Thanks for the reply. The waveform is a sine wave; the Back EMF of a Servo Motor. The HP FFT Analyzer reads .24%THD, and a Lab View application agrees exactly with the HP. The PicoScope, however, reads .14% when sampling at the exact fundamental frequency. When the sample frequency is a notch off of the fundamental, the PicoScope reads .19% THD (siginal is 19Hz. PicoScope frequency divisions go in steps of 19, and 21Hz). So, the actual error is 42%.
Also, the program produces an error when I attempt to save the FFT waveform as a .csv file, and crashes. The .psdata file saves OK, and the .csv file in scope function saves OK. I am using the new 6_r6_0_13 software.
Thanks for your help!