c. Frequency modulation (FM). Frequency modulation is the process of varying the frequency
(figure 9) of the carrier wave.
(1) In a frequency-modulated wave, the frequency varies instantaneously about the
unmodulated carrier frequency in proportion to the amplitude of the modulating signal. When the
modulating signal increases in amplitude, the instantaneous frequency increases; when the modulating
signal decreases, the frequency decreases.
(2) In an FM wave, the amplitude of the modulating signal determines the extent of departure
of the instantaneous frequency from the center, or rest, frequency. Thus, the instantaneous frequency
can be made to deviate as much as desired from the carrier frequency by changing the amplitude of the
modulating signal. This deviation frequency may be as high as several hundred kilocycles, even though
the modulation frequency is only a few kilocycles. The sideband pairs generated by frequency
modulation are not restricted, as in amplitude modulation, to the sum and difference between the highest
modulating frequency and the carrier.
(3) The first pair of sideband pairs in an FM signal are those of the carrier frequency plus and
minus the modulating frequency. Additional sideband pairs will appear at each multiple of the
modulating frequency. For example, if a carrier of 1 mc is frequency modulated by an audio signal of
10 kc, there will be several sideband pairs spaced equally on either side of the carrier frequency at 990
kc and 1,010 kc, at 980 kc and 1,020 kc, at 970 kc and 1,030 kc, and so on. As a result, a frequency-
modulated signal occupies a greater bandwidth than does an amplitude-modulated signal.
(4) As indicated above the FM wave consists of a center or carrier frequency and a number
of sideband pairs. When modulation is applied and the amplitude of the modulating signal is increased,
power is taken from the center-frequency component and forced into the sideband pairs.
(5) The FM signal leaving the transmitting antenna is constant in amplitude, but varying in
frequency according to the audio-modulating signal. As the signal travels between the transmitting and
receiving antennas, however, it is combined with natural and manmade 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 the limiter stage.