amplitude characteristic of the selected signal, while the phase detector detects the frequency deviation. When
the selected signal is not present at the AGC detector, there is no AGC output. The AGC detector output is
coupled to the AGC amplifier in the equalizer and frequency control unit, while the output of the phase detector
is coupled by way of the loop amplifier to the equalizer and frequency control unit. When there is no signal
present to input of the AGC detector, there is no output from the AGC amplifier. When there is no output from
the AGC amplifier, the equalizer and frequency control unit produces an alternating sweep voltage. The sweep
voltage is coupled to the VCO and causes the oscillator to sweep up and down in frequency. The sweeping
action continues until a 12-MHz IF is produced. This 12-MHz signal appears at the input of the AGC detector
and permits an AGC voltage to be developed. When the level of the AGC detector output exceeds the AGC
amplifier's threshold level, the AGC amplifier produces an output signal that eliminates the alternating sweep
voltage. The output of the equalizer and frequency control unit is then controlled by the output of the phase
detector. If the input to the phase detector drifts from 12 MHz, the equalizer and frequency control unit
produces a signal that causes the VCO to change frequency and return the IF signal to 12 MHz.
4. The bandwidth of the signal fed to the detectors is determined by the bandpass filter unit which contains six
different bandwidth filters, individually selectable according to the mode of operation. The bandpass of each
filter is centered at 12 MHz.
5. The four-stage 12-MHz wideband amplifier is capable of a gain of 60 dB and has an AGC range in excess
of 40 dB. Therefore, its output level of 0 dBm can be maintained essentially constant with input signal
variations from -20 to -60 dBm.
6. The 12-MHz IF signal is shifted 90 degrees in phase and coupled to the AGC detector, and is fed unshifted
in phase to the phase detector. Both of these detectors perform their functions by comparing the incoming 12-
MHz IF signal with a 12-MHz reference signal developed by the 12-MHz variable frequency oscillator and fed
to the detectors by way of the buffer stage and the 12-MHz reference transformer.
7. The PLL controls the frequency of the 12-MHz variable frequency oscillator for most of its modes of
operation. This phase-lock demodulation process permits demodulation of signals with C/Ns below the carrier-
to-noise thresholds. For conditions of severe signal loss, the PLL is disconnected from the variable frequency
oscillator and a crystal is used to stabilize he variable frequency oscillator's frequency. When the receiver is
operating under these conditions, the AFC loop that controls VCO becomes a narrowband loop that tracks the
carrier component of the incoming signal.
3-27
SS0031
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