LOCAL OSCILLATOR AFC/CRYSTAL REFERENCE SYSTEM.

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

1-15.

LOCAL OSCILLATOR AFC/CRYSTAL REFERENCE SYSTEM.

Correct operation of the receiver depends on a very precisely controlled local oscillator whose fre-

quency is maintained with minimal variation. At any frequency selected by the MC-TUNE-KC control,

the local oscillator is held to a tolerance of 3.5 kHz by the action of the crystal reference system.

Therefore, the crystal reference system (CRS) functions as an automatic frequency control (AFC)

loop.

A system block diagram showing CRS signal flow can be found on F0-11 in the back of this manual.

Refer to the foldout while reading the text in this paragraph.

The local oscillator's (1) output is applied to a high-pass Filter FL3002 (2) and then to Balanced Mixer

A3200 (3). At the same time, 12 frequencies (ie, 1 to 12 MHz), are applied to the balanced mixer

from the crystal-controlled Harmonic Generator A3100 (4).

The inputs to the balanced mixer are heterodyned, thereby producing 12 sum and difference fre-

quencies. Two frequencies closest to 53 MHz are passed by band-pass Filter FL3003 (5), while other

signals are attenuated. The two accepted frequencies are then applied to the CRS Second Mixer A3300

(6), and heterodyned with the output of the interpolation oscillator (7).

Within the interpolation oscillator (Crystal Switch A2000) are ten crystals. Each crystal corresponds to

a group of frequencies tunable by the MC-TUNE-KC control. When a frequency is selected, the

radio's gear train also selects one of ten crystals, which precisely controls the frequency of the inter-

The heterodyning process in the CRS Second Mixer A3300 (6) produces sum and difference frequen-

cies. 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 contained within the second mixer, to the first and

second IF Amplifiers A3400 (7). 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 1-26, covering the Amplifier and Limiter A4200 stages.

Output from the A3500 (10) is applied to the phase discriminator and hunt generator at the same time

to coarse tune, and then fine tune the local oscillator.

COARSE TUNING THE LOCAL OSCILLATOR

The A3500's output is applied to band-pass Filter FL3005 (11) to attenuate any unwanted frequencies

which may have passed through the limiter. From the 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 the

A4200 module described in paragraph 1-26. 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

component riding on it.

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

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

the local oscillator (1). The error signal biases the local oscillator close to 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.

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