and increasing the gain when the signal is at a low level. The circuit used to provide this gain control is called an
automatic gain control (AGC) circuit.
c. Control of IF amplifier gain may be accomplished either automatically or manually. A combination
of both methods is generally used.
Section II. FREQUENCY-MODULATION FEEDBACK RECEIVER
3-29.
GENERAL
The simplified FM receiver shown in figure 115 includes circuits used for AGC, AFC, and FMFB. The
preamplifier and the first conversion stages are omitted from this figure, but are similar to those in the receiver
shown in figure 116.
3-30.
BASIC OPERATION
a. Frequency conversion takes place in the mixer stages. Since this receiver uses three conversion
(mixer) stages, it is commonly called a triple-conversion receiver. The 60-MHz IF amplifier amplifies the input
from the first mixer. The amplified 60-MHz output is mixed with a frequency of 49.2 MHz in the second mixer
stage to produce a lower IF of 10.8 MHz. Before applying the 10.8-Maz IF to the third mixer stage, the IF is
amplified by the 10.8-MNz IF amplifier stage. The 10.8-Mz IF is mixed with the output of the second VCO to
produce an 800-kHz IF.
b. The 800-kHz output of the mixer has a variable bandwidth, which is controlled by mode selector
switches on the receiver control panel. The passband of the mixer is varied by the different resistive loads that are
placed across the mixer's tank circuit by the noise selector switches.
c. After the desired bandwidth is selected at the third mixer, a conventional stage of amplification
amplifies the 800-kHz IF signal and passes it to the limiting stage. The limiter operates on both the positive and
negative swings of the IF signal to control the amplitude of the signal applied to the discriminator. The
discriminator circuit converts the FM intelligence from the 800-kHz IF signal to usable audio or video signals.
The circuits in the video-amplifier stage filter, attenuate, and amplify the signal according to the selected mode
and bandwidth.
3-31.
AGC LOOP
The 800-kHz IF is also applied to the AGC detector and amplifier. The AGC amplifier provides an
amplified dc correction voltage to the 60-MHz IF amplifier. The AGC voltage is capable of varying the 60-MHz
IF amplifier's gain over a range of 20 db.
3-32.
FMFB LOOP
a. The FMFB loop uses degenerative feedback to effectively compress the FM signal deviation in order
to maintain a narrow IF bandpass. A demodulator using FMFB permits an increase in the output signal-to-noise
ratio above that
344 L3
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