LEARNING EVENT 22: AUTOMATIC GAIN CONTROL
1. Because of fluctuations in the propagation characteristics of free space and the Earth's atmosphere, the power
level of the received signals will not be constant. These fluctuations will cause undesired amplitude variations
in the demodulated signal.
2. The effects of these variations may be minimized by reducing the gain of the IF amplifiers when the received
signal is at a relatively high amplitude 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.
3. Control of IF amplifier gain may be accomplished either automatically or manually. A combination of both
methods is generally used.
SECTION VI. FREQUENCY-MODULATION FEEDBACK RECEIVER
LEARNING EVENT 23: FM RECEIVER
The simplified FM receiver shown in Figure 3-7, page 3-22, 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 3-8, on page 3-24.
LEARNING EVENT 24: BASIC OPERATION
1. 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-MHz IF to the third mixer stage, the IF is
amplified by the 10.8-MHz IF amplifier stage. The 10.8-MHz IF is mixed with the output of the second VCO
to produce an 800-kHz F.
2. 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.
3. 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 in 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.
3-21
SS0031
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