LOCAL OSCILLATOR AFC/CRYSTAL REFERENCE SYSTEM.

TM 11-5820-670-30

1-13. RECEIVER SIGNAL FLOW. (CONT)

The 11.5 MHz if. output from the fifth if. amplifier is applied to limiter module A4200 (13). The limiter

clips the positive and negative peaks of the input signal to ensure an output of constant amplitude. If

the limiter output is not of constant amplitude, a distorted audio output occurs from the discriminator

in module A4200 (14).

The clipped if. modulated signal is applied to the discriminator from the limiter. The discriminator

demodulates the frequency modulated if. signal and applies the resultant audio signal to the receiver

audio and squelch preamplifier.

The receiver audio and squelch preamplifier module A4300 (15) is a three-stage, direct coupled,

class A amplifier which amplifies the audio signal and applies it to audio amplifier module A5400 (16).

The audio signal is amplified in the audio amplifier and applied to low-pass filter FL5501 (17) and

squelch amplifier module A5200 (18). Unfiltered audio signals from audio amplifier module A5400

(16) are applied through relay K5501 to activate receiver squelch amplifier A5200 (18) and squelch

filter A5300 (19).

The squelch amplifier, depending on the setting of the squelch switch on the control unit, applies or

removes power from the receiver audio amplifier to squelch or unsquelch the radio set.

The squelch function is covered in greater detail in paragraph 1-32.

Audio signals below 3000 Hz pass through filter FL5501 and then are reapplied to the audio amplifier

for further amplification. Audio signals are then coupled to the control unit and interphone amplifier

where the headset/microphone is connected.

1-14. LOCAL OSCILLATOR AFC/CRYSTAL REFERENCE SYSTEM.

Correct operation of the receiver depends on an accurately controlled local oscillator whose frequency

is maintained with minimal variation. At any frequency selected at the control unit, the Iocal oscillator

is held to a tolerance of 3.5 kHz by the action of the crs. Therefore, the crs functions as an

automatic frequency control (aft) loop.

Refer to the system block diagram (FO2) showing crs signal flow while reading the text in this paragraph.

The output of iocal oscillators (1) is applied to high-pass filter FL3002 (2) and then to balanced mixer

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

crystal-controlled harmonic generator A3100 (4).

The inputs to balanced mixer A3200 are heterodyned, thereby producing 12 sum and difference

frequencies. Two frequencies closest to 53 MHz are passed by band-pass filter FL3003 (5), while the

other signals are attenuated. The two accepted frequencies are then applied to crs second mixer

A3300 (6), and heterodyned with the output of interpolation oscillator (7).

Ten crystals are within interpolation oscillator (crystal switch A2000). Each crystal corresponds to a

group of frequencies tunable at the control unit. When a frequency is selected, the radio's gear train

also selects one of ten crystals, which precisely controls the frequency of the interpolation oscillator.

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