c. Although the basic frequency of the VCO is determined by the setting of the selected VCO-bias-
adjust potentiometer, its exact frequency is controlled by the AFC voltage from the AFC loop. The 12-MHz
signal from the mixer is filtered by the selected filter in the bandpass filter network, amplified by the 12-MHz IF
amplifiers, and coupled to the AGC detector and phase detector in the phase-lock demodulator. The AGC
detector detects the 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 at the 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 IF 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.
d. 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.
e. 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.
f. The 12-MHz IF signal is shifted 900 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.
g. The phase-lock loop 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 carrier-to-
noise ratios below the carrier-to-noise thresholds. For conditions of severe signal loss, the phase-lock loop is
disconnected from the variable frequency oscillator and a crystal is used to stabilize the variable frequency
oscillator's frequency. When the receiver is operating under these conditions, the AFC loop that controls VCO
becomes a narrow-band loop that tracks the carrier component of the incoming signal.