The circuit useful for distortion measurements notches out the fundamental frequency of 1 kHz to allow measurement of the residual level of harmonics. First a true RMS meter is used to measure the 1-kHz input level Ein by setting Sa to the input position.

The Circuit Diagram of 1-Khz Harmonic Distortion Meter

Then, Sa is placed in the distortion position and the 2 k potentiometer is adjusted for a null. The residual reading is noted. The THD is then calculated based on a formula:

THD(%) = Vdist (100)/Vin