Harmonics on loopback test

I’m using a powered USB hub, and a heavy-duty USB cable - but also tested connected directly to my laptop USB port. Reset default settings, Looping L+ Out to L+In, with L-IN grounded.

There is no other equipment on, the location of the QA401 doesn’t matter. Cables don’t matter.

I can’t find the source of these harmonics - they also follow the fundamental, so as I change generator frequency, they move with it.

Measuring harmonics at 2kHz and 3kHz

Hi @voltist, what you are seeing is correct. Your THD is reported at -110 dB, and the spec is -108.5 dB around that signal level (see here). And if you compare your measurement to the Getting Started Guide you can see on page 8 another typical plot which is similar.

If you press ‘d’ to clear all the markers, and then click on the 3 peaks, you’ll be able to see in the purple summary the harmonic heights relative to the fundamental.

Your noise floor looks good too. The modest 60 Hz you see might be pushed down a bit further (or increased) by re-positioning the unit a bit. You can follow the Getting Started guide to make a noise floor measurement and see how you compare to the spec.

Still, kinda strange to see harmonics at 2k and 3k.

Hi @voltist, just curious, but why is it strange to see harmonics at 2 and 3k when your fundamental is at 1k?

What I was seeing seemed higher than I expected to see with a loopback test. My dScope M1 is -107dB at 2k with a 1kHz 0dbV input signal, on the QA401 I measure -102.2dBV.

THD on the dScope is 0.0005%, QA401 0.001%

Measuring the dScope’s generator on the QA401 yielded much closer results, with a peak of -106.7dB for the 1st harmonic, and THD of 0.0008%.

So, pretty close, and not as bad as I had expected or imagined in my head. We’re all good.