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

Abstract

Two noise contributors in FM systems are: (i) intermodulation noise due to transmission deviations; and (ii) intermodulation noise due to transmission deviations and AM/PM conversion, designated AM/PM intermodulation noise. Expressions for the second- and third-order AM/PM intermodulation noise are derived in terms of transmission medium coefficients and a continuous pre-emphasis characteristic, with the unpre-emphasized baseband signal being simulated by white Gaussian noise. These expressions have been programmed on a digital computer and representative noise responses and properties of AM/PM intermodulation noise were obtained. General properties and characteristics for the two noise contributors are documented in parallel for comparative purposes. It was found that AM/PM intermodulation noise can be a significant noise contributor in FM systems.