INPUT TRANSFORMER AND AUDIO LEVEL STRAPPING

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

4-138. The receive audio circuit receives detected audio

inhibited by the keyer control circuit.

(receive af input) from receiver rf module A3. The

receive af signal is amplified and passed through the

4-143. The following describes individual circuit func-

bandpass filter to the receive data audio output. The

tions of audio module A4. Refer to figure FO-21.

bandpass filter shapes the response of the signal so that

its amplitude does not vary by more than +1 to -3 dB

4-144. INPUT TRANSFORMER AND AUDIO LEVEL

from 16 Hz to 25 kHz. The receive af signal is also

STRAPPING. Transformer T1 (figure FO-21) termi-

applied to the compression amplifier. The compression

nates 150- or 600-ohm main audio input lines. Con-

amplifier maintains a constant output level for input audio

nected in series (P1-C to P1-D), the primary windings of

levels that vary as a function of the percent modulation of

T1 provides 600-ohm termination; individually, the

the receive rf signal. The constant level output signal

primary windings provide 150-ohm termination. Voice

from the compression amplifier is passed through a high-

audio applied to the main audio input is coupled through

pass filter followed by a low-pass filter to shape the audio

transformer T1 to the audio level strapping circuit where

response of the signal so that the signal amplitude does

voltage divider R2-R3-R4 attenuates the -15- to +10-dB

not vary by more than +1 or -2 dB from 300 to 3000 Hz.

mW audio signal by about 20 dB (normal strapping).

The audio level from the low-pass filter is adjusted by the

The attenuated audio signal (-35 to -10 dB mW) is

headset volume control and applied to the power

applied to the variable attenuator (R6-Q1) of the

amplifier to drive the headset. Also, the audio level from

compression amplifier circuit. For -35- to -15-dB mW

the low-pass filter is adjusted by the receive audio level

voice audio input signals, the optional -35-dB mW audio

control and applied to a second power amplifier to set

level strapping (strap E2 to E3 removed and strap E1 to

the audio level at the main audio output. Both outputs

E2 added) applies the output of T1 directly to the variable

are capable of 100 milliwatts of output power.

attenuator.

4-139. During transmit mode, transmit audio is detected

4-145. The microphone input is coupled directly to the

at the antenna (rf filter module A7) and fed back as

variable attenuator through coupling capacitor C1 and

sidetone audio. This audio is applied to the amplifier in

resistor R7. A -12 V dc applied through resistors R10

the compression amplifier circuit so that sidetone audio

and R11 provide dc current to the carbon microphone.

can be heard at the receive audio output (headset).

Resistor R10 provides a load to the microphone.

4-140. The squelch control and squelch gate provide

4-146.

COMPRESSION AMPLIFIER (TRANSMIT

receiver muting. AGC voltage that is related to rf signal

AUDIO). The transmit audio compression amplifier

strength is compared by the squelch control to the

(figure FO-21) consists of the variable attenuator circuit

squelch reference input (squelch potentiometer) that sets

(Q1), amplifier circuit (U1A), and detector circuit (U2A,

the squelch threshold. When the rf input signal to the

U2B, Q2, Q3). Functionally, the circuit maintains a

radio set decreases below the squelch threshold, as set

nearly constant audio output signal (about 180 millivolts

by the squelch potentiometer, the squelch gate mutes

rms at test point TP16) for audio input signals (test point

the receiver. The squelch on/off line disables the

TP6) that range from about 14 (-35 dB mW) to about 245

squelch gate whenever the squelch is turned off.

millivolts (-15 dB mW). Compression is achieved by

controlling the attenuation ratio of variable attenuator Q1

4-141. Keying information received at the main audio

which in turn controls the level of audio applied to

input is applied to the ptt keyer. The ptt keyer has

amplifier U1A. Positive or negative audio peaks from the

strapping options that allow selection of normal ground

output of amplifier U1A that exceed the threshold of the

ptt keying or optional 6-, 26-, or 48-volt keying. The

detector (about 250 millivolts peak or 180 millivolts rms)

remote ptt output of the keyer is connected to the

are converted into a dc control voltage and applied to the

REMOTE/LOCAL switch (front panel of chassis A10)

gate of variable attenuator Q1 to control the attenuation

which applies the remote ptt keying signal to the ptt input

ratio of the variable attenuator. As the audio input signal

of the keying control circuit whenever the radio set is

increases, the detector reduces the control voltage

operating in the remote mode.

applied to the variable attenuator to increase its

attenuation ratio. This reduces and maintains the audio

4-142. The keying control circuit receives ptt keying and

level at the input to amplifier U1A at the level necessary

radio set fault information and provides at its output two

to hold the output of the amplifier just at the detector

transmitter key lines and a fault switch circuit. The fault

threshold. The compression amplifier attack time is

switch circuit turns on the ready lamp (radio control for

about 4 milliseconds; release time is about 150

remote operation or front panel of chassis A10 for local

milliseconds.

operation). When a synthesizer fault (ptt fault input) or rf

filter servo fault (rf filter fault input) is detected, the ready

4-147. Variable Attenuator. The variable attenuator

lamp is turned off and the key 2 key line is

consists of resistor R6 and FET Q1 that together function

as a variable voltage divider. The drain to source

resistance of FET Q1 varies from infinity to

4-28