Automated non-coherent sampling THD meter with spectrum analyser

This paper describes an automated non-coherent sampling THD meter based on the 24 bit digitizer National Instruments PXI 5922. The THD meter is designed for a frequency range up to 1 MHz with analysis of the harmonics up to 6 MHz. Contrary to commercial devices developed THD meter displays a signal spectrum and gives user the possibility to evaluate impurities in the signal affecting THD calibration and allow the selection of an appropriate number of evaluated harmonics.