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I've been conducting a long-running personal project, designing and building a custom PC case. I've got round to the front IO panel and figured to design and make the circuitry myself instead of buying something that wouldn't quite look right. The panel will use a PCB with 2 USB A / 3.2 receptacles, power button and HD audio (and power LED).

During designing the PCB i've discovered a few things, one being that the differential signal lines (D+/-, SSTX/RX) require a specific 90 Ohm impedance. I understand some of the theory behind this, but not all, and am hoping someone can fill in my gaps:

I understand that impedance matching is required to prevent signals from "bouncing" at the boundaries, for instance between the usb port and the PCB traces or the traces and the header. I also understand that the traces need to be approx. identical lengths so that the differential remains "in sync". That all makes sense to me.

I've seen in numerous places that to acheive the required impedance, a "Ground plane" is required. This ground plane provides some sort of effect to the traces carrying the signal that affects their impedance. One question I have is: what is the role of the ground plane here?

Take for instance my USB 3 scenario:

USB Type A 3.0 and Type B 3.0 connectors

Image source: eTechnophiles - 10 USB Pinout Explained- USB A, B, C(Male and Female)

USB 3.0/3.2 Gen 1 has 3 sets of differential signals, V+, and 2 Ground contacts. Likewise, the USB PCB header on the motherboard will have ground contacts that correspond to the USB receptacle. Does the ground plane just connect to any contact named "ground", regardless of what component it is? Are there "types" of ground? Should I only connect the ground plane to a specific set of contacts?

I guess what I'm getting at here, is asking if all components can share a common ground. Will 2 USB ports + HD audio sharing the same ground lead to any complications?

Additionally, I have seen discussion about how the spacing/width of the diff. line traces, as well as their separation from the ground plane, affects the impedance. In these discussions there is reference to online calculators that allow you to calculate the impedance expected for a set of width/spacing/distance parameters, but upon using them, all of these calculators give different results.

Is there a quick cut rule for this sort of thing, in regards to typical width/spacing/distance from ground plane? Can anyone also fill me in on why there is such contention here also? The only part I can kind of understand is the "skin effect", which makes sense, but when I go to online calculators for AC resistance for the relevant factors (100 micron trace diameter, 10-20 mm length, 1GHz frequency), the resistance is shown as negligible. I'm guessing there's other factors at play, but at this point I'm stuck.

Very grateful for any clues or information anyone can give me here. It's been fun learning about all this stuff, but truth be told I am a little overwhelmed.

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So, first things first - even if you have a single PCB which has connectors for HD audio and USB, you do not have to connect audio connector section grounds together with USB ground section. You can just think you are making a separate HD audio connector PCB and separate USB connector PCB, but actually have single PCB with two unrelated sections.

And for USB data the impedance needs to match for the whole transmission line, so this includes PCB traces, vias, and connections from PCB traces to connectors.

A ground plane is not required as you can make a differential pair without it (think of unshielded CAT5 cable), but since you have a PCB, it is typical to make the differential pair over a ground plane, as it among other things acts as a shield for the data wires.

The motherboard USB3 cable specification lists which grounds should be used as shields for the data pairs and which grounds to use for power supply return.

If you are planning to make a PCB with impedance controlled traces, it basically means making a 2 layer PCB is impossible and you need at least 4 layer PCB, because the two layers on a two layer PCB are too far apart for having a sensible PCB trace width.

All PCB trace width calculators are just models and may use different formulas for different use cases, some more applicable in some situations. The key is knowing the PCB material and construction to get the effective dielectric constant, and distance between layers, to be able to calculate anything.

So no, there is unfortunately no simple quick cut rule, you are dealing with hard core PCB design requiring expert level knowledge and experience to make a decent USB3 interface board.

Extremely good designs even carefully handle how the connector area of PCB interfaces to the PCB traces, by opening up the ground planes to avoid stray capacitances to lower the impedance at the connector for example. Sophisticated designs basically need high performance PCB material and maybe EM field solving programs to simulate how good or bad the PCB design is.

So if possible, use an existing USB3 board to avoid designing something that may not work well with USB3 devices. Feel free to design the analogue HD audio IO and pushbutton/LED area as you wish.

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  • \$\begingroup\$ Thanks for the response, this has given me a lot to think about. I think I have a better understanding of what the ground plane actually is, and before I just heard the word "ground" and drew the wrong conclusion. As for the PCB, w/o going into specifics, it effectively takes the input from MoBo into the pcb via pin header, then from the header into 2 separate USB receptacles, which is why I got confused about grounding. Wouldn't any grounding for the usb just go back through the header to the motherboard? \$\endgroup\$
    – Mdd M
    Apr 17, 2023 at 21:35
  • \$\begingroup\$ Yes, but the motherboard USB3 connector has multiple ground pins - at least one defines for cable shield and at least one for supply return current. And same for the analg HD Audio connector, it will go to analog audio ground on motherboard. You don't want to connect noisy digital ground with sensitive analog ground. \$\endgroup\$
    – Justme
    Apr 17, 2023 at 22:03

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