[3] IEEE Standard 519-2022 covers the recommended practice and requirements for harmonic control in electric power systems.
The THD is usually expressed in percent or in dB relative to the fundamental as distortion attenuation.
[20] The International Electrotechnical Commission (IEC) also defines another term total harmonic factor for the "ratio of the RMS value of the harmonic content of an alternating quantity to the RMS value of the quantity" using a different equation.
It is usually measured by inputting a sine wave, notch-filtering the output, and comparing the ratio between the output signal with and without the sine wave:[22] Like the THD measurement, this is a ratio of RMS amplitudes[8][23] and can be measured as THDF (bandpassed or calculated fundamental as the denominator) or, more commonly, as THDR (total distorted signal as the denominator).
The model is described in the German standard DIN45631[26] For a given input frequency and amplitude, THD+N is reciprocal to SINAD, provided that both measurements are made over the same bandwidth.
The distortion of a waveform relative to a pure sinewave can be measured either by using a THD analyzer to analyse the output wave into its constituent harmonics and noting the amplitude of each relative to the fundamental; or by cancelling out the fundamental with a notch filter and measuring the remaining signal, which will be total aggregate harmonic distortion plus noise.
Different software can be used to generate sinewaves, but the inherent distortion may be too high for measurement of very low-distortion amplifiers.
[28] A number of methods have been developed to estimate the actual audibility of THD, used to quantify crossover distortion or loudspeaker rub and buzz, such as "high-order harmonic distortion" (HOHD) or "higher harmonic distortion" (HHD) which measures only the 10th and higher harmonics, or metrics that apply psychoacoustic loudness curves to the residual.
[1] For example, a pure square wave has THDF equal to The sawtooth signal possesses The pure symmetrical triangle wave has For the rectangular pulse train with the duty cycle μ (called sometimes the cyclic ratio), the THDF has the form and logically, reaches the minimum (≈0.483) when the signal becomes symmetrical μ = 0.5, i.e. the pure square wave.
[1] However, analytic computation of the THDF for complicated waveforms and filters often represents a difficult task, and the resulting expressions may be quite laborious to obtain.