An important porblem in receiving angle-modulated signals is the proper design of the filtering elements. Such filtering introduces distortion and crosstalk into the signal, but to date the formulations for their computation have not been useful at the required level of accuracy because of an inherent limitation. An angle-modulated signal with an arbitrary phase function is applied to a symmetrical, narrow band-pass filter and the phase of the output is expanded in a series that converges for all values of the input phase, subject to certain conditions. The expansion is unique in that its lending term is the linearly filtered input, not the usual quasistationary term. The spectrl analysis is performed by assuming a stochastic modulating signal and examining the third- and fifth-order terms of the output phase expansion. The results are given in the form of signal, distortion, and crosstalk components of the output spectrum; they appear to represent the principal contributions where total distortion is small.