Power spectral density (PSD) analysis is often undertaken assuming equally likely signalling input levels, and hence the different criteria of performance are evaluated with this condition in mind. In practice, however, cases of unequal probabilities of signalling input do arise due to choice of coder, and the study of these conditions is important. Power spectral densities are sensitive to variations in the probability density functions (PDFs), and both these curves and the fractional out-of-band spectra for multilevel FSK are treated in the paper. Different signalling inputs are used forM-ary CPFSK and the newly defined multi-hpartial response (MHPR) scheme. The influence of PDFs on creating spectral lines (for timing extraction) is also discussed.