier gain. When
4-153. HIGH-PASS FILTER (U3). The high-pass filter
audio is applied to the compression amplifier, the audio
(U3 of figure FO-21) shapes the low-frequency response
signal present at the output of amplifier U1A is coupled
of the wide bandwidth data applied at the data audio
to the inverting input of amplifier U2A through capacitor
input. The high-pass filter is a 3-pole, active filter with a
C4 and resistor R15 and to the noninverting input of
voltage gain of about 9 V/V and cutoff frequency of 16
amplifier U2B through capacitor C5.  Each amplifier
Hz. The three poles are determined by resistor-capacitor
amplifies the audio signal by a gain of about 16.2 V/V to
networks R26-C14, R28-C15, and R27-C16.  In the
produce an ac output voltage that is superimposed on
passband, the filter response varies less than +1 or -3 dB
the amplified bias voltage applied to Q2 and Q3. When
when referenced to the output at 1000 Hz. Because of
positive peaks of the output audio signal from U1A
the low cutoff frequency, high-pass filter U3 does not
exceed the detector threshold, the ac output voltage from
affect the frequency response of the voice audio signal
U2A forward biases Q2 for the period of time the peak
from high-pass filter U7B. It does, however, amplify the
voltage exceeds the threshold.  In the same manner,
voice audio signal by a voltage gain of about 9. Variable
when negative peaks exceed the detector threshold, the
resistor R107 allows the output of the high-pass filter to
ac output voltage from U2B forward bias Q3. When Q2
be set up for proper modulation level when transmitting
and/or Q3 are forward biased, they turn on and allow
wide bandwidth data.
capacitor C9 to discharge through resistor R23.  The
ratio of on time to off time determines the average
4-154.  PERCENT MODULATION CONTROL.  The
negative voltage developed across C9.  This voltage,
transmit audio output signal from audio module A4
when applied to the variable attenuator (gate of Q1),
(A4P1-13/A10J13-13) is fed to the percent modulation
controls the compression amplifier gain to maintain the
(% MOD) control located on the front panel of receiver-
peak voltage at the output of U1A at the detector
The percent modulation control allows
threshold. Through this circuit action, the compression
adjustment of the audio level applied to the modulator
amplifier output voltage is held at about 180 millivolts
circuit of power amplifier A8 to set up the modulation
(rms) for the approximate 14- to 250-millivolt range of
percent of the transmitted rf signal.
The percent
audio input signals.
modulation control (figure FO-16) consists of resistors
A10A1R6 and A10A1R7 and variable resistor A10A1R2.
4-150. Resistors R24 and R23 and capacitor C9 deter-
Resistor
A10A1R7
and
mine the attack and release time of the compression
4-29


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