Exact Asymptotic Behavior of Digital FM Spectra

The problem of calculating accurate power spectral densities for an extended class of digital FM schemes is considered. This extended class includes partial response as well as multilevel schemes. A method suitable for numerical calculation using a computer is developed. This new method consists of two parts. First, the spectral main lobe is calculated. Then, the spectral tail is calculated by a different approach. The accuracy of the spectral tail is that of a correction term to an analytical asymptotic tail formula. This method yields accurate power spectra down to a level considerably below -100 dB. Numerical results obtained from calculations are given. The power spectra can be calculated for an arbitrary modulation index value.