Is galvanic isolation of Hi-speed USB impossible?
There are definitely laws of marketing involved. :-)
Gigabit Ethernet and 10G-Ethernet have galvanic isolation. So, obviously it is possible and routinely done with today's technology.
A fiber-optic USB extender basically works a bit like an opto-coupler except that the light source and the light receiver are on separate chips. Combining the functions of a fiber extender into a single package should be cheaper, not more expensive. Using magnetic or capacitive coupling instead of optical coupling should be cheaper again.
USB is normally used for short distance (up to 5m) data connections where significant differences in ground potential do not exist and galvanic isolation is unnecessary.
There are a few applications, e.g. medical or low electrical noise, which require or benefit from galvanic isolation. All of those applications are specialized and the existing fiber extender solutions fully cover the galvanic isolation requirement. Additionally, wireless solutions like Bluetooth, Zigbee, etc also satisfy the isolation requirement (at slow speeds). In conclusion, there is probably not much of a market niche for USB isolators.
FWIW, I have used a fiber extender a few years ago during development work on a high voltage power supply sub-system. I only needed the isolation, the fiber remained coiled up on the bench.
Thanks for the links.
Edit: As for the part of the question "Are actual laws of physics involved, ..." No, there are many faster, galvanically isolated communications links such as Gigabit Ethernet, 10G Ethernet and even wireless solutions.
"... or is this merely an engineering challenge or cost issue?" Yes, as of 2018, the engineering challenge is less than it would have been a few years ago, but would still be a significant effort. But who would fund development of such solutions if the demand appears very limited?
There are ready-made isolated repeater chips for the 12Mbps USB transfer rate: ADuM4160 by Analog Devices or LTM2884 by Linear Technology. Surprisingly to me, both contain inductive coupling = miniature on-chip signal transformers as the coupling elements, interfaced to the outside world by silicon (CMOS?) buffered transceivers. Makes me wonder why the isolation is not optical these days...
Note that 100Base-TX Ethernet, SATA, PCI-e or RS422, all use a balanced pair in either direction, together comprising a 4-wire full-duplex link. Gigabit and 10Gb Ethernet works that way only on fiber optics I guess.
In contrast, USB low/full/high-speed uses a single balanced pair, in half-duplex mode, where the host and device take turns in talking on the bus, and have to tri-state the line driver when they're finished talking, to give a chance to the other party (somewhat similar to RS485, though many electrical and framing details are different).
Any galvanic isolator, including the chips mentioned above, has to respect that half-duplex direction-switching style of communication. A single signal trafo should theoretically work at 12 Mbps, except for the DC biasing resistors, and the framing possibly isn't "free of DC offset on average" either, making it difficult to just use a passive trafo. Attenuation aside.
Perhaps it's precisely this need for the active isolator to "turn the table" fast enough, to detect the end of transmission in the first place, that makes implementation of a "stupid USB repeater" at 480 Mbps impractical, even in today's silicon. There are supposedly some other changes in the electrical interface for high-speed USB 2.0 (constant current signaling) which may be another factor why high-speed USB doesn't lend itself easily to this kind of 485-style RX/TX switching in a dumb repeater.
Note that there's an alternative approach to the "direction switching" problem: rather than detect a high-Z on the line in an analog fashion, which brings along some inherent latency (lag), the isolator would have to understand the USB protocol, just like a USB hub does - so that it would know when to expect an end of the frame being currently received. And possibly, it would buffer whole frames, before relaying them onto the other side - just like a USB hub does. (Or does it?) Effectively the isolator would have to become a USB hub, with an isolation gap somewhere in there.
It is somewhat surprising to me, that there are no hub-style isolated repeaters. Possibly because ATMEL and friends make hubs, and Analog or Linear (or Avago?) make isolators, but the two gangs don't mix...
The problem of transporting the high bitrate over an isolation gap should not be all that difficult - yet even this area seems surprisingly "underdeveloped", or seems to suffer a gap of some sort. 10Gb Ethernet over fiber has been there for years, with bitwise base-band SERDES (bitstream), transmitted by a "laser" (at least a VCSEL) and received by a photodiode. Yet the DIL-packaged opto-couplers have barely reached 50 Mbps or so. Where does the gap come from? Well it seems to me that the guys making the DIL opto-couplers rely on relatively slow LED sources and photo-transistor receivers. While the guys making fiber stuff make their VCSELS and photodiodes suitable for coupling to a fiber - with adjustable bias current, with a local feedback diode strapped onto the VCSEL etc. Apparently noone got the idea to build an electric-to-electric photocoupler out of those high-grade parts. Note that the fiber-coupled gigabit stuff typically uses AC coupling on the electric interfaces, but that shouldn't be a big problem, if someone bothered to adapt the technology to classic DC-coupled TTL style.
Maybe it's just a conservative old-school view of the industry, on my part. Maybe the gigabit high-bandwidth tech has already moved to a new era, where you can only play in terms of standardized busses and interfaces, and there's no point in making discrete components capable of transferring a stupid simple logic 1/0 on a single signal. Maybe this is just my dinosaur-style thinking that you can still hack things together like that. The modern GHz era seems to "raise the bar" against casual hackers with a soldering iron. Electronics hacking has become a matter of closed labs with expensive equipment, only available to big industry-leading vendors. It's a closed club. From now on, all you can ever hack is software, or maybe some trivial antenna stuff.
Signal transformers are apparently only good into low hundreds of MHz. 1000Base-TX and especially 10GBase-TX take great pains of cunning modulation to squeeze the data into many "bits per symbol", on full-duplex-per-pair balanced lanes, at the cost of power-hungry DSP processing for all the modulation / local echo cancelation / pre-equalization... just to fit inside maybe 200 MHz of bandwidth available through the "magnetics" (signal transformers). If you're into TV antenna technology, you may have noticed that in the upper range, say 500-800 MHz and above, galvanic isolators are strictly capacitive. No matter what core material you choose, inductive transformers are just no good at those frequencies.
In the end... you know what? USB3 seems to use separate balanced pair transmission lines: one pair for TX, one pair for RX. Feels like coming home.
Sorry.
Answering literally: no, nowadays there is no limitation any more.
Still practical solutions <400$ are rare. This is something physical or rather electronic design thing again, not only marketing and volume production.
But a few years ago VCSEL were way to expensive, and parallelizing also rises the cost of insulation and has inherent protocol problems due to increased delay (we were glad when USB shifted from useless serial bus to something with some reliability).
Even 2015/16 the bit rate of available off-the-shelf digital isolators ICs is limited, to 150Mbit/s to my findings. I found only one company, see below, offering an USB2 480MBit/s chip.
Just look at the underlying principle of AD's iCoupler. They use pulse trains with pulse widths of 1ns, and reconstruct the original pulses by this, well, digitizing approach, with a transferrable bit rate of up to 150Mbit/s, which is not enough bandwidths for USB2 highspeed or USB3.
The nice thing of AD's icoupler is that they are able to transfer energy to provide the secondary side with power (not a lot, but still...), and a lot of their chips have e.g RX, TX and a Power coil. All you have to do is add some capacitors. So the wait will be worth while.
Corning uses true fibre optic techniqueswith VCSEL Lasers as emitters (alway has been a physically viable thing though no affordable way to go till recently).
At least corning optical USB3.0 cables are affordable, 110$ for the 10m version. You may need some powered USB3 hub afterwards for power hungry clients (if you need more than 200mA or so, but corning says it would transfer "no power"). And you may have bad luck (or low reliability) for some setups, be prepared to turn it back.
Sometimes we get infos from patents. But someone would have to pay license fees for using it, if not the owner. I found one by silanna.com , Australian Chip company, see google patents, WO2015104606A1. Aha, their USB2 Silicon on Saphire capacitive isolator based USB2 high-speed solution is out: http://www.silanna.com/usb.html So we wait for a protyping board with high-efficiency DC-DC-isolation included, as they claim to be working on.
You could surely argument that all lasers wear out, capacitors have their pitfalls, etc... And this is why AD uses magnetic coupling, among other reasons like common rail rejection. See http://www.analog.com/media/en/technical-documentation/frequently-asked-questions/icoupler_faq.pdf There you have to trade off isolation thickness vs. transmittable bandwidths. Let's wait for them to catch up with 5GBit/s, which would mean, to have acceptable jitter, internally they'd have to transmit like 20..30Gpulses/s, if they'd re-use the icoupler technique...
Hope I now have delivered more to the literals of the question...
For me, I will buy the corning, but will add my own DC-DC isolated power supply to have USB to power my digilent analog discovery 2, without any additional (wall) plugs. Since some optical cables are reported to be not USB2 compatible, perhaps some small (1port) USB Hub would have to be placed after the corning. Together, for now, this makes the approach clumsy and a patchwork of 3 modules.
Yours, Andi