Control systems in which the input and feedback signals of a plant are pulse-frequency modulated and the resulting pulses locked by a phase-locked loop are investigated. These systems are assumed to be regulator systems, and the input and feedback signals are converted into sequences of standard pulses using integral pulse frequency modulation (IPFM). These sequences of pulses are then compared in a phase comparator which produces a plant excitation signal that is dependent on the degree to which the feedback pulses track the input pulses.