Follett D H
Electronics and Bioengineering Development Unit, Department of Medical Physics, Bristol, England.
Clin Phys Physiol Meas. 1981 Nov;2(4):277-84. doi: 10.1088/0143-0815/2/4/004.
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.
一种常用的最大频率跟踪多普勒解调器使用一个可变频率的压控滤波器，该滤波器需要两个高质量的乘法器，并且可能难以设置和稳定。这里介绍的另一种方法具有成本低、易于设置以及在约25分贝范围内幅度无关性的优点，这为获得定量和定性结果提供了潜力。首先使用一个开关调制器将多普勒频谱转换为一对围绕调制频率分布的边带。不断调整调制频率，以使代表最高多普勒频率的下边带边缘刚好通过低通滤波器的裙边。滤波器输出经过整流和平滑后，为产生调制频率的压控振荡器（VCO）提供驱动，同时也形成流量信号输出。固定的低通滤波器形成一个稳定的反馈回路，在这个回路中可以使用高环路增益，因此线性度主要取决于VCO。特性斜率在高于阈值时与信号幅度无关，但增加信号会导致偏移电压增加。一个简单的信号整流器能够在很大程度上消除这种偏移，从而实现幅度无关性。
