Abstract: An electronic filter for automatically tracking and removing harmonically lated interfering electrical signals such as power line interference harmonics without attenuating other signals of interest even though the signals are frequency stable and/or near the interference signal frequencies. The filter comprises a very narrow band electronic commutated capacitor-bank comb-notch filter driven by a counter/decoder circuit which is in turn driven by a phase locked loop. The filter also comprises two narrow band analog filters tuned to the two lowest harmonics of the interfering signal. The summed output of these two filters is applied to the input of the phase locked loop. The phase locked loop locks to the proper multiple of the interfering signal and drives the comb-notch filter at a frequency which causes it to generate notches at unit multiples of the fundamental of the interference frequency.