THD Calculator

This THD Calculator calculates THD (Total Harmonic Distortion). The THD formula used in this Total Harmonic Distortion calculator is also mentioned. THD conversion formula to convert THD (%) to THD (dB) and vice versa are also used in this THD calculator.

 

What is THD?

• THD is the short form of Total Harmonic Distortion. It is the ratio of sum of harmonics to the fundamental signal component. Normally initial 5 to 6 harmomic components are used for characterization. In other words, it is the ratio of rms of fundamental signal to mean of root-sum-square of its harmonics plus all noise components (excluding DC). In FFT case, bandwidth is taken from DC to Fs/2. THD can be expressed in dB, dBc or percent.

The figure-1 depicts fundamental signal and six harmonic components in a frequency spectrum. The initial five to six harmonic components are significant in introducing distortion to the desired fundamental signal frequency. Hence these 5 to 6 harmonic components are used for THD calculation as mentioned below.

 

THD calculator #1

Following THD calculator #1 calculates THD (Total Harmonic Distortion) based on dBm values of fundamental signal and six harmonic signals. This calculation is approximate as noise and distortion levels are not considered.

 

Signal, dBm (Input)   :

Harmonic #1, dBm (Input)   :

Harmonic #2, dBm (Input)   :

Harmonic #3, dBm (Input)   :

Harmonic #4, dBm (Input)   :

Harmonic #5, dBm (Input)   :

Harmonic #6, dBm (Input)   :

 

THD in % :

THD in dB :

 

EXAMPLE of THD calculator #1:
INPUT: Fundamental signal (dBm) = 16.17, Harmonic #1 = -35.3 dBm, Harmonic #2 = -6.83 dBm, Harmonic #3 = -40.6 dBm, Harmonic #4 = -35.1 dBm, Harmonic #5 = -37.5 dBm, Harmonic #6 = -54 dBm
OUTPUT: THD in % = 7.0943, THD in dB = -22.9817

 

THD Formula or THD equations

Following THD formula or THD equations are used to calculate THD (%) in this Total Harmonic Distortion calculator.

 

 

After THD (%) in calculated using equation-1, following THD formula is used to convert THD (%) to THD (dB).

 

 

Refer relationship of THD with SNR and SINAD for more information on THD basics.

 

THD calculator #2

• If dBc values are mentioned for harmonic signals and fundamental signal is mentioned in dBm. One can calculate dBm values for harmonics and utilize above mentioned THD calculator.
• Fundamantal (i.e. Reference signal) = 15 dBm, H1 of -42 dBc can be taken as -27 dBm (i.e. -42 + 15).
• If fundamental and harmonics are mentioned in V(peak) then these can be converted to V(rms) values. One can use THD formula or equation 2 as mentioned above to calculate THD (%).

 

The Burosch audio NF Lab confirms these measurement results.