Power Density Spectrum of the Sum of Two Correlated Intermodulation Noise Contributors in FM Systems

Two noise contributors in FM systems are: (i) intermodulation noise due to transmission deviations, and (ii) AM/PM intermodulation noise. The first noise contributor is generated when an FM signal is passed through a linear transmission medium which has transmission deviations. The second noise contributor is generated when an FM signal is passed through such a medium which is followed by an AM/PM conversion device. These two noise sources are different, in general, but have in common the property that they are a function of the baseband signal. Therefore, one would expect that they are correlated to some degree. This would mean that combining the two noise power density spectra together assuming random addition (uncorrelated random variables), i.e., power addition, might not be sufficient in general. 2437 2438 THE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1967 In this paper, we will derive the power density spectrum for the sum of these two noise contributors. We will then examine the results, with the help of a digital computer program, for the conditions under which the two noise contributors are correlated. We will also present an example using a representative FM radio relay system. However, before considering the correlation problem, we will first briefly consider the two noise contributors individually.