b. FM is the process of varying the
carrier signal's frequency (rather than its
amplitude) in accordance with the variations of
the modulating signals. The amplitude or power
of the FM carrier does not vary during
modulation (Figure 1-9).
(1) The carrier signal's frequency,
when it is not modulated, is called the center or
rest frequency. When a modulating signal is
Figure 1-8. Amplitude-modulated system.
applied to the carrier, the carrier signal will
move up and down in frequency away from the
center or rest frequency.
(2) The amplitude of the modulating
signal determines how far away from the center
frequency the carrier will move.
This
movement of the carrier is called deviation; how
far the carrier moves is called the mount of
deviation. During reception of the FM signal,
the amount of deviation determines the loudness
Figure 1-9. Frequency modulation.
or volume of the signal.
(3) The FM signal leaving the transmitting antenna is constant in amplitude, but varying in
frequency according to the audio signal. As the signal travels to the receiving antenna, it picks up
natural and manmade electrical noises that cause amplitude variations in the signal. All of these
undesirable amplitude variations are amplified as the signal passes through successive stages of the
receiver until the signal reaches a part of the receiver called the limiter.
(4) The limiter eliminates the amplitude variations in the signal, then passes it on to a device
called a discriminator, which is sensitive to variations in the RF wave's frequency. The resultant
constant amplitude FM signal is then processed by the discriminator circuit, which changes the
frequency variations into corresponding voltage amplitude variations. These voltage variations
reproduce the original modulating signal in a headset, loudspeaker, or teletypewriter.
(5) FM is generally used by radiotelephone transmitter operating in the VHF and higher
frequency bands.
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SS0130
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