Spectral analysis is a broad topic. Most scientists and engineers who deal with signals are quite comfortable with the concepts of time-frequency relationships. They are familiar with the Fourier transform and the common theorems such as Parseval’s, convolution, delay, etc. The fast Fourier transform, or FFT, is widely used and understood. The FFT gives a value of the Fourier transform at all integer multiples of the reciprocal record length T. This spacing in frequency is also the resolution. The Nyquist frequency or folding frequency is half the sampling frequency. FFT coefficients between the folding frequency fs/2 and the sampling frequency fs are identical to those between –fs/2 and 0, due to the periodicity of the coefficients in the frequency domain.