# Harmonics on loopback test **URL:** \ **Category:** QA401\ **Created:** [May 8, 2020, 11:13pm UTC](https://forum.quantasylum.com/t/harmonics-on-loopback-test/80 "2020-05-08T23:13:27Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![voltist](https://avatars.discourse-cdn.com/v4/letter/v/9fc348/32.png) [@voltist](https://forum.quantasylum.com/u/voltist)\ **Post date:** [May 8, 2020, 11:13pm UTC](https://forum.quantasylum.com/t/harmonics-on-loopback-test/80/1 "2020-05-08T23:13:27Z")
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 ![image](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/1X/79cc3eced86a0b0edfa589bc92f914078a78fabf.png) ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://forum.quantasylum.com/u/matt)\ **Post date:** [May 9, 2020, 2:38am UTC](https://forum.quantasylum.com/t/harmonics-on-loopback-test/80/2 "2020-05-09T02:38:47Z")
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](https://cdn.shopify.com/s/files/1/1631/5609/files/QA401_Product_Sheet_RevD.pdf?v=1556811989)). And if you compare your measurement to the [Getting Started Guide](https://cdn.shopify.com/s/files/1/1631/5609/files/QA401_Quick_Start_RevB.pdf?v=1588785095) 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. ---
**Author:** ![voltist](https://avatars.discourse-cdn.com/v4/letter/v/9fc348/32.png) [@voltist](https://forum.quantasylum.com/u/voltist)\ **Post date:** [May 9, 2020, 3:48am UTC](https://forum.quantasylum.com/t/harmonics-on-loopback-test/80/3 "2020-05-09T03:48:22Z")
Still, kinda strange to see harmonics at 2k and 3k. ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://forum.quantasylum.com/u/matt)\ **Post date:** [May 9, 2020, 5:26am UTC](https://forum.quantasylum.com/t/harmonics-on-loopback-test/80/4 "2020-05-09T05:26:52Z")
Hi @voltist, just curious, but why is it strange to see harmonics at 2 and 3k when your fundamental is at 1k? ---
**Author:** ![voltist](https://avatars.discourse-cdn.com/v4/letter/v/9fc348/32.png) [@voltist](https://forum.quantasylum.com/u/voltist)\ **Post date:** [May 9, 2020, 10:05pm UTC](https://forum.quantasylum.com/t/harmonics-on-loopback-test/80/5 "2020-05-09T22:05:50Z")
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.