#### Onedust

##### Junior Member level 2

I have a question regarding understanding of the Total Harmonic Distortion (THD) and why is it a good merit for signal distortion measurement.

With the FOURIER Transform (let’s talk only about DFT) we can decompose any signal into sinusoids of different frequencies, phases and amplitudes. That means that any signal can be composed from the sum of these sinusoids.

Now, if we send a signal through an amplifier we would like to get the same signal on the output, but with the amplified amplitude. So if we check the spectrum before and after we can see, how good the amplifier is and if the signal was not only amplified, but also changed.

THD gives us a relationship between the power (amplitude) of the fundamental frequency, i.e sinusoid with the highest amplitude in the spectrum and the sum of powers of all

**harmonic components**of the signal. This way we can send a pure sine wave through an amplifier and see if it was changed and what

**harmonic components**(this is why it is total harmonic distortion) have changed the signal.

But what about other frequency components? The signal is not made of only harmonic frequencies. Why do we only check harmonic frequencies? I am sure I missed something fundamental.

Regards,

Den