Coherent FM Doppler system (US blood flow measurement)

A new Doppler approach to measuring the velocity of moving scatterers is presented. The technique is based on repetitive phase-coherent linear sweep signals (chirps), and promises to combine the high signal-to-noise ratio of continuous-wave (CW) Doppler with the ability to provide range information of pulsed-wave (PW) Doppler. Separate transmitting and receiving transducers are used. The receiver signal is demodulated with a frequency or time-shifted version of the transmitted signal, followed by a stationary echo canceller for removal of echoes from stationary reflectors. By appropriate choice of frequency shift and bandwidth of the lowpass filter, information from a small measuring volume, at a given depth, can be extracted. The position and velocity of each scatterer is encoded into the received chirp; specifically, the velocity is uniquely related to the Doppler-shifted sweep rate of the output signal from the stationary echo canceller.<>