Abstract: An apparatus for measuring the deformation parameters of a periodic signal concealed in noise. The signal is transmitted by a transmitter controlled by a stable clock and received by a receiver also controlled by a stable clock of the same frequency as the one of the transmitter. The receiver comprises a detector for detecting the signal transmitted by the transmitter, means for resetting to a predetermined value the voltage detected by the detector at each integral number of cycles but at any particular time in the cycle, and an analog to digital converter connected to the detector for sampling the signal detected by the detector a predetermined number of times per cycle for converting the analog signal into a digital signal.