RECEIVER OPERATION. (cont)

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

4-24. During receive mode, the incoming rf signal from

transmitter. The receive signal is also applied to the

the antenna is applied through the directional coupler to

compression amplifier.

The compression amplifier

the rf filter of rf filter module A7.

To eliminate

maintains a constant output level for variations in percent

interference from collocated equipment, the rf filter is

modulation of the receive signal. The output of the

tuned (as described in paragraph 4-19) to the radio

compression amplifier is passed through a high-pass

control frequency.

This prevents large out-of-band

filter and a low-pass filter that together shape the audio

signals from being applied to receiver rf module A3. The

response of the receive audio signal. The output of the

output from the rf filter is applied to the transmit/receive

low-pass filter is then applied to the receive audio (RCV

switch (actually a diode type switch) that also acts as an

AUDIO) control and to the headset volume (VOL) control

rf attenuator to provide front-end signal attenuation. The

located on the front panel of the receiver-transmitter.

amount of attenuation is controlled by the rf attenuation

The RCV AUDIO control is adjusted to set the receiver-

(AGC) signal from the AGC amplifier of receiver rf

transmitter audio output level, and the VOL control is

module A3 and is dependent upon rf signal strength.

adjusted to set the headset audio level. The output of

When the trans- mitter is un-keyed, the slow release of

the RCV AUDIO control is applied to the 100-milliwatt

key 1 causes the receive rf output of the rf filter module

power amplifier of audio module A4. The output of the

transmit/ receive switch to be routed to the uhf mixer of

power amplifier is transformed into two 150-ohm lines by

receiver rf module A3. Also, when the transmitter is un-

transformer A4T2 and then applied through chassis A10

keyed, the fast release of key 2 causes the receive

emi filter to the audio output of the receiver-transmitter.

injection output of the frequency synthesizer module

The output of the VOL control is applied to a second

transmitter receive switch to be routed to the uhf mixer.

100-milliwatt power amplifier in audio module A4. The

The receive injection signal is generated by the same

output of this amplifier is transformed into a 600-ohm

phase-locked loop that generates the transmit rf signal.

output by transformer A4T3 and then applied to the

However, when the transmitter is un-keyed, the key 1

headset jack on the front panel of the receiver-

key line and the 30-MHz decode circuit of frequency

transmitter through chassis A10 emi filter to headset

synthesizer module A2 cause the phase- locked loop to

audio out (J22-k).

shift down in frequency by 30 MHz. This results in a

receive injection signal that is 30 MHz less than the

4-26. During the transmit mode, sidetone audio is

selected radio control frequency. The slow release of

taken from the output of the forward power amplifier in rf

key 1 assures that the switch to receive mode has been

filter module A7 and applied to the SIDETONE control on

made before the phase- locked loop is shifted in

the front panel of the receiver- transmitter. The output of

frequency.

the SIDETONE control is applied to the compression

amplifier to provide sidetone audio in the output audio

4-25. In the receiver rf module, the uhf mixer mixes

signal or in the headset during transmit mode.

the receive rf signal (225 to 399.975 MHz) with the

receive injection signal (195 to 369.975 MHz) to produce

4-27. For receive squelch, the squelch sample signal

the 30-MHz if signal that is applied to the 30-MHz if

developed from AGC voltage is applied to the squelch

amplifier. The 30-MHz if signal is split into two signal

control circuit of audio module A4. Also applied to the

paths; one to the second mixer and one to the noise

squelch control circuit is the squelch reference input from

channel. The 30-MHz if signal that is applied to the

the SQUELCH control on the front panel of the receiver-

second mixer is mixed with the 19.3-MHz output signal

transmitter. The squelch control circuit compares the

from the injection amplifier to produce the 10.7-MHz if

squelch sample signal, which is a function of the level of

signal. Impulse type noise that is received at the

the incoming rf signal, to the squelch reference input.

antenna is routed through the noise channel where it is

The output of the squelch control circuit operates the

used to disable the 19.3-MHz oscillator input to the

squelch gate to mute the receive audio whenever the

second mixer. This action mutes the receive so that the

incoming rf signal is below the squelch threshold (as set

noise pulse does not appear at the receiver audio output.

by the SQUELCH control). When operating remotely

The output of the second mixer is applied to the 10.7-

through the front panel REMOTE/LOCAL switch, the

MHz if amplifier where the 10.7-MHz if signal is amplified

squelch can be disabled by the SQLCH ON/OFF switch

and applied to the detector and to the if jack of the

of C-7999/ GRC-171. When operating locally, the

receiver- transmitter. The detector demodulates the

squelch can be disabled by the front panel SQUELCH

amplitude modulated signal to produce AGC voltage and

ON/OFF switch.

the receive audio signal (or data signal) to audio module

A4. The AGC voltage is amplified by the AGC amplifiers

4-28. The output from the squelch control circuit of

and applied to the 10.7-MHz if amplifiers, 30- MHz if

audio module A4 is also applied to keyer module A9.

amplifiers, and to rf filter module A7 to control the gain of

When the squelch is turned on and the incoming rf signal

the receiver. The receive signal is applied to the

is above squelch threshold, the squelch com- parator

bandpass filter.

The bandpass filter shapes the

and relay driver circuit of keyer module A9 causes

frequency response of the receive data signal and

squelch relay A9K1 to energize.

The squelch

applies it to the data output of the receiver-

4-4