In this analysis, an angle-modulated signal having an arbitrary phase function is applied to a general linear filter, and the phase of the output is expanded in a series having the linearly filtered input as the leading term. The expansion is then specialized to the case of a narrowband signal applied to a narrow, symmetrical, bandpass filter. A spectral analysis is performed that provides the leading terms in the output spectrum. The formulas are used to calculate the distortion and crosstalk produced when an FM signal, having a flat baseband spectrum, is passed through a single-pole filter--an example of current interest because such a filter has been used in the forward path of a feedback FM receiver used for satellite communication. A number of cases are considered, and the results of the computations are plotted. 52 pp. Ref.