Signal processing apparatus and method

Processing circuitry is provided for processing signals received from, for example, sense coils forming part of a position encoder used to encode the relative positions of two relatively moveable members. The position encoder is such that each of the plurality of signals from the sense coils varies sinusoidally with the relative position of the members but out of phase with respect to each other. The processing circuitry comprises mixers for multiplying each of the received signals with one of a corresponding plurality of periodic time varying signals, each having the same predetermined period and a different predetermined phase, and an adder for adding the signals from the mixers. The phase of the mixing signals are chosen so that the output signals from the adder contains a single periodic component having the predetermined period whose phase varies with the relative position of the two members. Preferably, a reference channel is provided in order to allow for compensation of common phase errors in both channels. The period time varying signals multiplied with each of the signals from the position encoder preferably comprise a two or three level square wave signal having a number of transitions designed to reduce the low order harmonic content of the mixing signals.