Intermodulation Distortion in Freqency-Division-Multiplex FM Systems - A Tutorial Summary

In the design of FDM-FM communication systems, a great deal of importance is attached to an appreciation of those transmission irregularities that yield second- and third-order intermodulation spectra. Such spectra are readily generated by amplitude, phase, and AM/PM conversion anomalies within the FM transmission path. In this paper the results of the available journal literature have been reworked for presentation in a generalized form appropriate for use by equipment designers and system analysts. Intermodulation noise magnitudes and spectral distributions are expressed in terms of the transmission network and system modulation parameters with the AM/PM conversion characteristic extended to that of a second-order power series in radian frequency. To fully exploit the usefulness of the foregoing, the coherency between the intermodulation spectra of all distortion mechanisms is considered. The effects of this correlation is then exemplified for a tandemly interconnected set of distorters. It is found that all secondand third-order distortion mechanisms may be grouped under the classifications of: amplitude, delay, equivalent amplitude/delay, and quasi-equivalent amplitude/delay distortion. It is concluded that, as a result of the correlative structure of the distortion processes, an overall system measurement of intermodulation noise may exhibit an unexpected level of magnitude and/or have a spectral distribution significantly different from any of the inherent system distortions.