Abstract: In an ultrasonic Doppler flowmeter system for the measurement, imaging and mapping of blood flow, a transducer coupled to a patient's skin is caused to emit pulses of high frequency ultrasound waves. The resulting echo signals are received by the transducer, demodulated into quadrature components, sampled into a large plurality of channels, and digitized. An improved stationary filter circuit utilizing a finite iteration then suppresses unwanted masking "clutter", i.e., low frequency but large amplitude signals from stationary tissue interfaces, with significantly improved response time and dynamic range, while providing good resolution. The Doppler frequency period of the moving acoustic scatterers is obtained by passing the remaining signal through a zero crossing detector.