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The total harmonic distortion (THD or THDi) is a measurement of the harmonic distortion present in a signal and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. Distortion factor, a closely related term, is sometimes used as a synonym.
Total harmonic distortion, or THD is a common measurement of the level of harmonic distortion present in power systems. THD can be related to either current harmonics or voltage harmonics, and it is defined as the ratio of the RMS value of all harmonics to the RMS value of the fundamental component times 100%; the DC component is neglected.
A total harmonic distortion analyzer calculates the total harmonic content of a sinewave with some distortion, expressed as total harmonic distortion (THD). A typical application is to determine the THD of an amplifier by using a very-low-distortion sinewave input and examining the output.
For example, intermodulation distortion from the third order (IMD3) of a circuit can be seen by looking at a signal that is made up of two sine waves, one at and one at . When you cube the sum of these sine waves you will get sine waves at various frequencies including 2 × f 2 − f 1 {\displaystyle 2\times f_{2}-f_{1}} and 2 × f 1 − f 2 ...
Harmonic distortion may be expressed in terms of the relative strength of individual components, in decibels, or the root mean square of all harmonic components: Total harmonic distortion (THD), as a percentage. The level at which harmonic distortion becomes audible depends on the exact nature of the distortion.
When calculating the distortion, it is common to exclude the DC components. [1] Due to widespread use, SINAD has collected several different definitions. SINAD is commonly defined as: The ratio of (a) total received power, i.e., the signal to (b) the noise-plus-distortion power. This is modeled by the equation above.
An alternative technique, total harmonic distortion measurement, cancels out the fundamental with a notch filter and measures the total remaining signal, which is total harmonic distortion plus noise; it does not give the harmonic-by-harmonic detail of an analyser. Spectrum analyzers are also used by audio engineers to assess their work.
Typically, an amplifier's power specifications are calculated by measuring its RMS output voltage, with a continuous sine wave signal, at the onset of clipping—defined arbitrarily as a stated percentage of total harmonic distortion (THD), usually 1%, into specified load resistances. Typical loads used are 8 and 4 ohms per channel; many ...