I am having issues with a QA403 that I recently purchased.
It performs VERY POORLY when the Sampling rate is anything above 48k.
I’m testing it out in loopback mode (BNC connected between Left and Right Output + and Input +, and both negative BNC inputs are terminated with 50 ohm terminators).
When sampling rate is set to 48K, I’m measuring around -120db of THD on both channels. However, when set to higher sampling rates, it’s MUCH worse. (-15 db)
In order to debug further, I used the Oscilloscope tab on QA40xPlot and saw the following waveform when Sampling at 96k:
That looks bizarre. I think you’re seeing overload with the relay resetting or some USB or cabling issue. The app should be using an attenuation of 6 at that voltage (although 0 should work ok). Do me a favor and try that test turning off autorange on start and setting the attenuation to 42.
Are you seeing the same issues with the QA40x factory application?
When you switch to 48Khz does the skipping go away?
Mark
Postscript: I tried slightly overloading the inputs and a relay skip looks pretty different so it’s either your USB or cabling and cabling is way more likely.
As a next test I would disconnect the right channel BNC, test with just left +, then run BNC from both sides of the left channel output to both sides of the left channel input and test that. Keep the right channel disconnected (terminators or nothing) while testing. In all cases just leave attenuation at 42.
If those all fail then I’d check my USB. try a different cable and do not use a usb hub but plug straight into your PC, … Also, the factory application shows the USB power voltage. Check that it’s very close to 5V.
Your suggestion to check the USB turned out to be correct.
I reconnected to a different USB hub and the unit is working far better at higher sampling rates.
My laptop only has USB-C ports (and I don’t have any USB-A to USB-C cables) so I can’t connect the QA403 directly to my laptop. Nonetheless, I was using an (expensive) CalDigit USB docking station that I would have expected to provide a clean USB connection to the QA403. Swapping to a non-powered USB hub (with nothing else connected to the hub) seems to have solved my problem for now.
Thanks very much for your help! And thanks for your wonderful QA40xPlot software!
Is it possible that you’ve got your laptop set to lower performance to save power when it’s on battery? I think that’s the default for lots of laptops. Check the battery settings in windows.
That could cause what you’re seeing if it’s running more slowly or having the ports sleep more quickly.
