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.
In many technical applications, this characteristic value is used to describe the deviation of a real signal from an ideal sinusoidal shape. The more an electrical system operates non-linearly, the more clearly additional frequency components appear. These additional signal components cause the spectrum of a signal to change and its quality to be measurably affected.
TDH 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 is, the “cleaner” the signal is and the closer it is to an ideal sinusoidal signal shape.
In signal theory, this characteristic value therefore describes the extent to which a system generates additional harmonic signal components. This consideration is particularly relevant wherever high signal quality is required – for example, for precise measurements or when transmitting sensitive information.
The TDH results from the ratio between the RMS values of the harmonic signal components and the fundamental oscillation. The calculation is based on the RMS value of the individual signal components, as this is the most reliable way of describing the energetic strength of an electrical signal. The decisive factor here is how strong the individual amplitudes are in relation to the fundamental. For the calculation, the effective values of the individual components are considered, as they describe the energetic strength of a signal most reliably.
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 recognize exactly which frequencies – for example at a reference frequency of 1 kHz – are contained in the signal.
In practice, the entire spectrum is analyzed. In addition to the fundamental frequency, additional lines appear at integer multiples of the frequency, for example at 2 kHz, 3 kHz or higher ranges. These frequency components are caused by non-linear effects in the system.
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 the harmonic signal components. With detailed measurements, frequency ranges up to several tens of kHz can also be analyzed, which is particularly important in audio technology or high-frequency applications.
In practice, harmonic distortion is caused by non-linear components such as semiconductors, transformers or loudspeakers. These components generate additional frequency components that can change the original signal.
Typical areas of application:
Audio amplifiers: A THD value of less than 0.1% is ideal for the most unadulterated sound reproduction possible. This value is measured over a wide frequency range, especially for high-quality hi-fi devices.
Power supplies: High values can affect the power grid, for example by placing additional load on lines or transformers.
Communication technology: Clear signals with low distortion are crucial for reliable data transmission. Even small additional frequency components can influence the signal quality.
In industrial applications, such as frequency converters or switching power supplies, strong distortion can also have a negative impact on the electrical power quality. Here, additional frequency components are often caused by switching operations of electronic power components.
The term distortion factor is often used. Strictly speaking, however, there are differences between the two terms.
The distortion factor generally describes the ratio of the non-linear distortion to the total signal. Total harmonic distortion, on the other hand, looks specifically at the additional frequency components in relation to the fundamental frequency. This makes it possible to determine more precisely which additional frequencies are present in the signal.
Another common parameter is THD+N (Total Harmonic Distortion plus Noise). This parameter also takes into account the noise within a certain frequency range. This parameter is used in audio technology in particular because it provides a more realistic picture of the actual signal quality.
The level of THD depends on various factors:
The effective value of the voltage or current can also have an influence on the resulting distortions. At higher signal levels, many electronic components operate less linearly, which can result in additional frequency components.
THD is also influenced by the shape and stability of the applied voltage. A constant and clean signal makes a significant contribution to reducing unwanted distortion.
Symmetrical signal paths, active filters, linear amplifiers and negative feedback are used to reduce THD. Careful layout planning, a clean power supply and high-quality components can also help to improve the signal quality.
Depending on the area of application, different guide values are considered acceptable. These limit values depend heavily on how sensitive a system is to additional frequency components.
| 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. These standards ensure that electrical systems maintain a certain level of signal quality and network compatibility.
THD is a key measure for assessing signal quality in electrical and electronic systems. It describes the amount of harmonic distortion caused by additional frequency components in the signal and is usually expressed as a percentage or in dB.
Low THD stands for high signal purity – and is therefore essential in areas such as audio technology, measurement technology, communication and energy technology. Today, modern analysis methods make it possible to determine this parameter very precisely, for example by analyzing frequencies up to the kHz range.
Total harmonic distortion can be effectively reduced through targeted circuit development, suitable measurement methods and high-quality components. It therefore remains an important quality indicator for the performance and reliability of electronic systems.
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