NOISE CHANNEL AGC AMPLIFIER

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

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