SQUELCH CIRCUIT - TM-11-5820-815-140077

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

4-173. Detector. The detector circuit functions as a

about +0.29 volt for minimum setting of squelch

peak detector to reduce compression amplifier gain

potentiometer A10A1R4 (maximum squelch sensitivity)

whenever negative audio peaks at the output of amplifier

to about +1.35 volts for maximum setting of A10A1R4

U5A try to exceed the detector threshold (approximately -

(minimum squelch sensitivity). For no rf input signal or

2.24 volts). Resistor R74 of voltage divider R75-R74

for rf input signals below squelch threshold, the absolute

sets the detector threshold by developing a bias voltage

value of the negative AGC voltage is less than the

of about +1.54 V dc that reverse biases transistor Q13.

positive squelch reference bias. This results in a positive

With Q13 off, -12-V dc power supply voltage applied

voltage at the summing point of R79 and R80 and, thus,

through resistor R76 charges capacitor C36 to about -12

a negative output voltage from U5B (approximately -10

V dc to turn FET Q11 completely off. When audio is

Vdc). This forward biases squelch gate Q12 to turn it on.

applied to the compression amplifier, output audio from

When on, Q12 shunts receive audio to ground to mute

U5A appears at the base of transistor Q13

the receiver output. When the rf input signal exceeds

superimposed on the bias voltage.

When negative

the squelch reference, the voltage applied to U5B goes

audio peaks exceed about -2.24 volts, the voltage

negative. This causes the output of U5B to go positive

applied to the base of Q13 exceeds the bias voltage plus

(approximately +10 V dc) to reverse bias Q12 and

base-to-emitter junction voltage of Q13. This brings Q13

unmute the receive audio. To prevent random muting of

into conduction to allow capacitor C36 to discharge

the receiver when the rf input is right at squelch

through resistor R117 for the length of time Q13 is turned

threshold, resistors R84 and R85 form a positive

on. The ratio of on time to off time determines the

feedback path around U5B to provide about 20 millivolts

average negative voltage developed across C36. This

of hysteresis to the input signal.

voltage, when applied to the variable attenuator (gate of

Q11), controls the compression amplifier gain to

4.176. A ground on the squelch on/off line disables the

maintain negative peaks at the output of amplifier U5A at

squelch circuit by grounding the base of squelch gate

about -2.24 volts. Through this circuit action, the audio

Q12. Thus unmutes the receive audio independently of

output of the compression is held at about 1.6 V rms

the output of squelch control U5B.

over the range of the variable attenuator which is

approximately 25 dB. Diodes CR10 and CR11 clip

4-177. HIGH-PASS FILTER (U6A). The high-pass filter

positive peaks that exceed approximately 3 volts.

(U6A of figure FO-21) shapes the low-frequency

Capacitor C72, resistor R121, and diode CR14 comprise

response of the output signal from the receive audio

an averaging detector which reduces the reverse-bias

compression amplifier. The high-pass filter is a 3-pole,

voltage on Q13 when receiving a high peak-to-average

active filter with unity gain and cutoff frequency of 300

receive audio signal. This regulates the compression

Hz. The three poles are determined by resistor-capacity

amplifier output and compensates for average power

networks R86-C39, R88-C40, and R87-C41. In the

output changes that would occur between high peak-to-

passband, the output varies less than +1 or -2 dB when

average and low peak-to-average receive audio signals.

referenced to the output at 1000 Hz. In the stopband,

the output is down more than -10 dB at 10 kHz.

4-174. Resistors R117 and R76 and capacitor C36

determine the attack and release time of the compres-

sion amplifier. When audio is first applied to the

4-178. LOW-PASS FILTER (U6B). The low-pass filter

compression amplifier or the level of audio increases, the

(U6B of figure FO-21) shapes the high-frequency

fast discharge of capacitor C36 through resistor R117

response of the output signal from the compression

sets the attack time at about 4 milliseconds. When audio

amplifier that is passed through the high-pass filter

is removed or the level of audio decreases, the slow

(U6A). The low-pass filter is a 3-pole, active filter with

charge of C36 through R76 sets the release time at

unity gain and cutoff frequency of 3000 Hz. The three

about 150 milliseconds.

poles are determined by resistor-capacity networks R-

89-C43, R90-C45-C44, and R91-C46. In the passband,

4-175. SQUELCH CIRCUIT. Refer to figures FO-21

the output varies less than +1 or -2 dB when referenced

and 4-7. The squelch control (U5B) compares AGC

to the output at 1000 Hz. In the stopband, the output is

voltage to a squelch reference input voltage from the

down more than -10 dB at 10 kHz.

squelch potentiometer (A10A1R1 of figure 4-7) to mute

the receiver whenever the rf input signal to the receiver is

4-179. HEADSET VOLUME AND RECEIVE AUDIO

below squelch threshold as determined by the squelch

LEVEL CONTROLS. The output signal (audio to gain

potentiometer setting.

Summing network R79-R80

controls) from audio module A4 (A4P1-S/A10J13-S) is

compares negative AGC voltage (squelch sample) to the

fed to the headset volume control (VOL) and the receive

positive squelch reference bias developed across

audio level control (RCV AUDIO) located on the front

resistor R82 and applies the resulting sum to the

panel of the receiver-transmitter. The headset volume

inverting input of squelch control U5B.

Squelch

control (A10A1R1 of figure FO-15) allows adjustment of

reference bias developed across R82 of voltage divider

the audio level to the headset. The receive audio level

R82-R83-R81

varies

from

control

(A10A1R3

figure

FO-15)

4-32