USB cable characteristic impedance

Question

Does differential impedance between D+ and D- not only depends on separation between and wire gauge of D+ and D-, dielectric constant of the insulation, but also the GND wire and shielding?

I want to connect two EVMs which use PCIe connection by using the impedance controlled USB cable at hand.

However, I don't know if I should also connect the GND and shielding of the USB cable(of course, I will use common ground for two boards). If the differential impedance between D+ and D- does not depend on separation between D+, D- and GND, shielding, I think I can leave these two wires floating.

Answer 1

For USB, Zdiff = 90 Ohm and for PCI-Express and most other diff cables, Zdiff = 100 Ohm. So they're not equal, and you'll need matching circuit for it. It is better to take SerialATA or HDMI or Ethernet cable for your task, as they all 100 Ohm.

Cable should normally be grounded. This does not affect Zdiff in theory, but greatly improves noise performance.

(Additional theoretical note) In general case it does affect specific impedance if conductor is grounded or floating (e.g., triaxial cable with inner shield grounded or not), however for symmetric differential cables it does not matter.

Answer 2

Use a USB3.0 cable, which includes an appropriate shielded pair for both TX and RX. DP/DM can be used for REFCLK. It’s not perfect but it will work. This is exactly what a ‘GPU Miner’ slot adapter uses. Example: https://www.parallelminer.com/product/usb-3-0-pcie-riser-card-expansion

If you want your cable to be exactly PCIe compliant, consider SFF-8639 (U.2), iPass, Firefly - all designed to carry PCIe. SFF-8639 carries up to 4 lanes.

You can also consider USB Type C, with the appropriate signal set used for PCIe. This is what Thunderbolt 3 uses. It can carry 2 lanes if you need that.

In any event do connect the shield grounds, as they prevent common-mode noise from each pair from getting into the adjacent signals. Differential impedance is set within the pair by wire size and insulation, which is mainly what you care about.

Answer 3

Does differential impedance between D+ and D- not only depends on separation between and wire gauge of D+ and D-, dielectric constant of the insulation, but also the GND wire and shielding?

The Characteristic impedance of a wire (more correct: transmission line) depends on the (signal) conductor size and what other conductors are nearby. So yes, insulation and shielding influence the Characteristic impedance.

But I sense that you're thinking that you can influence that Characteristic impedance by (not) connecting the shield to ground. That is not the case! The Characteristic impedance is a property of the wire itself, irrespective of how you connect it.

If the Characteristic impedance of the impedance controlled lines and the USB cable are the same and you're able to make a "nice" connection between the two then that can work and you must connect the shielding of the USB cable to ground because that's how it is supposed to be used. If you leave the shield floating the capacitance between shield and signal lines will cause extra coupling between the 2 differential signal lines. Grounding the shield will minimize this effect.

However, unless you're going to use this at a very low speed I doubt that this is going to work properly as I am unsure that the Characteristic impedance of a USB cable and PCIe lines are the same. Also connecting the USB cable to those PCIe lines without making it such that signal reflections will be an issue, will be a challenge.