Equipment to quantify USB 2.0 highspeed signal integrity - Page 1
Forum section: electronics / projects

author: parawizard
author_rank: Regular Contributor
timestamp: January 05, 2025, 10:24:20 pm
content: Hello everyone!I've been working on troubleshooting an issue with an USB 2.0 highspeed HID device with an integrated cable. I want to be able to quantify signal integrity but it is looking pretty dang expensive. I don't have anything better than 200MHz on hand so I have been shopping for scopes capable of possibly doing USB 2.0 compliance. Looking for something broken to fix as this is a side project. I can probably spend up to $5000 CAD. I am also open to out of the box ideas. I was able to produce a fullspeed eye diagram with persistence and a mask made to USB-IF standards on my Picoscope.Agilent 54846+Tektronix TDS7254 or TDS7404I was also thinking I might be able to do some other kinds of measurements instead of eye diagrams. Not sure what could be done with new VNAs? Was also thinking about doing some TDR measurements to see if I can find issues with the cabling/connectors.How would you go about quantifying issues with signal integrity on a USB 2.0 Highspeed device with integrated cable?

author: Njk
author_rank: Frequent Contributor
timestamp: January 05, 2025, 11:25:05 pm
content: As an engineer who had designed a USB 2.0 highspeed HID and Audio class devices with integrated cable, I can suggest that it would be better (and perhaps cheaper if a used would be OK) to look for a specialized instrument, such as USB analyzer from a reputable vendor like Lecroy, etc.BTW, out of curiosity I'd attempted to use a sub-standard cable to find out how sensitive the design is to the cable quality. The design was of real high power (500 mA). Tried a thin and long low speed mouse cables, a cable made of two pieces connected by a hand-wrapping the wires, etc. No adverse effects in the form of increased BER and protocol errors were observed in the lab environment. It seems it's actually not so critical.

author: parawizard
author_rank: Regular Contributor
timestamp: January 06, 2025, 12:44:52 am
content: Quote from: Njk on January 05, 2025, 11:25:05 pmThanks for your reply. I've got an inexpensive protocol analyzer on the way already and I am looking at fancier ones. I was using Saleae Logic and Picoscope both fine with Fullspeed. I agree it's a good suggestion. I'm not sure if it's going to tell the whole picture. I should be able to tell if something is not being ACKed that there is an issue. Or if stuff goes really weird.Quote from: Njk on January 05, 2025, 11:25:05 pmThe wire in question here is multicore with an OD of 2mm. It's also polling at up to 125us. There is a wire to board connector as well implicated. Some of the devices seem worse than others so there may be some kind of fault at play. I figure using a TDR and comparing cables maybe show differences between them.Edit: I would like an analyzer with n external trigger and a scope that could at least see the signal correctly.

author: DaJMasta
author_rank: Super Contributor
timestamp: January 06, 2025, 01:09:28 am
content: I've seen some less expensive LeCroy PeRT3s on ebay, but a BERT is the tool of choice to characterize high bandwidth signal integrity.A scope can be better for troubleshooting in-situ, and a sampling scope is well suited to a lot of these kinds of faults, so you don't necessarily need a realtime scope, but a BERT will give you an eye, a pattern generator capable of saturating the full bandwidth of the channel, and probably some simulated impairment options to find out how error tolerant your system is.

author: parawizard
author_rank: Regular Contributor
timestamp: January 06, 2025, 06:55:59 am
content: Quote from: DaJMasta on January 06, 2025, 01:09:28 amInteresting. I hadn't see those before. I will have a look on eBay and do some reading.

author: Njk
author_rank: Frequent Contributor
timestamp: January 07, 2025, 11:13:33 pm
content: This is what I'd used for tracing HS traffic on the bus.https://www.akribis.co.uk/catc-universal-protocol-analyzer-system-upas-2500hSeems not so expensive now.

author: parawizard
author_rank: Regular Contributor
timestamp: January 08, 2025, 08:38:52 am
content: Quote from: Njk on January 07, 2025, 11:13:33 pmI am looking at Lecroy Mercury T2 and a Ellysis USB Explorer 200 <$500 USD used. But also there are a couple other cool devices like:https://greatscottgadgets.com/cynthion/https://github.com/ataradov/usb-snifferThe latter seems to be sold on ebay and aliexpress for <$50 CAD. So I have one of these on the way and they use wireshark to view the captured data. The Cynthion I haven't read much about yet but could be interesting as well.Any of these will be helpful to see what is going on but I definitely would like to quantify what might be going on signal wise. I am putting in some bids on some broken 2.5GHz+ oscilloscopes in hopes to be able to do that.

author: tggzzz
author_rank: Super Contributor
timestamp: January 08, 2025, 09:11:06 am
content: What do you mean by "signal integrity"?To me it means ensuring the analogue waveforms on the wire will be correctly interpreted as digital signals by the receiver.It requires a scope with sufficient bandwidth to observe the transitions (clock rate is irrelevant). It doesn't require a scope with a high real-time bandwidth; repetitive sampling and eye-diagrams are sufficient.It does requires a scope, probes, and probing technique that don't unduly change the analogue waveform.The probes and probing technique are critical, and you will run into the philosophical question of whether it is better to see "what is" (with the probes/scope attached) or "what ought to be" without the probes/scope being attached.A general principal is firstly to ensure signal integrity, and secondly - as it looks like you are doing - then flip to the digital domain.

author: parawizard
author_rank: Regular Contributor
timestamp: January 08, 2025, 05:52:32 pm
content: Quote from: tggzzz on January 08, 2025, 09:11:06 amThis is how I would understand it. If I probe on the host side and look at signal generated by the host I would want it to look the same on the device side. Also the inverse.Quote from: tggzzz on January 08, 2025, 09:11:06 amI managed to pick up a broken Agilent Infiniium DSO9254A 2.5GHz 20GSa to see if I can fix it. It was a bit spendy but has good features for what I would like to do built right in.Quote from: tggzzz on January 08, 2025, 09:11:06 amI think that there is a bit of flexibility here in what I see as I can should be able to compare signals at both the device and host. I am having to look into getting probes now and the active differential agilent ones are definitely expensive. Active singles less so. Agilent is recommending >2.5GHz ones. Not sure what I could get away with in this respect.Quote from: tggzzz on January 08, 2025, 09:11:06 amThis sounds in line with my general thinking.

author: Njk
author_rank: Frequent Contributor
timestamp: January 08, 2025, 10:58:50 pm
content: With USB 2.0, in simple cases, when the chip is located close to the connector, it's very hard for the designer to do something wrong causing a signal integrity issues. It's definitely not like HDMI at 340 MHz pixel clock, where you've to be careful with the PCB routing, etc. A more complex cases, when the chip is very far from the connector on the board, can be another matter. I don't know what's you're building. Anyway, eye diagrams and stuff can be viewed with any decent general-purpose scope (which can be borrowed/rented specifically for that quick check, BTW). But USB is actually a quite complex bus and possible problems (both in HW and SW) during the design time can be countless. Everyone can make a mistake, don't expect every usb controller on the chip is bug-free. Imagine you'd checked the integrity, all fine but the problem persists. It's little more you can do with the scope. That's why a protocol analyzer is more useful instrument here. There are many of them available. Sorry for saying the obvious, but don't expect comparable levels of sophistication and convenience from the instruments of very different price. Cat see was originally an independent company which was later acquired by Lecroy. Back at that time, it must be good enough for Lecroy to worth buying. Among the other things, they'd developed excellent PC software (the trace viewer) which was a big pleasure for me to use.

author: tggzzz
author_rank: Super Contributor
timestamp: January 08, 2025, 11:33:59 pm
content: Quote from: parawizard on January 08, 2025, 05:52:32 pmDon't underestimate how easily suboptimal probing can affect what you see on the scopeA quick and dirty spice model including relevant parasitic components might indicate where you need to be careful. So, of course, would scope manufacturers' application notes on USB testing.One crude gross sanity check would be to see whether the digital signal tools detect any difference when a scope probe is attached.

author: parawizard
author_rank: Regular Contributor
timestamp: January 11, 2025, 11:52:26 am
content: Quote from: Njk on January 08, 2025, 10:58:50 pmThe device has a rather exotic integrated cable so I will be glad to have an idea of how good the signal integrity actually is. Protocol Analyzer will definitely be more useful if it's a firmware issue. I have a opensource prebuilt coming from China that uses wireshark.I managed to grab a broken Agilent DSO9254A for about $6000 CAD. I am feeling pretty confident I can fix it. This should work pretty well for the signal integrity side of things.Quote from: tggzzz on January 08, 2025, 11:33:59 pmI have to be more careful using a set of single active probes but I think they should work well enough. I can spring for the differential but it's another $2000 CAD used.I got in contact with nctnico and asked about his DIP1400 differential probe which I think should do the trick. Much less spendy than Agilent's one. I would likely be following the USB 2.0 signal integrity test instructions. Ideally with a test fixture. But some idea is better than no idea at this point. Some relative comparisons may be useful too.