A frequency shift maximum frequency follower Doppler demodulator with amplitude correction.

A commonly used maximum-frequency follower Doppler demodulator uses a variable-frequency voltage controlled filter that requires two good quality multipliers and can be difficult to set up and stabilise. An alternative presented here has the advantage of cheapness, easy setting-up and amplitude independence over about 25 dB giving the potential for quantitative as well as qualitative results. A switching modulator is first used to convert the Doppler spectrum into a pair of sidebands disposed about the modulating frequency. The modulation frequency is continually adjusted so that the lower sideband edge, representing the highest Doppler frequency, just passes the skirt of a low pass filter. The filter output, after rectification and smoothing, provides drive to the voltage controlled oscillator (VCO) generating the modulation frequency and also forms the flow signal output. The fixed low pass filter results in a stable feed-back loop in which high loop gain can be used so that linearity depends mostly on the VCO. The characteristic slope is independent of signal amplitude above a threshold, but increasing signal causes an increasing offset voltage. A simple signal rectifier enables this offset to be largely cancelled, giving amplitude independence.