USB audio cracked... finally!

[opus101]

6 hours ago, manisandher said:

 

I've never understood why people think transformers prevent noise from passing through them. They prevent DC, for sure, and are certainly bandwidth limited. But in this case, they must pass (edit - really need to get my M and k sorted out!) 500MHz or so, so any noise under (edit) 500MHz will get through. If it didn't, the USB signal wouldn't either.

 

Transformers are good for rejecting common-mode noise (they have good CMRR). They must pass 500MHz (or whatever the bandwidth) as a differential-mode (often called 'normal mode') signal but when there's equal noise on both signal lines (seeing we're talking differential transmission) then the trafo's going to reject it.

Of course in practice the noise is never quite equal on the two lines so some noise is going to get through as a normal-mode signal.