6.4.5 Power Contained In An FM Wave
Because the instantaneous amplitude of an FM signal remains constant, the total transmitted power of the wave is also constant and equal to the average power of the unmodulated carrier. Although the amplitude of the carrier component itself varies—as power is exchanged between the carrier and its sidebands—the sum of the powers in all frequency components remains constant at all times.
As shown in Equation (6.33), an increase in sideband amplitude corresponds to a reduction in carrier amplitude, and vice versa, such that the total envelope power is conserved. This constant-power property is a defining characteristic of FM and enables FM transmitters to use efficient nonlinear amplifiers, such as TWTAs and SSPAs in satellite communications, without distortion.
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