AUDIO MODULE A4.

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

rf filter module. The series attenuator begins to provide

transistor Q19 off. This turns mute switch Q18 on.

attenuation to the rf signal when rf attenuator (AGC)

When on, Q18 applies about 9 V dc to the 10.7-MHz

voltage exceeds about 3.4 V dc and provides maximum

AGC line which, in turn, causes AGC amplifiers U7B and

attenuation after the voltage exceeds approximately 5 V

Q17 to apply about 10 V dc to the 30-MHz AGC line and

dc. Refer to the circuit description of rf filter module A7

about 11 V dc to the rf attenuator (AGC) line. This high

(paragraph 4-221) for circuit operation of the series

AGC voltage mutes the receiver by turning the receiver

attenuator. The gain reduction by the 10.7-MHz if

gain down to provide maximum attenuation to the rf

amplifiers and the gain reduction by the series attenuator

signal.

overlap so there is a smooth transition at the crossover

point.

4-135. AUDIO MODULE A4.

4-132. For AGC voltage above -2 volts, the third stage of

4-136. Refer to the block diagram of figure FO-7 and

gain reduction is controlled by rf attenuator (AGC)

schematic diagram of figure FO-21 while reading the

voltage applied to the shunt attenuator in the rf filter

following circuit description. Unless otherwise specified,

module and by 30-MHz AGC voltage applied to shunt

reference designators apply to components of audio

attenuators (CR1 and CR6) in the receiver rf module.

module A4.

Refer to the circuit description of rf filter module A7

(paragraph 4-221) for circuit operation of the shunt

4-137. GENERAL. Audio module A4 is composed of a

attenuator in that module. In receiver rf module A3, the

transmit audio circuit, receive audio circuit with squelch,

shunt attenuator (CR1) at the output of the uhf mixer

a ptt keyer circuit, and a keyer control circuit. The

(U1) functions as follows: When forward biased, pin

transmit audio circuit receives voice audio from the main

diode CR1 acts as an rf attenuator by shunting part of

audio input and microphone input or wide bandwidth data

the 30-MHz if signal to ground through capacitor C21.

from the data audio input. Voice audio that can range in

The amount of attenuation is controlled by the amount of

level from -15 to +10 dB mW is coupled through the input

forward bias applied through the parallel combination of

transformer to the audio level strapping circuit. For

diode CR2 and resistor R25. For 30-MHz AGC voltage

normal strapping, the voice audio is attenuated by 20 dB

below about 3.8 V dc, negative voltage developed across

and applied to the compression amplifier. For voice

resistor R26 of voltage divider R24-R26 reverse biases

audio levels that range from -35 to -15 dB mW, optional

CR1 so that it offers no attenuation to the 30-MHz if

audio level strapping couples the signal directly to the

signal. As the 30-MHz AGC voltage increases above 3.8

compression amplifier without attenuation. To prevent

V dc, pin diode CR1 becomes forward biased to begin

overmodulation of the transmitter, the compression

attenuating the 30-MHz if signal. Maximum attenuation

amplifier maintains a constant output level for input

(approximately 25 dB of receiver gain reduction) is

levels that vary from -35 to +10 dB mW depending on

attained when the AGC voltage exceeds about 4.4 V dc.

the strapping option used. In addition, the compression

In a similar manner, pin diode CR6 attenuates the 30-

amplifier amplifies the microphone input to hold

MHz if signal at the output of the second if amplifier

microphone audio at the same constant output level.

(Q12).

The constant level output signal from the compression

amplifier is passed through a low-pass filter followed by a

4-133. The 30-MHz AGC voltage is developed as

high-pass filter to shape the audio response so that the

follows: When the AGC voltage is -1 V dc, AGC

modulation amplitude does not vary by more than +1 to -

amplifier Q17 amplifies the output of U7B to produce a

2 dB from 300 to 6000 Hz. The output of the high-pass

30-MHz AGC voltage of about 3.4 V dc. This is just

filter passes through a second high-pass filter to the

below the turn-on point of the shunt attenuators. As

percent modulation control. The percent modulation

AGC voltage increases above -1 V dc, pin diodes CR1

control sets the modulation audio level to modulate the

and CR2 of the shunt attenuators begin to turn on to

transmitter at the desired modulation percentage. The

provide receiver gain reduction. Once the receiver input

output of the percent modulation control is passed

signal increases to the point where the AGC voltage

through a clipper to remove audio peaks that could

exceeds about -2 V dc, the 30-MHz AGC voltage

overmodulate the transmitter. The output of the clipper

exceeds 4.7 V dc and the shunt attenuators (including

passes through a second low-pass filter to provide

the shunt attenuator of rf filter module A7) take over to

transmit audio to the modulator circuit of power amplifier

provide receiver gain reduction. The gain reduction of

module A8. The second high-pass and low-pass filters

the series attenuator overlaps the gain reduction of the

have no effect (other than signal amplification) on the

shunt attenuators so there is a smooth transition of the

audio response of the voice audio since they have cutoff

crossover point.

frequencies of 16 Hz and 25 kHz respectively. These

filters shape the frequency response of the wide

4-134. RECEIVER MUTE. The receiver mute circuit

bandwidth data so that the modulation amplitude does

(Q19 and Q18 of figure FO-20) turns down the receiver

not vary by more than -1 to -3 dB from 16 Hz to 25 kHz.

gain to mute the receiver whenever the transmitter is

The modulation level of the wide bandwidth data is set by

keyed. When the transmitter is keyed, the ground

a potentiometer at the data audio input.

applied through resistor R98 turns mute

4-27