Measuring eye movements with a search coil: non-linear filter allows simultaneous recording of horizontal and vertical eye position by means of the phase modulation method

Measuring eye position from the phase angle of the signal induced by a rotating magnetic field in a search coil presents a number of advantages. So far, however, existing schemes using this principle either record eye position in one dimension only or require two search coils. We describe here a simple analog circuit which singles out a signal of given frequency from a frequency composite without changing the signal's phase. This makes it feasible to measure horizontal and vertical eye position with a single search coil by using two orthogonal rotating fields of different frequency.