QA402 harmonics in loopback

Hi, first post, sorry it has to be about a problem.

I have a 402 and when I run with short BNC loopbacks, I see fairly high harmonics.

QA40x version: 0.9999
Firmware Version: 42

I can only post 1 image, so here’s the left channel with 6" RG-174 shunt. I’ve tried 6" RG-174s and 3’ RG58.

The right channel is even a bit worse, with a -92 dB 3rd harmonic.

With all inputs 50 ohm shunted, the harmonics are all <= -120 dB.


This is likely ADC distorting at higher input levels. Try lowering the generator level. On my 401 the best THD is around -114dB in loopback at about -15 dBV generator level. Check out the QA480, it is there to work around this type of issues.

Thank you. yes, that looks like it. It seems my best SNR is about 100 dB with -10 dBV output and 0 full-scale input.

The SNR is a different metric. It actually increases with the level as the ratio Signal/Noise increases and noise stays the same in the ideal case. However you don’t always get SNR dBV improvement per level dBV increase unfortunately as there appears to be some amount noise floor modulation in place with these analyzers, but unlike the THD it should still be better at 0dBV than at -10dBV. Here you can see how THD worsens and SNR improves when the generator level is increased 15dBV: