Hi @moto and @Claudio, thanks for sharing, I’d not seen before. Yes, I think the approach in the PDF looks very solid and well reasoned. I’d note on page 3 the BNC shroud should NOT be shorted to the housing. The lower housing has a good connection to the USB ground via press-fit 3mm screw bosses. The upper housing does not (due to paint). And while all the BNC shrouds are tied together, you should measure an “open” with your DVM between a BNC shroud and the USB shell (which is USB ground, and that turns into earth ground on a PC when plugged in).
That’s a relief! I was having similar trouble with my brand new QA403, where the right channel noise floor measured about -86dBV while the left channel was -118dBV, even though a loopback SNR looked normal. It didn’t occur to me to check the terminators, so that was a great suggestion. After swapping the terminators, the high noise channel also swapped, proving that was the issue. After a couple of minutes of trial and error, I found that one of the inexpensive terminators was apparently bad. Thanks!
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