[SI-LIST] Re: Diff.Pairs - Return current distribution
- From: "Charles Grasso" <cgrassosprint1@xxxxxxxxxxxxx>
- To: <a.ingraham@xxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 14 Oct 2003 22:55:57 -0700
Referring to Eric Bogatins book (SI simplified)
there are nice pictures that show the current distribution
in a pair of coupled transmission lines over a ground plane.
Fundemetally, when the coupling between the signal line
and the return plane is much larger than the the coupling
to the adjacent signal, there are seperate and distinct
return currents with very little overlap in the plane.
(to paraphrase from the book)
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Ingraham, Andrew
Sent: Tuesday, October 14, 2003 8:03 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Diff.Pairs
> Lastly, concerning the effect of plane breaks, differential mode currents
> are not affected by the break because they cancel, and the plane acts as a
> virtual equipotential surface or plane.
I would assume that this is true ONLY when the differential traces are on
the same layer and adjacent where they cross the plane break.
If they were routed as independent single-ended signals, even with identical
length constraints, the return current cancellation would be much less
effective if it happens at all (except at low frequencies).
For a broadside coupled pair that is sandwiched between ground planes, the
return currents would not cancel very effectively unless you provided plenty
of paths (vias) from one plane to the other in the vicinity of where the
pair crosses the plane break, on both sides of the break.
Regards,
Andy
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