Total Harmonic Distortion (THD) analysis utilizing the FFT capabilities of modern digital storage oscilloscopes

This paper shows how to analyze Total Harmonic Distortion (THD) using the fast Fourier transformation (FFT) capability of low-cost digital sampling oscilloscopes. It was found that THD measurements using this method are accurate within less than a few percent of absolute error. A high level of accuracy can be achieved as long as the measured THD is sufficiently large (> 3.2 %) in order to not subceed the dynamic range of common 8-bit digital storage oscilloscopes. The method presented herein is particularly convenient if only a coarse, relative indicator of THD improvement or degradation is needed.