Hi @Briks,
The procedure for checking out your QA40x is at the link 1) below, with lots of plots so you can verify your device is working as expected.
For large FFT at 192ksps, there are issues on some machines with packets being lost. You can see more at the thread in 2) below. In that thread, you can see how to set up on automated test to capture 1000 points, and then from the graph you can see how many of those captures had an issue because the THD will be wildly incorrect. This week there will be a fix released that should fix the problem. The issue is that previously 2 32K buffers are handed to the OS to perform the async IO with the USB. That has been increased to 4, which has fixed the issue on a low-spec fanless PC at larger FFT sizes. And I think in retrospect, the success of machines with small FFT, but failure at large FFT should have been indicated what the problem was sooner.
Is there a way to play a continuous wave from QA403 output ?
Yes, you can push the front-panel IDLE button (under the run/stop button). This tells the QA40x to play the GEN1 setting as a continuous tone when not actively making measurements.