Measuring the Spectral Content of a Signal: An Introduction [Instrumentation Notes]

Fourier analysis is often thought of as a technique for measuring the spectral content of a signal over the period in which the signal was measured. In this column, we discuss techniques that allow you to use Fourier analysis to provide accurate measurements of the instantaneous frequency of a signal. We discuss the short-time Fourier transform and its main problem; we explain why it gives a very coarse estimate of the frequency. We show how it is possible to use the phase information provided by the discrete Fourier transform (DFT) to produce a much sharper estimate of the frequency. Finally, we show how the improved estimate can be used to build a virtual FM demodulator.