QA403 issue ? (stop output when runing a fft)


I see maybe an issue with my QA403.
I run a simple FFT analysis on left channel, output connected in diff on my DUT, input connected in diff on my DUT. Right channel not used/disconnected.
I run QA403 the simplest way: 192ks, FFT 128k, input set on 18dBv (same issue when 24dBv).
Sometime I think my DUT crash, ie FFT become ugly. I start investigation and finally find that the QA403, sometime, make a ‘break’ inside a run.
=> The run start normaly, ie QA403 play the right signal and then stop and restart few moment later. This comes often when FFT size is big, ie the QA403 has to play a long run.
I share screen shoot showing the issue, QA403 output on channel 2 of the scope, on part of my DUT on first channel. Second picture is the setup.

My DUT is from my design, so… but input of these amp is a 10KR resistors in serie, so there is a 10KR resistor between QA403 output and DUT. Output of DUT is directly connected to QA403 input, and off course this is a low impedance ouput from DUT point of view.

So good news finally my amps may works, bad news my wonderful QA403 is broken ?

=> Is there a way to play a continuous wave from QA403 output ?

and SW v1.176 runing on a win 10 laptop with a 4.771V usb voltage (show in QA403 SW) and 0.898A

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.

  1. Verifying Calibration · QuantAsylum/QA40x Wiki · GitHub
  2. QA403 giving DAC underrun errors and ADC overruns - #11 by mhuth1776

I test my QA403, all looks good (except section with DVM because I don’t own a good DVM).

Finally I find the mistake, my own, and my QA403 works great!
I disconnected my laptop PSU when running long FFT tests… With my laptop charger back no issue reported even @ 1024k FFT!
And thanks for IDLE button, can now test ‘long’ terms run on my design.

Thanks for fast answer.