6.4.1 Frequency Deviation
The frequency deviation (∆f) of an FM waveform is the maximum shift of the carrier frequency from its resting frequency caused by the modulating signal. For example, if an FM system has a deviation of ±50 kHz, a 100 MHz carrier varies between 99.95 MHz and 100.05 MHz.
From Equation (6.22) we can see that the deviation is proportional to the amplitude of the modulating signal voltage and the frequency-deviation constant such that:
Unlike AM, which has a defined upper limit, FM has no theoretical upper bound on deviation. In practice, however, every system specifies a rated system deviation , to limit the occupied bandwidth and ensure compatibility with channel allocations. Typical deviations range from tens of kilohertz in narrowband systems to several megahertz in satellite or video links.
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