Abstract: Accurate estimates of the instantaneous mean frequency of a time varying signal are achieved at favorable and adverse signal-to-noise ratios by an improved time domain approach. Demodulated in-phase and quadrature signals are sampled and delayed. Noise in both terms of a simple arc tangent algorithm, whose denominator has products of time samples shifted at least one time period with respect to itself, is uncorrelated and removed by low pass filtering. One embodiment is a pulsed ultrasonic Doppler system with quadrature detection to measure and display blood velocity as a function of time.