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The combined effects of filter distortion and the associated intersymbol interference on coherently detected biphase and quadriphase PSK signals are studied in white Gaussian noise. A determination is made of the system performance degradation from matched-filter detection resulting from the use of a linear channel model that focuses attention on the critical filters in the communication link. The critical filters that affect performance consist of transmission filters in the transmitter and/or transponder, receiver predetection filters, and data filters associated with the detector. Numerical results for adjacent symbol interference are given for detection using both integrate-and-dump filters as well as more general data filters, particularly an undumped 2-pole Butterworth data filter. The numerical results include symmetrical band limiting, broad-band filtering with a frequency offset, mismatched data filtering, cascaded filter chains, and the effects of pure phase distortion.