0
\$\begingroup\$

I have a question about USB 3.0 wire differential pairs length matching.

As we know, the length matching is required and can be calculated when designing the USB3.0 PCBs. But what about the wires?

If I make a cable manually to connect the USB3.0 signals and PWR. The PWR wires can be defined according to the PWR and AWG.

I think I should also take care of the wire length of the differential pairs. It is hard to keep the wires the same length. It would be appreciate it if you could tell how to calculate the required length matching between the differential pairs of the wires.

\$\endgroup\$
4
  • 1
    \$\begingroup\$ Yes they must be matched equally well. That is why there are good cables which actually manage to support the USB standard, and bad ones that don't. But I have to ask: Why would you try and make a USB3 cable yourself? \$\endgroup\$
    – tobalt
    Commented Jan 4, 2023 at 11:18
  • \$\begingroup\$ because of our application. I need to try thinner wires AWG 34 or even thinner ones. The normal usb cable are too thick for us. Could you tell the tolerance of the difference of the differential pairs? \$\endgroup\$
    – Xiyao
    Commented Jan 4, 2023 at 12:11
  • \$\begingroup\$ I can't. But for such stuff better consult the USB standards (usb.org ): especially this one: usb.org/sites/default/files/… \$\endgroup\$
    – tobalt
    Commented Jan 4, 2023 at 14:30
  • \$\begingroup\$ Thanks for your help. I learnt a lot from the links. \$\endgroup\$
    – Xiyao
    Commented Jan 5, 2023 at 14:04

1 Answer 1

1
\$\begingroup\$

For USB 3.x SuperSpeed (5Gbps), the maximum intra-pair timing skew is 15ps. For High Speed (480Mbps), the maximum intra-pair timing skew is 50ps. There are varying rules of thumb for propagation delay (it depends on your εr) but a typical value is 100ps/inch or 40ps/cm, which would imply 3.75mm of intra-pair skew on the superspeed lines.

You will also need to pay attention to the characteristic impedance specifications, which must be 45Ω ±15% single-ended and 90Ω ±15% differential. Achieving that with AWG 34 wire might be tricky.

\$\endgroup\$
3
  • \$\begingroup\$ Thanks a lot for your answer. It helps a lot. If I begin to try it with the AWG34 wire, could you give me some more hint? \$\endgroup\$
    – Xiyao
    Commented Jan 5, 2023 at 14:06
  • \$\begingroup\$ All I can really advise is that you read the USB cabling specs and attempt to meet the requirements they set out there. SuperSpeed cables are nontrivial to design and you'll likely need to do quite a bit of research and design work. \$\endgroup\$
    – Polynomial
    Commented Jan 5, 2023 at 18:45
  • \$\begingroup\$ Thanks a lot!I will try. \$\endgroup\$
    – Xiyao
    Commented Jan 9, 2023 at 8:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.