(2) The discriminator is a frequency detector. The center frequency of the
discriminator is the same as the IF. Any deviation from this frequency
(above or below) will be detected, and the discriminator will produce a
voltage proportional to the amount of deviation.
This voltage is
filtered to eliminate the IF carrier, leaving only the dc voltage
variation to be amplified by the audio stages.
The output of the
discriminator is considered to be the AF component of the dc voltage.
2-6.
FREQUENCY SYNTHESIZERS
In modern communication, it is common practice to simultaneously transmit
several hundred telephone messages over one radio channel.
To prevent
interference, each telephone message must have its own carrier signal. Each signal
requires a different frequency which must be extremely stable. The most practical
means of producing these signals is by a method known as frequency synthesis. The
circuit that produces these stable frequencies is known as a frequency synthesizer.
a. Frequency Synthesis Methods.
The basic principle of frequency synthesis
is shown in figure 2-18.
Each of the several methods or combinations of methods
used to synthesize signals follows this principle.
Some of the various methods
used to synthesize frequencies are circuits designed to add, subtract, multiply,
and divide a stable basic frequency.
b. Uses For Synthetic Frequencies.
The signals that are produced by
frequency synthesizers may be used in a variety of ways.
They may be used as
carrier frequencies, testing signals, synchronizing signals, or as alarm or ringing
signals.
The intended use of the signal will in many cases determine which of
several methods will be used to synthesize the signals.
Figure 2-18.
Basic frequency synthesis principle.
309 L2
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