Detection of weak Doppler signal in a strong carrier: an equivalent high-Q band-elimination filter

In the paper, weak Doppler-shifted components of reflected ultrasonic signals were measured following the application of continuous-wave ultrasound for the measurement of blood flow velocity using the Doppler effect. The component of the emitted frequency was suppressed by subtracting an extracted signal from the reflected signal obtained by nonlinear closed loops. Phase information of the signal was obtained with a phase-locked loop, while the amplitude was controlled by an integrated loop to make the component of the emitting frequency minimum after subtracting. In principle, the circuit could be considered as a sufficient narrow band-elimination filter. The Q of the fitter was 10⁴-10⁵; its elimination depth was larger than 40 dB; its frequency stability was better than 10^-5 because the phase-locked loop used a crystal oscillator as the voltage-controlled oscillator. Applying this circuit to a telemetry system, an example of measuring carotid blood velocity in an exercising man is given in the paper