Hi Antonix, I think the article SABristol is referencing might be below. In short, use the ability of the QA401’s sweep to learn the best settings for the measurements. When distortion is bad, SABristol is spot on that long FFTs just waste your time. As distortion levels drop, you’ll need larger FFTs to reduce the noise floor and see the harmonics. But the sweeps will help familiarize yourself with the tradeoffs to be made.
In short, if speed is important, start with a small FFT and increase it until you see the 2H, 3H fully emerge. And then go from there.
I’d be interested is seeing your results posted on here.
Certainly when I modelled it, increasing the bias current reduced the distortion with a law of diminishing returns, so in practice my decision where to set it in practice was mainly driven by how hot it got when idling.