I am designing an 4-port USB 3.0 hub and using fpc 0.5mm connector to connect it to other usb devices. Will using fpc 0.5mm cable have interference because of high data rate? I am talking about the interference inside the cable itself from D+,D- to SSRX or SSTX wire. I have been reading and only found out that it cause interference to wireless devices but nothing about interference inside the wire.
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2\$\begingroup\$ I think you should use a ground pin between differential pairs (but not within pair!). Like ...GND D+ D- GND... But I'm far from expert on that topic \$\endgroup\$– IlyaCommented Mar 12, 2021 at 10:19
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\$\begingroup\$ I have already inserted ground between pairs (not within). Seems like this is the only caution I can take. I will just increase the number of ground wires...that I can modify. Thank you. \$\endgroup\$– Creet intrimCommented Mar 12, 2021 at 10:29
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\$\begingroup\$ I would wait for someone really competent to answer. I'm really NOT the person to give a good advice on this topic, just know this and that. And I've never worked with frequencies anywhere close to USB 3.0 myself, so ¯\_(ツ)_/¯ \$\endgroup\$– IlyaCommented Mar 12, 2021 at 10:32
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\$\begingroup\$ @Creetintrim you should probably read this and understand the motivation for people providing free help and giving you answers. \$\endgroup\$– Andy akaCommented Mar 12, 2021 at 10:53
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3 Answers
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Wow, this is an ambitious approach I think! I am already scared to do such an appraoch on USB 2.0 high speed. I think using a 2-layer flex cable (bottom side as a ground layer) instead of using a standard FFC cable will allow you to match the 90 ohm impedance and to get much better results. But I have to admit that I'm not a high-speed layout specialist as well.
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Flexible cables are used everywhere in modern laptops and chromebooks. As the screen size gets bigger and mainboard size gets smaller due to increasing level of integration, the USB ports within laptop body appears to be well apart from processor, so flex cable is cheaper than PCB. The connectors, however, are pretty wide, 30-40 pins. The signal distribution is usually in groups GND-S-S-GND, GND are not shared with adjacent link. So it is G-S-S-G-G-S-S-G....
The cable MUST maintain 80-90 Ohm differential impedance, otherwise the signal will be killed by reflections. In many cases the cable has a solid ground reference plane. The 90-Ohm is a must. To find such a cable will be a challenge.
answered Mar 14, 2021 at 2:38
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The short answer is yes, there will be interference. Whether that will be acceptable for your design depends on a number of factors, primarily the length of the FFC. I can’t tell you what length is likely to be ok but perhaps another contributor can...
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\$\begingroup\$ The wire length is 5 cm so it is not that long but I don't know if it is short enough. Thank you. \$\endgroup\$ Commented Mar 12, 2021 at 10:32
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\$\begingroup\$ I’ve run USB2.0 over a moderate length of FFC (150mm) but I think that USB 3.0 will need proper impedance control over the full length of the connection. If you can find an FFC cable that has approximately the right impedance then you might be ok but you’d have an easier time if you could use purpose-designed connectors and cables. \$\endgroup\$– FrogCommented Mar 12, 2021 at 21:14