Total Harmonic Distortion(THD) is a measure of the amount of non-linear distortion in electrical signals. It plays a decisive role in audio, measurement and energy technology in particular, as it allows conclusions to be drawn about the purity and quality of a signal.
Total Harmonic Distortion (THD) refers to the ratio of harmonics to the fundamental frequency of a signal. These are multiples of the fundamental frequency that would not occur in an ideal signal form – a pure sine wave. These additional frequencies are caused by non-linear distortions in components or circuits.
THD is usually given as a percentage and shows how much a signal deviates from the ideal. A THD value of 1 %, for example, means that the sum of the harmonics corresponds to 1 % of the fundamental. The lower this value, the “cleaner” the signal.
The total harmonic distortion results from the ratio between the effective values of the harmonics and the fundamental. The decisive factor for the calculation is how strong the individual amplitudes of the harmonics are in relation to the fundamental.
To analyze the signal, it is broken down into its frequency components using a Fourier transform. The most common method is the Fast Fourier Transform (FFT). This makes it possible to identify exactly which frequencies – for example at a reference frequency of 1 kHz – are contained in the signal.
Modern measuring devices display the THD value directly after internal calculation. In addition to the percentage, the frequency spectrum is often also displayed in order to visualize the distribution of harmonics.
In practice, harmonic distortion is caused by non-linear components such as semiconductors, transformers or loudspeakers. These components generate additional frequency components that can distort the original signal.
Typical areas of application:
Even in industrial applications, such as frequency converters or switching power supplies, a high THD can have a negative impact on the electrical power quality.
The term distortion factor is often used synonymously with total harmonic distortion. Strictly speaking, however, there are differences. The distortion factor often refers to the ratio of the non-linear distortion to the overall signal, while THD specifically considers the harmonics in relation to the fundamental frequency.
There is also the extended measurement parameter THD+N (Total Harmonic Distortion plus Noise), which takes noise into account in addition to harmonics. This specification is particularly common in audio and measurement technology, as it provides a more realistic representation of the actual signal distortion.
The level of THD depends on various factors:
THD is influenced by the shape and size of the voltage applied to the system. A constant and clean voltage signal contributes significantly to minimizing distortion.
Among other things, balanced signal paths, active filters, linear amplifiers and negative feedback are used to reduce THD. The use of high-quality components also helps to reduce distortion to a minimum.
Depending on the area of application, different THD guide values are considered acceptable:
| Application | Typical THD value |
|---|---|
| Hi-fi audio amplifier | < 0,1 % |
| Mains electricity (public supply) | < 5 % |
| Radio technology / Mobile communications | < 1 % |
| Industrial plants / grid load | < 8-10 % (permissible) |
Standards such as IEC 61000 or corresponding DIN regulations define limit values, particularly for industrial applications or the mains connection of appliances.
Total harmonic distortion is a key measure for assessing signal quality in electrical and electronic systems. It describes the proportion of harmonic distortion caused by harmonics and is expressed as a percentage or dB.
Low THD stands for high signal purity – and is therefore essential in areas such as audio technology, measurement technology, communication and energy technology. Thanks to modern analysis methods and targeted system design, THD can now be efficiently measured and minimized – a decisive factor for quality, efficiency and safety.
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