Patent ID: 7406393

Claim:
A frequency determining system comprising: a signal generator operable, when energised by an alternating excitation signal, to generate an alternating response signal of predetermined but unknown frequency and whose amplitude varies in dependence upon the difference in frequency between the excitation signal and the response signal; an energising circuit operable to generate first and second excitation signals at first and second different excitation frequencies for energising said signal generator; a receiver operable to receive a first response signal generated by said signal generator when energised by said first excitation signal and a second response signal generated by said signal generator when energised by said second excitation signal; a processing circuit operable to process each of the first and second response signals received by said receiver to generate first and second amplitude measures indicative of the amplitudes of the corresponding response signals; and a determining circuit operable to determine a frequency measure indicative of the frequency of the alternating response signals generated by said signal generator using said first and second amplitude measures.