A measurement system is provided which comprises: source circuit for receiving feedback signals and for providing respective signals at respective discrete frequencies in a prescribed microwave frequency range, wherein the respective provided signals at respective discrete frequencies are substantially phase locked to at least one downconverted signal in response to the feedback signals; downconverting circuit for linearly downconverting the respective provided signals and for providing the at least one respective downconverted signal; and phase detector circuit for receiving the at least one respective downconverted signal and for providing the feedback signals.