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 ha rmonics, wide-band noise, commutation notches, and other impulsive dis turbances without causing any phase shift on the primary sinusoidal wa veform, Our novel signal-processing system is a cascade of a median fi lter and an adaptive sinusoid predictor, followed by up-sampling and i nterpolation, The three-point median filter effectively removes impuls es, and the predictor provides wide-band noise attenuation while compe nsating for delays in the other processing steps, The predictor adapts to possible line frequency variations within the specified range by c hanging the set of coefficients, based on an estimate of the instantan eous line frequency, The adaptive approach allows the use of highly se lective IIR bandpass predictors.