Total harmonic distortion (THD) is a measurement that tells you how much of the distortion of a voltage or current is due to harmonics in the signal. THD is an important aspect in audio, communications, and power systems and should typically, but not always, be as low as possible.
Figure 1. A sinusoidal voltage and a square wave voltage in the time domain.
Figure 2. A sinusoidal voltage and a square wave voltage in the frequency domain; only the square wave has peaks at the harmonic frequencies.
component. This component can be factored out, and since it appears in both the numerator and denominator, it actually cancels out, which leaves the expression for THD of a square wave as follows:
is the value of
when n is 1. Since this value is 1, the summation in the THD expression can be re-written as:
Measuring Total Harmonic Distortion
Example THD Measurement
Figure 3. A system that introduces crossover distortion into a signal.
Figure 4. Frequency spectrum of sinusoidal voltage with crossover distortion.
and since the
occurs in all terms, it can be factored out and cancelled).