Phase Discriminator - TM-11-5820-670-300044

TM 11-5820-670-30

1-34. MASTER OSCILLATOR FREQUENCY CONTROL LOOP. (CONT)

Phase Discriminator

The phase discriminator performs the following four functions:

Phase comparison

Correction voltage generation

Demodulation

Filtering

Two rf signals are applied to phase discriminator (8) for comparison. One is the 11.5 MHz signal from

first and second if. amplifiers (6). This frequency may not be exactly 11.5 MHz. The other input is an

11.5 MHz signal from 11.5 MHz modulator (7). This signal contains audio intelligence and is pre-

cisely controlled at 11.5 MHz by a crystal-controlled oscillator.

A difference in frequencies of rf signals applied to the phase discriminator causes it to generate a dc

correction voltage which is applied to the master oscillator circuitry. The dc signal forces the master

oscillator back to exactly 30 MHz. At that instant, the difference frequency produced by mixer (4)

becomes almost 11.5 MHz, and the phase discriminator no longer outputs a correction voltage.

In actual operating conditions, master oscillator drift causes a constant plus or minus dc correction

signal to be generated by the phase discriminator.

The audio intelligence is demodulated by the phase discriminator and rides on the dc correction

voltage. This audio signal is applied to the master oscillator to modulate its rf output.

The 11.5 MHz rf signals applied to the phase discriminator are shunted to ground by a series resonant

rf trap circuit.

COARSE TUNING

The coarse tuning process is initially identical to the fine tuning process and follows the same signal

path up to mixer. However, at the mixer, a different chain of events begins to occur.

Mixer

The 11.5 MHz output of the mixer is applied to receiver if. amplifiers (10).

Receiver IF. Amplifiers

Receiver if. amplifier module A4000 is used during reception and transmission. Two 11.5 MHz filters

wired in series with the if. amplifiers attenuate rf signals other than 11.5 MHz. Therefore, the if,

amplifiers produce highest amplitude signal from an 11.5 MHz input. As the input varies above or

below 11.5 MHz, the if. amplifiers' output drops in amplitude.

When the master oscillator is running almost exactly on proper frequency, the mixer's difference

frequency is very close to 11.5 MHz. This signal, applied to if. amplifiers (10), results in minimum

attenuation by the filters and maximum amplifier gain. The high amplitude output of the receiver's

fourth if amplifier is applied in parallel to hunt the cutoff detector within module A4200.

1-34