Abstract: A signal processing system and technique is disclosed for detecting the pence of a desired frequency signal particularly that of a sine wave, in a noisy environment. The signal mixed with noise is received and analyzed by a Fast Fourier Transform spectrum analyzer the output of which is indicative of instantaneous signal phase. The output of the spectrum analyzer is digitally processed to produce a series of average instantaneous signal values based upon the instantaneous signal phase and a statistical estimate of its rate of change, the values being representative of the relative signal strengths for a progression of phase reference values between 0.degree. and 360.degree.. The desired signal is detected when an average instantaneous signal value exceeds a threshold value determined empirically based upon a certain probability of detection and false alarm rate.