In a synchronous detector the carrier of a binary differentially coherent phase shift-keyed signal is recovered by decomposing the carrier information signal into its conjugate in-phase and outof-phase components which contain in their arguments a term delta (t) representing the phase difference between the carrier and a local oscillator signal. The conjugate components are multiplied to generate a product signal which is sampled every other time slot to produce an error signal proportional to sin(2 delta (t)). This signal is applied to the local oscillator to phase lock the local oscillator signal to the carrier of the information signal.