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The USB IF spec mentions the following for USB 3.0:

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This has me scratching my head. In my experience, \$Z_{diff} < 2 Z_0\$ -- typically around 1.6-1.8. So how is it possible to meet both the single and double ended requirements if the single ended requirement needs to be around 50\$\Omega\$ (readily achievable) to get the 90\$\Omega\$ differential impedance?

Some of the resources I've found even explicitly say 50\$\Omega\$, such as this Toradex high speed layout guide (PDF):

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And this TI USB 3.0 hub reference design, which uses ~4.5mil trace, 5 mil space on 1oz cu, 3.7mil dielectric thickness also does 50/90:

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...but this is straight from the horse's mouth.

Most information I find seems to emphasize the 90\$\Omega\$ requirement more than the 45 ohm requirement. Is it really important to get 45\$\Omega\$ or is 50\$\Omega\$ preferred? Can someone set the record straight for me?

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I think they're talking about two different sets of wires in different generations of the standard.

First sentence refers to SDP, the two Shielded Differential Pairs introduced in USB 3.0 for 5 Gbps throughput. These are usually shielded twisted pairs. Their \$Z_{diff} < 2 Z_o\$.

Second sentence is talking about Enhanced Superspeed, the 10 Gbps signalling introduced with USB 3.1. These can be (high-grade) shielded twisted pairs, but they can also be individual micro-coax wires - one each for the direct and inverted signal of each differential pair. In this latter case because there is little or no mutual coupling between the + and - signals, \$Z_{diff} \approx 2 Z_o\$.

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  • \$\begingroup\$ As a caveat: I'm only somewhat confident in this answer. I'm not familiar with USB 3.x. \$\endgroup\$ – pericynthion Jun 16 '17 at 15:25
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Single ended impedance is the trace impedance with reference to ground.

Differential Impedance is the impedance between two differential pair signal traces.

So i think both needs to be matched if you want to work at rated high frequency. Also need to be within tolerance range as in USB case it is 15%.

Reference : http://www.ti.com/lit/an/slla414/slla414.pdf (Page # 6)

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