COMPRESSION

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

VR3-VR4. Pad R65-R66 provides a 10-kilohm load to

ready lamp (remote operation) or chassis A10 ready

the receive data output line. Zener diodes VR3 and VR4

lamp (local operation). Figure FO-16 shows ready lamp

provide protection to the audio circuit against high-

circuit. When off, key 2 inhibit transistor Q9 allows keyer

voltage noise spikes that may be present on the receive

transistor Q10 to be turned on and off by key driver

data audio output line from the radio set.

transistor Q6. Under the above conditions, a ground

applied to the ptt input (P1-K) turns key driver transistor

4-169.

COMPRESSION

AMPLIFIER

(RECEIVE

Q6 on which, in turn, turns keyer transistors Q7 and Q10

AUDIO).

The receive audio compression amplifier

on to ground the key 1 (P1-5) and key 2 (P1-M) key

(figure FO-21) is composed of the variable attenuator

lines. When a fault is detected, the logic 0 fault signal

circuit (Q11), amplifier circuit (U5A), and detector circuit

applied to the p11 fault or rf filter fault inputs turns fault

(Q13). Functionally, the compression amplifier maintains

switch Q8 off. This ungrounds the ready lamp circuit to

a constant (within 3 dB) audio output signal (about 1.6

turn off the ready lamp. In addition, Q8 turns on key 2

volts rms at test point TP13) for voice audio input signals

inhibit transistor Q9 to turn off keyer transistor Q10. This

(receive af input, P1-7) that vary as a result of the

ungrounds the key 2 key line to unkey the power

percent modulation of the received rf signal.

amplifier. Key 1 key line remains grounded to mute the

Compression begins when the input signal exceeds

receiver.

about 0.22 volt rms.

4-164. Capacitor C21, diode CR6, and resistors R46

4-170. Compression is achieved by controlling the

and R47 provide fast attack-slow release keying to the

attenuation ratio of variable attenuator Q11 which in turn

key 1 keyer circuit. When Q6 turns on (ptt input

controls the level of audio applied to amplifier U5A.

grounded), the fast charge of capacitor C21 through CR6

Negative audio peaks that exceed the threshold of

provides fast attack to ground key 1. When Q6 turns off,

detector Q13 (about 2.24 volts peak or 1.6 V rms) are

the slow discharge (relative to charge time) of capacitor

converted into a dc control voltage and applied to the

C21 through R46 and R47 provides slow release to

gate of variable attenuator FET Q11 to control the

unground key 1.

attenuation ratio of the variable attenuator. As the audio

input signal increases, detector Q13 decreases the

4-165. Capacitor C22 and resistors R45 and R50 pro-

control voltage applied to the gate of variable attenuator

vide slow attack-fast release keying to the key 2 keyer

Q11 to increase its attenuation ratio. This reduces and

circuit. When Q6 turns on, the charge of capacitor C22

maintains the audio level applied to the amplifier input at

through resistor R45 provides slow attack to ground key

the level necessary to hold the output of the amplifier just

2. When Q6 turns off, the fast discharge (relative to

at the detector threshold. The compression amplifier has

charge time) of capacitor C22 through R50 provides fast

an attack time of about 4 milliseconds and a release time

release to unground key 2.

of about 150 milliseconds.

4-166. BANDPASS FILTER/'AMPLIFIER. The band-

4-171. Variable Attenuator. The variable attenuator is

pass filter/amplifier (U4 of figure FO-21) amplifies and

composed of resistors R67 and R68 and FET Q11 that

shapes the frequency response of the received wide

together function as a variable voltage divider. FET Q11

bandwidth data signal from receiver rf module A3

acts as a variable resistor that changes from infinite

(receive af input, P1-7). The filter is a 6-pole, active filter

resistances to some finite value as the detector-

with a voltage gain of about 4.2 V/V and cutoff

developed control voltage applied to its gate decreases

frequencies of 16 Hz and 25 kHz. The three poles that

from -12 V dc toward 0 volt.

control the low-frequency response are determined by

resistor-capacitor networks R57-C23, R56-C24, and

4-172. Amplifier. Amplifier U5A provides a voltage gain

R58-C25.

The three poles that control the high-

of about 48 V/V. Together, the variable attenuator and

frequency response are determined by resistor-capacitor

amplifier provide a variable compression amplifier gain

networks R112-C69, R113-C70, and R114-C67. In the

that varies from about 7.1 V/V to less than 0.64 V/V. The

passband, the output varies less than +1 or -3 dB when

minimum gain depends upon the minimum turn-on

referenced to the output at 1000 Hz.

resistance of FET Q11. In addition, amplifier U5A

provides a voltage gain of about 3.7 V/V to sidetone

4-167. Bias voltage of about +0.64 V dc developed

audio. During transmit mode, transmit audio detected at

across resistor R59 of voltage divider R60-R59 biases

the antenna (rf filter module) is fed back and inserted

the output voltage of U4 to approximately 5 volts with no

into the receive audio path at this point so that it appears

input audio signal. This allows the output of U4 a

as sidetone in the audio output. Receive audio is muted

maximum swing of approximately 17 volts when an

by receiver rf module A3 during this time. During receive

audio input signal is applied.

mode, sidetone mute switch Q18 is turned on by the

logic 1 key 1 key line. This grounds the sidetone input to

4-168. Capacitor C32 couples the output of U4 to the

amplifier U5A.

receive data audio output (P1-15) through impedance

matching pad R65-R66 and transient protection circuit

4-31