A HIGH-PERFORMANCE DIGITAL SYNCHRONOUS NOISE-FILTER

A digital synchronous filter for rejecting periodic noise such as that arising from the mains power supply is described. With a stationary n oise signal the filter achieves a rejection ratio of 127 dB. With the somewhat variable mains, rejection ratios obtained in practice are typ ically 60 dB or better. The rejection notches are very narrow and the passband response is flat, leaving the signal of interest essentially undistorted. The performance of a hardware implementation of the filte r is examined in detail and alternative implementations are discussed.