Glossary definition of 'Total Harmonic Distortion'(THD) Not to be confused with THD+N (Total Harmonic Distortion plus Noise), this is a specialised measurement of the level of the distortion harmonic components in a signal. Whereas the THD+N measurement filters out the fundamental and measures what is left, including noise, hum, etc, along with the distortion components, the THD measurement selectively band pass filters the distortion harmonics and measures them in isolation. The result is usually expressed in percent or dB relative to the fundamental and should state the number of harmonics included in the measurement.
A THD measurement is most easily implemented in the frequency domain using an FFT based spectrum. The below screenshot shows the spectrum from a 1kHz signal driven into clipping. The purple trace shows the response of an FFT filter set to filter in the first 9 harmonics (i.e., harmonics 2 to 10).
This type of measurement will tend to give a lower reading than a THD+N measurement as it contains far less noise. Using the same technique it is also possible to measure the level of only the odd harmonics, only the even harmonics, or to measure individual harmonics in isolation. This measurement can also be swept over frequency, amplitude or both to give a more detailed picture of the performance of the EUT.