An important problem in the design and analysis of communication systems is determining the distortion experienced by a modulated signal when it is passed through a linear bandpass filter and then demodulated. In this paper, the input signal is assumed to be simultaneously amplitude and phase modulated and the bandpass filter may be unsymmetrical and mistuned. The demodulator output is the magnitude of the filtered signal for an AM receiver or its phase for a PM receiver. The results are in the form of an expansion in which the leading term is the linear (i.e., undistorted) reproduction of the input AM and PM and the successive terms represent the distortion. In both cases, the significant effects are given by the first few terms of the expansions because the applications of interest are those in which the interference is small. Finally, a spectral analysis is performed on the output of an AM receiver.