Hi @chrichap, you have the measurement start at 1 Hz, and there’s a large rise near DC present in your signal. Is there a reason you want the sub 10 Hz in there?
Try some experiments with a second tone and the QA401 in loopback. You can then use the second tone to sweep around and understand what is getting notched out for the THDN measurement.
For example, in the plot below, you can see Gen2 is set to 284 Hz and it’s NOT part of the measurement. So, from that you can learn the notch for the THDN measurement extents from at least 100 Hz up to 284 Hz.
And then, as we increase Gen just beyond 300 Hz, note how the THDN suddenly jumps to -20.
This second generator will let you figure out the “notch” used for the THDN. The challenge is the strategy that makes sense at 1 kHz doesn’t usually make much sense at very low or very high frequencies.
Some semi-related discussion is at the link below: