Automatic power-line interference canceller for geophysical instruments

Abstract

Power-line interference is one of the main interferences for many geophysical measurements. Especially for instruments operating in the low frequency range, possessing broad frequency bandwidths and being very sensitive to phase changes, such as time-domain electromagnetic (TEM) instruments, any conventional linear filter used to suppress the power-line interference will cause distortion to the desired signals because of its unwanted phase (and amplitude) contribution(s). Normally, the 'comb' filtering effect of a TEM instrument provides excellent suppression of power-line interference. But in the vicinity of an industrial area, the power-line interference is sometimes so strong that it may saturate the linear circuits of the instrument or modulate the desired signal, and thus spoil the 'comb' filtering effect. The completely self-contained power-line interference canceller introduced in the following is developed to cancel the strongest frequency component, very often the fundamental one, of the power-line interferer. Two or more such cancellers can be used together, if more than one frequency component of the interferer need to be cancelled.