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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 movable members. The position encoder is such that each of the plurality of signals from the sense coils varies s

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 movable 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.

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1. An apparatus for processing a plurality of input signals each of which vary sinusoidally with the value of a variable and out of phase with respect to each other, the apparatus comprising:a multiplier operable to multiply each of said signals with a respective one of a corresponding plurality of

1. An apparatus for processing a plurality of input signals each of which vary sinusoidally with the value of a variable and out of phase with respect to each other, the apparatus comprising:a multiplier operable to multiply each of said signals with a respective one of a corresponding plurality of periodic time varying signals, each having the same predetermined period and a different predetermined phase; a first combiner operable to combine the signals from said multiplying means to provide an output signal; wherein said predetermined phases of said periodic time varying signals are determined so that the output signal from said first combiner contains a single periodic component having said predetermined period whose phase varies with said variable; first processing circuitry operable to process said output signal from said first combiner to generate an output signal having a value which monotonically varies with the phase of the output signal from said first combiner and hence with the value of said variable over one period of said sinusoidal variation; second processing circuitry operable to process a periodic time varying signal having said predetermined period to generate an output signal having a value which monotonically varies with the phase of the periodic time varying signal processed; and a second combiner operable to combine the output signal value from said first processing circuitry with the output signal value from said second processing circuitry to provide a combined output signal having a value which monotonically varies with the value of said variable over one period of said sinusoidal variation. 2. An apparatus according to claim 1, wherein the predetermined phases of said periodic time varying signals are determined such that their magnitude equals the phase of the corresponding input signal with which it is multiplied.3. An apparatus according to claim 1, wherein each of said plurality of input signals amplitude modulates a periodic time varying carrier signal having a period less than said predetermined period of said other periodic time varying signals.4. An apparatus according to claim 3, wherein said multiplier is arranged to multiply each of said input signals with a periodic time varying signal having a period equal to the period of said carrier signal.5. An apparatus according to claim 4, further comprising a waveform generator which is arranged to generate each of said periodic time varying signals.6. An apparatus according to claim 1, wherein each of said periodic time varying signals are two or three level digital signals.7. An apparatus according to claim 6, wherein said waveform generator is arranged to combine the two different periodic time varying signals prior to multiplication with the respective input signal.8. An apparatus according to claim 6, wherein said multiplier comprises at least one CMOS IC switch.9. An apparatus according to claim 1, further comprising a filter operable to filter said output signal to remove components not having said predetermined period.10. An apparatus according to claim 9, wherein the output of said filtering is substantially sinusoidal having said predetermined period, and wherein said apparatus further comprises a signal converter operable to convert said sinusoidal signal into a square wave signal having said predetermined period and said phase which varies with said relative position.11. An apparatus according to claim 10, wherein said converter comprises a comparator for comparing said sinusoidal signal with a reference signal.12. Apparatus according to claim 1 wherein the modulus of the phase of each of said periodically varying signals is given by (iπ)/n, where n is the number of received signals.13. An apparatus according to claim 1, wherein said transitions are located within said period in the vicinity where said fundamental component and at least said third harmonic component add together.14. An apparatus according to claim 1, wherein said periodic time varying signals each comprise a three level digital signal and wherein said multipler comprises at least two switches.15. An apparatus according to claim 14, comprising a digital waveform generator operable for generating at least two control signals for controlling a respective one of said switches based upon said three level digital signal.16. An apparatus according to claim 1, wherein said first processing circuitry comprises:a comparator for comparing the output signal from said first combiner with a reference voltage to generate a square wave signal whose phase varies with the value of said variable; a first circuit responsive to the leading edge of said square wave signal to generate a first signal having a value which monotonically varies with the phase of the output signal from said first combiner and hence with the value of said variable over one period of said sinusoidal variation; a second circuit responsive to the trailing edge of said square wave signal to generate a second signal having a value which monotonically varies with the phase of the output signal from said first combiner and hence with the value of said variable over one period of said sinusoidal variation; and an adding circuit or a subtracting circuit operable to add or subtract the first and second output signal values from said first and second circuits to provide a resultant output signal having a value which monotonically varies with the value of said variable over one period of said sinusoidal variation. 17. An apparatus according to claim 16, wherein said first and second circuits each comprise a latch circuit which is operable to output a pulse width modulated time varying signal whose DC level monotonically varies with the value of said variable.18. An apparatus according to claim 17, further comprising a filter operable to filter out the time varying component of said pulse width modulated signal to provide a DC output whose value monotonically varies with the value of said variable.19. An apparatus according to claim 16 wherein said first and second circuits each comprise a counter which is operable to output a count which monotonically varies with the value of said variable.20. An apparatus according to claim 1, wherein each of said plurality of periodic time varying signals are two or three level digital signals having a fundamental frequency component corresponding to said predetermined period and higher order harmonics, and wherein said first and second processing circuitry each comprise a filter operable to filter out components above said fundamental frequency component from the output signal from said first combiner.21. An apparatus according to claim 1, wherein said first and second processing circuitry comprises a comparator for comparing the output signal from said first combiner with a reference voltage to generate a square wave signal whose phase varies with the value of said variable.22. An apparatus according to claim 21, wherein each of said first and second processing circuitry comprises a pulse width modulation signal generator responsive to the square wave signal output from said comparator, for generating a pulse width modulating signal whose duty ratio varies with the value of said variable.23. An apparatus according to claim 22, wherein said pulse width modulation signal generator is responsive to the leading edge of said square wave signal to generate a first pulse width modulated signal whose duty ratio varies with the value of said variable, and wherein each of said first and second processing circuitry further comprises a second pulse width modulation signal generator responsive to the trailing edge of said square wave signal output by said comparator for generating a second pulse width modulated signal whose duty ratio varies with the value of said variable.24. An apparatus according to claim 22 wherein each of said pulse width modulation signal generators comprises a latch.25. An apparatus according to claim 23, wherein said second combiner is operable to combine the signals from each of said pulse width modulation signal generators to generate a combined output signal having a value which is less sensitive to voltage offsets in said comparator.26. An apparatus according to claim 1 wherein said signal processed by said second processing circuitry is a reference periodic time varying signal having said predetermined period and a predetermined phase.27. An apparatus according to claim 1 wherein said signal processed by said second processing circuitry comprises a single periodic component having said predetermined period, the phase of said component varying with the value of said variable and wherein the variation of the phase of the signal processed by the first processing circuitry varies in the opposite sense to the variation in phase of the signal processed by the second processing circuitry.28. An apparatus according to claim 27 wherein the signal processed by said first and second processing circuitry are obtained from different input signals.29. An apparatus according to claim 27 wherein the signal processed by said first and second processing circuitry are obtained using at least one common input signal.30. A position detector comprising:first and second relatively movable members; said first member comprising a plurality of sensing circuits, each extending over a measurement path and being offset from each other over said measurement path; said second member comprising a signal generator operable to generate a signal in each of said sensing circuits which varies sinusoidally with the relative position between said signal generator and said sensing circuits, whereby the phase of each of said sinusoidally varying signals is different due to the offset between each of said sensor circuits; and an apparatus according to claim 1 for processing the signals from said sensor circuits to identify the relative position of said first and second members. 31. A position detector according to claim 30, wherein said plurality of sensing circuits are inductively coupled to said signal generator.32. A position detector according to claim 30, wherein said sensing circuits are capacitively coupled to said signal generator.33. A position detector according to claim 30, wherein said sensing circuits extend over a linear path.34. A position detector according to claim 30, wherein said sensing circuits extend in a radial rotary path.35. A method for processing a plurality of signals each of which vary sinusoidally with the value of a variable and out of phase with respect to each other, the method comprising:multiplying each of the signals with a respective one of a plurality of periodic time varying signals, each having the same predetermined period and a different predetermined phase; combining the signals generated by said multiplying step to provide an output signal; wherein said predetermined phases of said periodic time varying signals are determined so that said output signal from said combining step contains a single periodic component having said predetermined period whose phase varies with the value of said variable; using first processing circuitry to process said output signal from said combining step to generate an output signal having a value which varies with the phase of the output signal from said combining step and hence with the value of the variable over one period of the sinusoidal variation; using second processing circuitry to process a periodic time varying signal having said predetermined period to generate an output signal having a value which varies with the phase of the periodic time varying signal which is processed; and combining the output signal value from the first and second processing circuitry to provide a combined output signal having a value which varies with the value of the variable over one period of the sinusoidal variation.