Digital filtering for robust 50/60 Hz zero-crossing detectors

An improved digital filtering method for line frequency zero-crossing detectors is proposed. The multistage filter efficiently attenuates harmonics, wide-band noise, commutation notches, and other impulsive disturbances without causing any phase shift on the primary sinusoidal waveform. Our novel signal-processing system is a cascade of a median filter and an adaptive sinusoid predictor, followed by up-sampling and interpolation. The three-point median filter effectively removes impulses, and the predictor provides wide-band noise attenuation while compensating for delays in the other processing steps. The predictor adapts to possible line frequency variations within the specified range by changing the set of coefficients, based on an estimate of the instantaneous line frequency. The adaptive approach allows the use of highly selective IIR bandpass predictors.