in 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 S/N.
2. 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 is 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.
3. 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.
LEARNING EVENT 27: AFC LOOP
1. 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.
2. 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 VII. PHASE-LOCK RECEIVER
LEARNING EVENT 28: GENERAL
The simplified receiver, shown in Figure 3-8, 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
3-23
SS0031
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