Multistage adaptive filters for in-phase processing of line-frequency signals

We describe two adaptive multistage digital filters for line-frequency signal processing in zero-crossing detectors and synchronous power systems. These filters combine a median filter with adaptive, predictive filters, thus making it possible to extract the sinusoidal signals from noise and strong disturbances without phase shifting the primary frequency component. The median filter is used as a prefilter because it can efficiently remove deep commutation notches from the waveform. Adaptation allows the filter to track the time-varying line-frequency and avoids the misadjustment problem encountered with a fixed predictive filter. An FIR (Finite Impulse Response) filter-based predictor is attractive when there are large frequency variations, and an IIR (Infinite Impulse Response) filter-based cascade predictor has better attenuation characteristics for undesired frequency components.