Abstract: The apparatus detects the presence of long or short pulse phase shift keyed (PSK) modulated signals and determines the underlying clock frequency and modulation code associated with such signal, in real time, using an instantaneous frequency measurement (IFM) receiver as its signal source, without affecting the band-width or sensitivity of this receiver. It monitors the output of the IFM correlators for transients which occur in the correlator's outputs when a PSK signal is present. Logic circuits are used to declare the PSK signal is present, determine its clock rate, a record the modulation or total number of pulses.