More on Intermodulation Noise in FDM-FM Radio-Relay Systems

Intermodulation distortion in FDM-FM radio-relay systems is investigated from an aspect that appears not to have been analyzed in appropriate depth previously, viz., determining more or less exactly the bandwidth within which intermodulation noise is sensitive to transmission imperfections of the radio channel. To this end an analysis of the distortions of linearly filtered anglemodulated signals is presented with the result that intermodulation noise is governed not by transmission deviations directly, but by "weighted deviations," i.e., convolution of the transmission deviations and the spectral density function of the transmitted anglemodulated signal. Although it is intuitively trivial that the spectral distribution of the angle-modulated signal has direct bearing on intermodulation distortion, no previous paper seems to show this relationship explicitly. Further-by a series expansion of the weighted deviation function with respect to suitably defined orthogonal polynomials-specific weights are attributed to sections of the transmission band. The weights-rolling off beyond some distance from the carrier-indicate the frequency band looked for. Simultaneously, a simple computation method of intermodulation noise power is developed, permitting calculation in terms of groupdelay and differential gain variations caused by transmission deviations alone or together with AM-PM conversion and AM compression.