I have captured an FFT of a 1 kHz signal that I am generating using a WM8904 codec running at 48k samples/second connected via I2S to a ARM microcontroller (Atmel SAME70). It appears to have some jitter in it, which I believe is due to a small delay in restarting the DMA after each block (which I know how to fix, but I was more interested at this point in the particulars of taking this measurement). I am using a Agilent MSO7054B to make the measurements.
The fundatmental is clearly visible at 1.0 kHz. But the next peak is at 2.1 kHz, not 2.0; the one after that at 3.2 kHz, etc. Is that due to the jitter?
Can I still use those peaks to calculate a THD value? When doing so, I got a value of 4.12%. Does that seem reasonable?