Abstract: A problem with phase modulators particularly when modulating high frequency signals with signals of audio frequency is that noise is generated during the production of a free running ramp signal and noise is present on the modulating signal so that accurate detection of the modulating signal is not possible. In order to avoid this problem a fast ramp signal is generated by charging a low value capacitor (12) from a constant current source (13) in response to the edge of an applied timing signal. The starting voltage of the ramp signal is related to the phase of a modulating signal derived from a source having a low output impedance (25). The ramp signal is applied to a level detector (16) having a defined switching level, which detector produces an output edge signal in response to the ramp signal traversing the switching level in a given direction.