100% Distortion At <20.3Hz In Loopback

I’ve noticed that when looping back and measuring THD, any frequency below about 20.3Hz produces a 100% distortion reading. The FFT is 1024K, and it doesn’t seem to change with windowing. Single ended vs differential doesn’t change it, the operating level is 0dBV and sampling rate is 192k. Changing sampling rate to 48k lowers the reading to around .0012%. I know that 20Hz at 192k may be pushing the capabilities of the QA402, but it would be nice if this wouldn’t happen.

Hi @Rammis, yes, the lowest you can currently go is around 23 Hz with 1M FFT. For others trying to replicate, you need to make sure the green “F” is drawn below your fundamental. That can be done by setting RMS starting measurement to a number suitably below that (the default is 20 Hz, but that might skew the measurement, so a bit lower would be preferable depending on the skirt.

I think we can make some improvements here. I added to github issues.

I’d be happy with going down to 20Hz since that is what is commonly the lowest frequency measured on power amps.