MASTER OSCILLATOR FREQUENCY CONTROL LOOP.

TM 11-5820-670-30

1-34. MASTER OSCILLATOR FREQUENCY CONTROL LOOP.

For proper radio transmitter operation, the fm output of the master oscillator must be within 3.5 kHz

of the selected channel frequency. To prevent master oscillator frequency drift beyond these limits, a

frequency control loop is built into the radio. The electronic modules that comprise the frequency con-

trol loop (automatic frequency control) are shown in FO-6.

Correction of master oscillator frequency drift is accomplished in two ways. Slight frequency drift is

fine tuned to proper frequency by a dc correction voltage from the phase discriminator. Extreme

frequency drift is coarse tuned to proper frequency by a sawtooth voltage from the hunt generator.

Fine tuning and coarse tuning of the master oscillator are done as follows.

FINE TUNING

Master Oscillator

Master oscillator (1) outputs rf signals at a frequency corresponding to the channel indicated by the

radio's tuning dials. For this discussion it is assumed that the channel selected is 30.00 MHz. Due to

master oscillator drift, its rf output is not precisely 30 MHz. This signal is applied to mixer (4) through

buffer amplifier (2) and isolation amplifier (3) which prevents excessive loading of the master

Mixer

Receiver mixer (4) is used by both the receiver and transmitter for the purpose of heterodyning rf

signals. During transmission, the receiver-mixer heterodynes the 30 MHz master oscillator output,

which is not necessarily 30 MHz, with the 41.5 MHz output from receiver local oscillator (5). Sum and

difference frequencies are generated by the mixer. The difference frequency, which approximates

11.5 MHz, is applied to first and second if. amplifiers (6). The other frequencies are filtered.

Local Oscillator

The local oscillator performs an identical function during reception and transmission. During trans-

mission, when the radio is set to 30.00 MHz, the local oscillator outputs a precise 41.5 MHz signal.

Since the local oscillator is prevented from drifting by the crs, the oscillator's output is extremely

accurate. The 41.5 MHz signal is applied to the receiver-mixer and heterodyned with the master

oscillator's 30 MHz signal to produce an if. signal which varies around 11.5 MHz.

First and Second IF. Amplifiers

First and second if. amplifiers (6) increase the amplitude of the 11.5 MHz intermediate frequency to

make the signal strong enough for processing by phase discriminator (8).

11.5 MHz Modulator

The 11.5 MHz modulator (7) provides the fixed 11.5 MHz audio modulated input to the phase

discriminator.

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