Fourier analysis tells us that any signal can be represented as a sum of signals of various frequencies. Therefore, any signal can be represented as an infinite sum of these distinct signals as follows:

A pure sinewave is such that for only one value of n. In other words, all energy is concentrated at one frequency. This characteristic, unique to the sinewave, is one of the reasons that sinewaves are used extensively when testing for distortion.

Modem and audio transformer applications are particularly sensitive to the effects of harmonic distortion. Harmonic distortion is due to nonlinearities in the amplitude transfer characteristics. In particular, it is the nonlinear change of magnetization in the material that causes distortion. The typical output signal contains not only the fundamental frequency but integer multiples of the fundamental frequency. For harmonic distortion measurements, a single sinusoidal signal is supplied to the transformer primary winding and wave analysis of the secondary winding at the harmonic frequencies determines the distortion (see Fig 1).