of a conventional FM demodulator at low-signal levels. The increase in the signal-to-noise ratio is obtained by
processing the signal input to the demodulator so that it will pass through a smaller IF bandwidth than would be
required by an unprocessed signal. The reduction in the IF bandwidth allows a smaller amount of noise power to
be delivered to the limiter and the discriminator and, therefore, produces a better output signal-to-noise ratio.
b. The process by which the FM signal deviation is reduced consists of varying the injection frequency
into the third mixer in the same direction as that of the received FM signal deviation. The resulting mixer-output-
frequency deviation is the difference between the input signal deviation and the injection-frequency deviation.
This signal with reduced deviation is filtered and amplified in the 800-kHz IF amplifier and then demodulated in
the discriminator. The resulting signal, at audio or video frequencies, is then returned to the second VCO and is
used to vary the oscillator's frequency with the incoming signal's frequency deviation. This feedback process is
necessary for proper reduction of signal deviation which, in turn, is required when using a narrow IF bandwidth.
c. The FMFB loop is closed only during certain demodulation modes. In the other modes of operation
of the demodulator, the FMFB loop is open and the second VCO, not receiving a tracking voltage from the FMFB
loop filter, acts as a conventional local oscillator. When the FMFB loop is open, the receiver operates as a
conventional FM receiver.
3-33.
AFC LOOP
a. A portion of the discriminator's output is applied to the differential amplifier stage. When the input to
the differential amplifier is 0 volt, the differential amplifier permits the first VCO to operate at its center
frequency of 14 MHz. When the input is other than 0 volt, the differential amplifier changes the operating
frequency of the first VCO.
b. The 14-MHz output of the first VCO is mixed with the 35.2-MHz crystal oscillator output to provide
the 49.2-MHz injection voltage for the second mixer. The AFC circuit controls the 49.2-MHz injection voltage
which, in turn, controls the 10.8-MHz IF.
Section III. PHASE-LOCK RECEIVER
3-34.
GENERAL
The simplified receiver shown in figure 116 is representative of the receivers designed for use with
present near synchronous satellites. This receiver is capable of operating on any one of four preset 2.5-MHz-wide
channels in the 50- to 90-MHz frequency range. The operating channel of the receiver is selected by means of the
channel select signals from the receiver control circuits. In addition, the receiver can be operated in any one of
nine modes of operation selected by means of the mode-select signals from the receiver control circuits. The
receiver consists of a preamplifier section, a converter section, an amplifier-converter section, a preselector
344 L3
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