Patent ID: 8108167

Claim:
A method for ascertaining harmonic components of a periodic analog signal at a fundamental frequency which changes over time, the method comprising: using the analog signal to produce an analog target signal at a target fundamental frequency which is constant over time, the analog target signal being produced by, modulating the analog signal using a first carrier frequency to form two frequency sidebands, the first carrier frequency being chosen to be relatively greater than the fundamental frequency of the analog signal, filtering one of the two frequency sidebands out of the modulated analog signal, modulating the filtered-out frequency sideband using a second carrier frequency to form two frequency sidebands again, the second carrier frequency being greater than the fundamental frequency of the analog signal, and a difference between the second and first carrier frequencies respectively being the same as a discrepancy between the fundamental frequency and the target fundamental frequency, and outputting one of the two frequency sidebands as the produced analog target signal; digitizing the produced analog target signal, using the target fundamental frequency as a sampling frequency, to produce digitized samples; and fast Fourier transforming the digitized target signal to ascertain the harmonic components using the digitized samples.