Hey Coen,
Maybe it could be nice (well for me) if you can more clearly talk about what you see. For example, your "the former" seems to refer to two (!) read wires while I think you mean one of them, which now makes not much sense (to me).
Also, you might put some attention to
"i can see that the former is connected to a (one) film capacitor which in its turn is connected to one of the old xtal pcb islands."
which to me seems wrong in the first place. Not sure, but I'd say that this "former" needs to be connected to the old xtal island which from there goes to ground through the cap. Or ?
This is all not crucial of course, and it seems that I try to help you which I don't think is necessary at all. However, I am always eager to find messed up things (designs) and this seems to be one.
Btw and might that help, an active osciallator (not voltage controlled assumed) has "inputs" for voltage and ground, and for output the one clock wire;
Running the output through a transformer might not be the best idea for the signal itself (I keep on saying that any analog device is too slow for this) but may imply groundloops again (somewhere at the other side). But the supplies (gnd and V) can sure go through the transformerS.
For real isolation it needs three transformers, but I'd still wonder what happens with the still not-isolated ground at the other side.
We (or I) can also say : this never can work unless far (!) more complicated schemes are applied, and they won't lend themselves to apply on existing designs. That's what I think of it.
Mr. Pang (I think is his name) may have found himself for the same half-decisions. Any good for the better ? I again don't think so because the result will be out of (veryfiable) control ?
Peter
Sorry for being so cryptic about the clock signal isolation transformer.
If you still want to byte into it here's the deal:
In short the clock isolation transformer has:
- a primary winding with one leg connected to the clockboard ground and
- a secondary winding with one leg connected to the clockboard ground.
There is also:
- a black wire form the isloation transformer that is probably connected to the shielding of the transformer. It is soldered to clockboard ground.
- the second wire of the primary is connected to the output of the crystal oscillator (wich is dc decoupled by a small capacitor first).
- the second wire of the secondary is connected (via a film type capacitor) to - only - one of the connections where the original crystal has been on the USB3 board near the NEC chip.
The groundplane of the clockboard is connected to the USB board ground ONLY via the ground of the 5V supply (the twisted silver and red wire).
This was kind of a relief since now it is NOT an alternative groundpath of the regular USB card circuitry.
In short primary, secondary and shield all share the same clockboard ground. I think in this case clocksignal return currents have to travel however via the 5V ground wire. This must be some kind of trade off since I think the way that Nick solders both secondary wires to the original clockmounts (don't you Nick?) would be the way to go...
As far as I have been able to investigate the 5V is supplied via the MOLEX and is filtered by a pi filter with a "big" HF choke (top of the board) before being offered to the pins. Marketing rap says this will increase the quality of the data transfer (less loss, higher speed). So forget about a similar noise spectrum here....
I'm gonna let this brew in my head for a little while.
regards, Coen
p.s. before anyone overlooks it: the above is NOT how to create galvanic isolation (if one should want to persue that). Your batteries and special 5V clock supplies will be connected to the USB board ground anyway.