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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


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