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UHF MIXER
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TM-11-5820-815-14 Service and Circuit Diagrams RADIO SET AN/GRC-171 Manual
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10.7-MHz CRYSTAL FILTER


TO 31R2-2GRC171-2
TM 11-5820-815-14
NAVELEX 0967-LP-544-5010
4-112. NOISE CHANNEL AGC AMPLIFIER. The noise
4-115.  NOISE BLANKER GATE.  The noise blanker
channel AGC amplifier (Q5 and Q6 of figure FO-20) is a
gate (Q15 of figure FO-20) is a transistor switch that
2-transistor, dc coupled, high-gain amplifier.
The
turns the injection amplifier (Q13) off whenever there is a
amplifier amplifies small changes in carrier-derived dc
noise blanking pulse generated by the one-shot.  The
voltage developed across resistor R19 into the proper
noise blanking pulse applied through R36 causes
level to control the gain of noise channel if amplifiers U2
transistor Q15 to turn on. When on, Q15 shunts base
and U3. Resistor R13 is test selected to set the noise
current from transistor Q13 to turn it off. With Q13 off,
channel if amplifier gain for optimum noise blanking.
the 19.3-MHz injection signal to the second mixer is
Capacitor C46 filters the audio component super-
removed causing the receiver to mute for about 8
imposed on the dc voltage.  Noise blanking can be
microseconds.  This prevents the noise pulse which
disabled by applying a ground to the external noise
initiated the triggering of the one-shot from appearing at
blanker disable line (P1-15). This turns transistor Q6 off
the receiver output.
which, in turn, causes maximum AGC voltage to be
applied to U2 and U3 to mute (disable) the noise channel
4-116. Applying a ground to the noise blanker disable
if amplifier.
line or installing a wide bandwidth 10.7-MHz crystal filter
(FL1), which has a special grounding pin, disables the
4-113. NOISE PULSE DETECTOR. The noise pulse
noise blanker gate making it nonresponsive to noise
detector (Q7 of figure FO-20) processes impulse type
blanking pulses from the one-shot.
noise to provide a negative-going trigger pulse to the
one-shot (Q9, Q8). Capacitor C49 and resistor R22 form
4-117. FILTER/DELAY. The filter/delay circuit (FL2 of
a differentiator that is responsive to the fast rise time of
figure FO-20) is a triple-tuned interstage filter that
impulse type noise but nonresponsive to the slow rise
terminates the first if amplifier in the main if channel.
time of voice audio.  This allows impulse noise to be
The filter/delay provides about a 1.2-microsecond delay
coupled to the base of transistor Q7 to turn it on. When
to the 30-MHz if signal so that the noise blanking pulse
Q7 turns on, capacitor C50 (which is charged to about
can disable the second mixer before the noise pulse
+12 V dc) discharges through Q7 to develop a negative
arrives there. Variable capacitors C14, C16, and C18
trigger pulse at the base of one-shot transistor Q9.
tune the filter/delay to pass the 30-MHz center frequency
Negative voltage developed across resistor R29 of
and provide about 54-dB rejection of signals at 1.5 MHz
voltage divider R29-R28 speeds up the discharge of
from center frequency.  Capacitor C28 couples the
capacitor C50 to produce a fast rise time trigger pulse,
output of the filter/delay circuit to the second if amplifier.
and therefore, faster triggering of the one-shot.
4-118. SECOND 30-MHz IF FILTER. The second 30-
4-114. ONE-SHOT. The one-shot circuit (Q8 and Q9 of
MHz if amplifier (Q12 of figure FO-20) is a temperature-
figure FO-20)  is  a  monostable  multivibrator  that
compensated rf amplifier that compensates for gain
generates a pulse at the collector of Q9 whenever there
variations within the amplifier and second mixer. The
is a negative-going trigger pulse applied to the base of
room temperature gain is about 8 dB. Thermistor RT1 is
Q9. In the stable state, +12 V dc applied through resistor
the temperature-sensitive component that varies the gain
R32 biases Q9 on, and the negative voltage developed
of the amplifier.  As the temperature increases, the
across R29 and applied through resistor R30 biases Q8
resistance of RT1 decreases.  This causes the ac
off. During this time, C51 charges to approximately 12 V
impedance of the emitter circuit of Q12 to decrease
dc. Whenever a noise pulse is detected, capacitor C50
which, in turn, causes the gain of the second 30-MHz if
couples a negative-going pulse to the base of Q9
amplifier to increase with temperature. Transformer T3
causing it to turn off. When Q9 turns off, capacitor C52
couples the output of the second 30-MHz if amplifier to
couples the rising collector voltage of Q9 to the base of
the second mixer.
Q8, turning it on. Transistor Q9 collector voltage applied
through resistor R33 keeps Q8 biased on once the
4-119.
19.3-MHz OSCILLATOR AND INJECTION
collector voltage reaches its final value of +12 V dc.
AMPLIFIER. The 19.3-MHz oscillator (Q14 of figure FO-
With QB on, capacitor C51 applies a negative voltage to
20) is a crystal-controlled oscillator that generates the
the base of Q9 to keep it turned off and the one-shot in
19.3-MHz local oscillator frequency for the second mixer.
its unstable state.  Transistor Q9 remains off for the
Variable capacitor C71 adjusts the oscillator frequency to
length of time (about 8 microseconds) required for C51
exactly 19.3 MHz. The injection amplifier (Q13) amplifies
to discharge through R32 and Q8. When C51 begins to
the 19.3-MHz signal to about 2 V rms and applies it to
charge positive, the positive voltage applied to the base
the local oscillator port of the second mixer.
of Q9 turns Q9 on which, in turn, turns Q8 off; and the
one-shot reverts back to its stable state.  The pulse
4-120. SECOND MIXER. The second mixer (Q10 and
generated at the collector of Q9 is the noise blanking
Q11 of figure FO-20) is an active balanced mixer that
pulse applied to the noise blanker gate.
mixes the 30-MHz if signal with the 19.3-MHz injection
signal to produce the 10.7-MHz if signal
4-24


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