Intermodulation Noise in FM Systems Due to Transmission Deviations and AM/PM Conversation

Intermodulation noise is produced whenever a phase modulated signal is passed through a linear transmission medium whose amplitude and phase characteristics are nonlinear functions of frequency. The output signal from this medium is both envelope and phase modulated, with the phase modulation being a distorted replica of the input phase function. The envelope modulation and phase modulation functions are similar in that both consist of first (linear), second-, third-, and higher-order functions of the input phase function. They differ in that the coefficients of the terms making up the two modulating functions are related in different ways to the transmission medium characteristic. The distortion terms higher than first order, in the output phase * Portions of this paper were presented at the 1966 I E E E International Communications Conference in Philadelphia, Pa., June 16, 1966. 1749 1760 THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 196(5 modulating function, produce intermodulation noise. This source of noise has been the subject of much work over the past ten to twenty years. The envelope distortion terms directly produce no degrading effects in linear systems. However, when the linear transmission medium is followed by a device that converts envelope variations at its input to phase variations at its output then a different noise-generating mechanism exists. This latter source of noise will be designated as "AM/PM intermodulation noise" to distinguish it from the intermodulation noise produced directly by transmission deviations.* The two phenomena are illustrated in Fig.