RECEIVER SIGNAL PATH-continued

TM 11-5820-401-34-3/0967-LP-432-3060

1-14.

RECEIVER SIGNAL PATH. (CONT)

Very low amplitude rf is applied to the first rf Amplifier A1200 module (2) which provides the first stage

of amplification. A second stage of amplification is provided by the second rf Amplifier A1300

module (3).

Amplified rf is then applied to the Mixer A1400 module (4), where the signals are heterodyned

(mixed) with the output of the local oscillator (5) in the A1500 module. For all MC-TUNE-KC settings

in the

band range, the local oscillator runs exactly 11.5 MHz above the frequency to which the

receiver A1100 module (1) is tuned. For all MC-TUNE-KC settings in the

band range, the local

oscillator runs exactly 11.5 MHz below the receiver's tuned frequency.

The A1400 module (4) produces sum and difference frequencies as a result of the mixing process. All

frequencies, except the 11.5-MHz intermediate frequency, are rejected by the 11.5-MHz Crystal

Filter FL4001 (6).

The intermediate frequency (if) is amplified by the first and second IF Amplifier A4100 module (7),

filtered by FL4002 (8) to eliminate any unwanted frequencies, and further amplified by the third and

fourth IF amplifiers in the A4200 module (9).

In the fifth IF Amplifier and Limiter A4200 (10), the 11.5-MHz rf is again amplified and if noise spikes

are clipped. The limiter clips noise, that is, excessive amplitude, from the if to prevent distortion in

the demodulated audio output of the Travis-type discriminator. Both positive and negative spikes are

clipped, keeping the if amplitude constant. Several if amplification stages are used to insure that the

signals applied to the limiter are strong enough to reach the limiter's minimum clipping amplitude. If

the signals applied to the limiter are too low in amplitude, the limiter cannot clip the upper and lower

wave peaks; therefore, the if applied to the discriminator can vary in amplitude, resulting in distorted

audio.

SILICON VERSION THIRD IF AMPLIFIER/LIMITER/DISCRl MINATOR A4200A

The silicon version of the A4200 module contains silicon, instead of germanium transistors. Discrete

fourth and fifth if amplifier stages are not required because an integrated circuit quadrature detector

is used which incorporates amplification, limiting, and discriminating stages. The quadrature

detector also simplifies alinement.

Demodulated audio output from the discriminator is applied to Audio and Squelch Preamplifier A4300

(12) for initial amplification. The A4300's output is fed to Audio Filter FL5001 (13), and by a parallel

circuit to Squelch Amplifier A5200 (14). However, in the OFF positions of the front panel SQUELCH

switch, the squelch amplifiers and related circuits are essentially inoperative. The reason for this is as

follows:

Audio signals are amplified by the Monitor Amplifie r A5100 (15), whose output is not volume con-

trolled, and by Audio Amplifier A5100 (16), the output of which is determined by the position of the

VOLUME control. Both amplifiers require 16 vdc to operate. In the OFF position of the SQUELCH

switch, the amplifiers get their 16-vdc power directly from the SQUELCH switch (17); therefore, the am-

plifiers operate independent of the squelch circuits. The squelch function is covered in greater detail

Output from the audio amplifier (16) is applied to Audio Transformer T5100 (18) which provides

unmuted audio, and muted audio through Muting Circuit R5117 (19).

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