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This paper is concerned with detection and diagnosis of faults in feedback control systems in which the conventional analogue controller is replaced by a digital filter. The difficulties associated with locating faults in such a hybrid control system using well established frequency domain test techniques are examined. Digital and analogue/hybrid computer simulations are used to investigate phenomena peculiar to these hybrid systems, caused by the marriage of sharply defined on-or-off mode digital elements and relatively fuzzy mode analogue elements. The information content of a beatlike phenomenon, observed with sinusoidal excitations of frequency falling in the troublesome range, close to half sampling rate, is shown to offer a simple and rapid method for detecting a faulty sampling mechanism.