Abstract: This invention concerns a carrier synchronization system for a four-phase coherent PSK demodulator. The four-phase PSK signal is demodulated by an in-phase reference carrier and a quadrature reference carrier both of which are locally generated by a reference carrier source. The in-phase and quadrature components of the four-phase PSK signal that result from these demodulation operations are delayed a time interval substantially equal to T/2 seconds, T being the signaling period. The product of the delayed in-phase component and the sign of the quadrature component is compared with the product of the delayed quadrature component and the sign of the in-phase component, to produce a phase error signal. This error signal is used to control the adjustment of the reference carrier source.