8.4.1 Signal to Harmonic Distortion

Harmonic distortion arises when a signal passes through a nonlinear circuit. The spectrum of the output of a nonlinear circuit includes not only the frequency components that appeared at the input, but also integer multiples (harmonics) of the input frequency components. Harmonic distortion is often measured with a single tone test signal, that is, a sine wave at a particular frequency (specified in the data sheet). To save test time, distortion can be measured in parallel with absolute gain using the FFT results from the gain test.

When passing a single test tone through the circuit under test, the harmonic distortion components appear at integer multiples of the test tone's frequency, F _t. F _t is often referred to as the fundamental tone (not to be confused with the fundamental frequency of the sampling system, F _f). Distortion that is symmetrical about the x axis gives rise to only odd harmonics (3 F _t, 5 F _t, 7 F _t, etc.). Asymmetrical distortion, such as clipping on only the upper or lower portion of the waveform, gives rise to both odd harmonics and even harmonics (2 F _t, 4 F _t, 6 F _t, etc.).

Signal to total harmonic distortion is defined as the ratio of the RMS signal level of the test tone divided by the total RMS of the odd and even harmonic distortion components. Signal-to-distortion is often expressed...