COARSE TUNING THE LOCAL OSCILLATOR

TM 11-5820-670-30

LOCAL OSCILLATOR AFC/CRYSTAL REFERENCE SYSTEM. (CONT)

The heterodyning process in crs second mixer A3300 (6) produces sum and difference frequencies.

Two of the difference frequencies are very close to 5.625 MHz. The frequency closest to 5.625 MHz is

coupled across a 5.625 MHz tuned tank within the second mixer, to first and second if. amplifiers

A3400 (8). Other frequencies are greatly attenuated by the resonant tank circuit.

After amplification, the if. signals are filtered by FL3004 (9), which is sufficiently selective to

attenuate any unwanted frequencies. The third if. amplifier and limiter A3500 (10) functions in a

manner similar to that described in paragraph 1-29, covering amplifier and limiter A4200 stages.

Output from amplifier and limiter A3500 (10) is applied to the phase discriminator and hunt generator

at the same time to coarse tune, then fine tune the Iocal oscillator.

COARSE TUNING THE LOCAL OSCILLATOR

The output from amplifier and limiter A3500 is applied to band-pass filter FL3005 (11) to attenuate any

unwanted frequencies which may have passed through the limiter. From the band-pass filter, the

signals are applied to the hunt discriminator A3600 (12).

The hunt discriminator contains a triple tuned travis-type discriminator similar to the one used in

module A4200 described in paragraph 1-29. Large deviations away from the discriminator's 5.625

MHz center frequency cause the discriminator to output a plus or minus dc signal with an ac com-

ponent riding on it.

Both dc and ac outputs are applied to damping network (13), where the ac component is damped or

attenuated. From the damping network, a positive or negative dc error signal is applied to local

oscillator (1). The error signal biases the local oscillator close to the correct frequency by a hunting

action. That is an initial large dc error signal followed by smaller error signal voltages until the

oscillator is close enough to its correct frequency for the phase discriminator to begin fine tuning.

The hunt discriminator can coarse tune the Iocal oscillator when its frequency error is within

400 kHz.

FINE TUNING THE LOCAL OSCILLATOR

The hunt discriminator is sensitive to large errors in local frequency, while phase discriminator (14)

responds to small errors. The phase discriminator compares the fixed output of reference oscillator

(15) with the variable output of limiter (10). When two signals are of unequal frequency (due to

incorrect local oscillator frequency), the phase difference causes the phase discriminator to output a

dc error voltage which is applied to the local oscillator to bias it into correct frequency operation.

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