Re: [SI-LIST] : Q: Plane-jumping return currents

Eric Goodill (ericg@cisco.com)
Wed, 22 Sep 1999 16:51:47 -0700

Mike Jenkins wrote:
>
> Eric,
>
> One line of your question, "My system is running pretty fast
> (> 1 Gbps)", caught my eye. At that speed, which I assume might
> be Fibre Channel or Gigabit Ethernet, you may well be running
> differential. (If not, good luck to you.) But if your lines
> are dif'l, they carry their own return current. Depending on
> geometry, there is some discontinuity, but MUCH less than
> single-ended. If your lines are, in fact, differential, and
> if you wish me to elaborate, I will.

Mike,

Yes, differential. However, we're using edge-coupled pairs, and it's my
understanding, though I've done no analysis, that about 10% - 15% is about
as much coupling as you can get between edge-coupled lines. Thus, there is
still a strong coupling between the trace and it's reference place.
Therefore, I suspect that there's non-ignorable amount of return current in
the reference planes. I'd be interested to see a
return-current-distribution plot for a diff pair both in the reference
planes and the coupled traces.

-Eric

-- 

Eric Goodill Cisco Systems M/S SJ-N2 mailto:ericg@cisco.com 170 W Tasman Dr voice: (408) 527-3460 San Jose CA 95134-1706 fax: (408) 527-3460 (yes, the same)

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