b. As the IF decreases, the output from the discriminator assumes an average voltage which is negative.
The average negative voltage causes the local-oscillator frequency to decrease and the IF to increase toward the
desired value. As the IF approaches the desired value, the voltage output from the discriminator approaches an
average value of zero.
LEARNING EVENT 21: PHASE-SENSITIVE AFC
1. Except for the phase detector and the reference oscillator, the phase-sensitive AFC loop shown in Figure 3-6
is essentially the same as the frequency-sensitive AFC loop shown in Figure 3-5.
Figure 3-6. Phase-sensitive AFC loop.
2. The phase detector senses the changes in the phase of the IF with reference to the output signal's phase from
the reference oscillator. These phase changes cause an average DC output from the phase detector. The
reference oscillator is a highly stable oscillator that operates at the same frequency as the IF.
3. An increase in the IF is sensed as an advance in the phase of the IF. The phase detector will produce an
average DC output that is proportional in the amount of the phase change. This output voltage is applied to the
local oscillator as in the frequency-sensitive AFC loop. A decrease in the IF is sensed as a lag in phase by the
phase detector which will produce an average output of opposite polarity from that generated by an advance in
phase. When there is no phase difference, the output is zero. The phase-sensitive AFC circuit is similar to a
PLL demodulator.
SS0031
3-20
Previous Page
