Abstract: Method and apparatus for demodulation of a phase shift keyed carrier in which a plurality of auxiliary signal frequency sources are provided and a substantially constant phase difference is maintained therebetween. Each auxiliary signal frequency is mixed with the modulated carrier and the mixer outputs are combined to form the product thereof to yield a control signal for regulating the auxiliary.The sum and differences of the mixer output may be formed prior to forming the product thereof.Input signals to the product forming circuit are amplitude limited to limit variation of the control signal r between -1 .ltoreq.r .ltoreq.+1.A filter may be employed to smooth the control voltage and may be incorporated with switching means to periodically interrupt the control signal to limit evaluation only during the established keying phase.