3-38.
PRESELECTOR
The preselector section of the receiver consists of the bandpass filter, the O-MHz IF amplifiers, and the
variable-tuned circuit.
a. The 70-MHz input signal is coupled by means of a coaxial cable from the RF patch panel to the input
of the bandpass filter. The bandpass filter passes the signals in the 50- to 90-MHz frequency range with an
attenuation of less than 1 db. The filtered signal is then fed to the 70-MHz IF amplifiers.
b. The wideband five-stage amplifier provides a gain of up to 50 db (100,000) in the 50- to 90-MHz
range. The second, third, and fourth IF amplifier stages are controlled by an AGC signal which is developed by
the AGC detector and amplifier. The AGC signal is capable of maintaining the output level constant to within 2
db with an input variation of as much as 50 db above the threshold level.
c. The amplified 50- to 90-MHz signal is coupled to the variable-tuned circuit. The variable-tuned
circuit is a voltage-controlled tuned circuit with a 2.5-aHz bandpass. The center frequency of the tuned circuit is
determined by the voltage applied to it from the channel-frequency potentiometers. The voltage applied to it, and
thus its frequency, is determined by the setting of the selected potentiometer.
3-39.
DEMODULATOR
The demodulator section of the receiver contains an AGC loop, an AFC loop, and a phase-lock loop, and
consists of a mixer, a difference amplifier, a driver amplifier, a VCO, a buffer amplifier, bandpass filters, 12-MHz
IF amplifiers, a phase-lock demodulator, and an equalizer and frequency control circuit.
a. The AFC loop signal, which controls the frequency of the VCO can be either a sweep signal or a
doppler tracking signal. The AGC signal controls the gain of the second and third 12-MHz IF amplifiers, and also
turns the AFC sweep signal on and off. The phase-lock signal controls the frequency of the variable frequency
b. The selected signal from the preselector section is fed to the mixer, where it is mixed with the output
of the VCO to produce a 12-MHz IF signal. The basic frequency of the VCO is set by the voltage from one of
four channel-frequency-adjust potentiometers in the preselector section, and one of them is selected at the same
time that a channel-frequency potentiometer is selected. The selected VCO-bias-adjust potentiometer is adjusted
so that the frequency of the VCO is 12 MHz above the frequency which was tuned with the selected channel-
frequency-adjust potentiometer. The voltage from the VCO-bias-adjust potentiometer is coupled to the VCO by
way of the difference amplifier and the driver amplifier. The difference and driver amplifiers maintain the
linearity of the frequency-to-voltage characteristic of the VCO for all portions of the band.
344 L3
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