Power density spectrum of the sum of two correlated intermodulation noise contributors in FM systems

In the recent literature, two noise contributors in FM systems have been analyzed: (i) intermodulation noise due to transmission deviations, and (ii) AM/PM intermodulation noise. Even though different, these two contributors have the same property of being functions of the baseband signal. Hence, one would expect them to be correlated to some degree. In this paper, we derive the expression for the power density spectrum for the sum of these two noise contributors. The resulting expression has been programmed on a digital computer. It has been found that, under certain conditions, the correlation can be quite significant. In fact, an example using a representative FM radio relay system shows that the correlation can result in greater than 4 dB error if the two contributors are assumed to be uncorrelated.