Total harmonic distortion (THD) is a representation of the effect that multiple harmonic spurs throughout a spectrum band have on the fundamental tone.

You can calculate THD by summing the power in each of the harmonics and dividing by the total power of the fundamental. As a general principle, a signal becomes visibly distorted when the THD approaches -30 dB.

The equation for THD is:

T H D = V h 2 2 + V h 3 2 + V h 4 2 + … + V h N 2 V f u n d × 100 % T H D = V h 2 2 + V h 3 2 + V h 4 2 + … + V h N 2 V f u n d × 100 %

As this equation suggests, the THD specification evaluates the power in harmonic spurs from the second through the nth harmonic. In practice, typical signal generators feature THD specifications for harmonics two through six. The following figure shows an example plot of a generated signal and illustrates the concept of THD specification in signal generators.

Figure 4. Total Harmonic Distortion Example


The previous figure shows an example plot of a 20 kHz sinusoid generated with an arbitrary waveform generator. Notice the power levels of the harmonic spurs, indicated with circles. This generator features -77 dBc or better of THD for the second through sixth harmonics.

THD generally deteriorates as the generated signal increases in frequency. When working with a signal generator, consider the THD throughout the bandwidth of the generator.