Cancellation of coherent line-frequency noise in evoked potential recordings

A stimulator for evoked potential recording can become synchronized with the fundamental or harmonics of the line frequency, thus introducing interference which cannot be removed by averaging. Synchronizing the stimulus alternately to the positive-going and negative-going zero-crossing of the line will reduce 60 Hz and odd harmonics, but will not reduce higher even-harmonics. This communication describes a circuit that synchronizes the stimulus to the alternate phases of the line, but also introduces a random delay between zero and one-half the line period, thus reducing the fundamental and higher harmonics as well.