If we dig deep, both variants - RF and LF shielding (connect shield to ground on two ends and on one end) has its pros and cons.
That's why there is twisted pair, it allows to eliminate ground loop issues by using single side shield grounding and keep high speed signal protected from RFI.
(...)
No no no, not so fast -- I need you to check your assumptions, do actual research. It's not enough to rattle off generics. This is a specific case, in a specific implementation. No wishy-washy needed! I chose the list of questions very particularly, and they are objectively answerable.
The USB standard is open and available:
https://www.usb.org/document-library/usb-20-specificationcrack open that ZIP and grab the "usb_20.pdf" and start perusing.
USB 3.0+ of course adds more, but since we're talking the four-wire shielded case, this covers everything.
If you would like a more concrete example, let's say we have...
- A desktop PC (metal enclosure and all that)
- 2m long USB cable to an instrument
- Instrument consists of a plastic enclosure, PCB with ground plane design, mains power (a more-or-less normal off-the-shelf isolated SMPS, with adequate filtering to meet EMC otherwise), general construction is "double insulated" (so that grounding through device connections doesn't matter), and, let's say it's a, like, USB-connected SMU or something, so there is a wired connection doing something useful.
- And let's say the wired connection is 2m long, and the device under test is grounded.
--> So, PC is earthed through its cord, and EUT is grounded through DUT (bonded) and USB (to PC). Only mains ports are (galvanically) isolated.
- And, this is all within a semi-anechoic test chamber, so when I say "grounded", I mean the DUT is
bonded to that, and the PC and instrument are elevated on an insulated table, as is standard, let's say as in CISPR 22.
- And let's say both PC and EUT are plugged into the wall via separate LISNs each.
I could go on to describe (or provide crude drawings for) cable placement and construction, relevant line filter sections (if mains circuit impedance should be relevant), USB cable construction (well heck, let's just put that in real quick anyway: say it's outer braid 90% coverage, power pair twisted, foil over data pair twisted; braid serves as drain wire for foil), etc., but at this point, you get the idea, and this should be sufficiently well constrained that anything left undefined, you can freely make assumptions about -- whether extrapolating from common practice, or to try and game a solution.
And we assess EMC in this setup by putting current clamps or CDNs on all cables, as well as measuring the LISNs, to cover conducted band; and there's an antenna in the usual configuration (variable altitude, both polarizations) to cover the radiated band.
You should be able to answer the numbered questions; if not, let it be known in this thread that your answers on this topic are not authoritative. But so, too, you have opportunity to improve your craft, to better your radio skills, to diversify on topics (how exciting!).
It is with regret that I do this confrontationally; I realize such approach has a strong chance of simply shutting down subsequent response, rather than generating the nuanced analysis that is required on this topic, and for this I apologize. I hope however that this leads to fruitful discussion, and learning experiences (not just you but readers too). These really are tough topics, and one must approach them with care and humility; it is only from decades of experience that I would bring such confidence to bear here -- and even so, I would still be ready for surprises in testing real hardware of this general description. I have a pretty good idea of what would happen here, what approximate levels such a system might pass or fail by (but I wouldn't dare stake my reputation on anything more precise than 10dB or thereabouts--), and what ways it might fail in; but also be open to surprises. After all, it takes only one little gap to emit a couple millivolts of RF and spoil the whole thing.
Tim
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