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A general solution to the problem of calculating the distortion imposed on the instantaneous frequency of a frequency-modulated wave in passing through a transmission network is obtained by a direct operational method. Approximate formulas for the cases of large and small deviation ratios are derived, and it is shown that a range of overlap exists in practical cases. For very large deviation ratios the distortion is entirely nonlinear in character and depends on the maximum frequency deviation, while for very small deviation ratios the distortion is entirely linear and is independent of the frequency deviation. The nature of the distortion is examined with particular reference to intermodulation distortion. When the modulating wave consists of two sine waves of different amplitudes and frequencies, intermodulation distortion takes the form of a frequency modulation of the small high-frequency component by the large low-frequency one. The application of negative feedback to a frequency-modulation receiver is considered. Numerical examples are worked out.