Strange FFT / FFT not stable

[kawal]

@JP-Huijser

loscope signal of QA403 app looks clean.
Using a real scope (Picoscope), the FFT is also ok-esh, except the FFT on my scope does not go to such a low noise floor (only about -75dBV).

I wonder if its the same issue I am seeing with FFT display - I use it on analog class A/B amp and saw it when changing settings in the SW. I saw some issue also in QA40Xplot. So maybe its a HW issue with the USB buffer or some sort of issue like this.

Here is the issue i seen

Some bug in the spectrum display which fixes itself over time

Happens when some settings are changed in the SW like sampling rate or FFT depth