Power Switching Noise Removal in Sensitive Doppler Applications

The velocity profile of fluids flowing in industrial pipes can be detected by pulsed-wave Ultrasound Doppler, where bursts of ultrasounds with frequency in the range 300 kHz-10MHz are transmitted every pulse repetition interval. To detect the weak echoes present in attenuating fluids and/or large pipes highly sensitive front-ends (gain ~50dB) are employed. These are sensitive to the noise produced by power switching regulators, whose working frequency is within the input working bandwidth. EMI filters and spread-spectrum modulation help, but unacceptable artefacts can still be present in the Doppler spectrum. This work proposes an adaptive method that, based on a mathematical model of the noise, is capable of concentrating the switching noise in the low-frequency region of the Doppler spectrum, which is eliminated by the clutter filter together with the echoes from still or slow-moving targets (e.g. pipe walls). Experiments with a Doppler system show the effectiveness of the method.