What is Harmonic Distortion : Types and Its Causes

When we are applying an input signal in the form of sinusoidal (or any form of signal) to any electronic circuit then its output should be the same type of signal. It means the output also must have the same form of signal is sinusoidal. If in case, the output is not the same replica of the input signal or if the output does not equal to the input signal then the difference is called distortions. Due to these distortions, the output does not equal the input. The harmonic distortion can be defined by using this example. When the 5V input signal is applied to the circuit, then the output signal will have only 2V voltage. It indicates the signal loses its voltage because of distortion. This will be occurred in amplifiers, power amplifiers & modulation techniques, etc. There are various techniques to decrease this distortion and few methods & formulas are available to calculate the distortion level. This article discusses what is harmonic distortion, definition, analysis, causes, etc

What is Harmonic Distortion?

We can understand the word harmonic like the integer which multiplies fundamental frequencies is known as “Harmonics”. Here, harmonic is a type of signal whose frequency is an integral multiple of the reference signal. In another way, it can be defined as the ratio between the frequency of the signal and the frequency of the reference signal. For example, X is an input AC signal which has the frequency f Hz.

Harmonic-distortion-input- signal
Harmonic-distortion-input- signal

When the signal X is displayed on the CRO then the signal X will appear to repeat for every f Hz. Here, signal X is the reference signal & the signal is showing on CRO have frequencies like 2f, 3f, 4f and so on. Theoretically, the signal includes infinite harmonics. Below two figures indicate the input signal & the distorted output when input is applied to any circuit.


If the signal having an equal time period of positive cycle and negative cycle, then such a signal is called symmetrical signal & odd harmonics may appear (multiplies 3rd, 5th, etc of the fundamental frequency). If the signal doesn’t have an equal time period of positive cycle and negative cycle, then such a signal is called asymmetrical signal and even harmonics may appear (multiplies 2nd, 4th, etc of the fundamental frequency) and DC components also may appear in the asymmetrical signals.

In the above figure, we can notice the fundamental signal frequency as 100Hz and their harmonics will exist at different frequencies for the reference signal frequency like 100 Hz.

Harmonic-distortions-in -signal
Harmonic-distortions-in -signal

If the signal has harmonic distortions while harmonic frequency components exist then to find the percentage of these distortions at the particular harmonic level is,

%nth harmonic distortion = [Pn]/[P1} * 100

[Pn] = amplitude of nth frequency component

[P1] = amplitude of fundamental signal frequency

Distortions may occur because of nonlinear characteristics of the components which are used in an electronic circuit. These components may exhibit nonlinear characteristics this results in the generation of distortions in the signal.  There are five different types of harmonic distortion in power systems. They are

  • Frequency distortion
  • Amplitude distortion
  • Phase distortion
  • Intermodulation distortion
  • Cross over distortion

Harmonic Distortion Analysis

The analysis of this distortion is a unique type of analysis. In this type, a single frequency sinusoidal signal is applied to the circuit and its output with distortion to be measured and analyzed.

When the input signal is applied to the circuit, due to nonlinear characteristics of the components the distortion may develop in the output signal. Because of this, the reference signal may appear in the output at different frequency points. If we analyze the distortions with total harmonic distortion measurement technique we can know the value of the total harmonic distortion (THD), total harmonic distortion plus noise (THDN), signal to noise and distortion (SINAD), signal to noise ratio (SNR) and nth harmonic value with respect to the fundamental frequency. By this total harmonic-distortion measurement method, we can know the input and output voltages and input and output power.

Harmonic Distortion Causes

The main reasons for the harmonic-distortions are the nonlinear load and nonlinearity characteristics of the electronic components. Nonlinear load changes the impedance with the applied input voltage. This leads to distortions will develop in the output signal. And the components which are using in the circuit also show the nonlinearity characteristics. This also leads to the development of the harmonics in the output. Because of the harmonic-distortions circuit gets heat and output not equal to the input. This effect is harmful to any circuit.

Harmonic Distortion Analyzer

Finding the harmonic distortion factor is most important for any circuit. We can analyze these distortions by this value. Total harmonic distortion (THD) is the most useful technique to find the total harmonic-distortion for the current signal and total harmonic distortion for voltage signals.

THD can be defined as the ratio between RMS values of all harmonic signals to the RMS value of the fundamental signal frequency.

Current THD – As per the above statement total distortion for current is indicated by THDi


Here, In is the RMS current for the nth harmonic signal and I1 is the RMS value of the fundamental signal.

Voltage THD – same as THDi, total harmonic-distortion of voltage is denoted by THDv.


Here, Vn is the voltage of nth harmonic and V1 is the voltage of the fundamental signal. Total harmonic distortion (THD) also analyzes the nonlinear behavior of the system with the Fast Fourier transform (FFT).

Total harmonic distortion plus noise (THDN) is defined as the ratio of the RMS value of the fundamental signal to the RMS value of the harmonics along with noise components.

Thus, this is all about Harmonic distortion. From the above information finally, we can conclude that this is the most considerable important parameter in the system because it can violate the output signal. And this can be analyzed by the THD factor and can be decreased by the techniques and devices which are available in the market.  Here is a question for you, what are the applications of harmonic distortion?


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