[SI-LIST] R: Differential traces over a split in the ground plane
- From: <gianguida@xxxxxxxx>
- To: <tom_cip_11551@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 22 Dec 2006 09:20:05 +0100
My answer is yes...
in the real case you elimnate the simmetry
of the ideal case ...( for the ideal case I assume that also the gnd plane is
ideal and infinite...)
Question is: how much the cut will affect the beahaviour
of the differential pair, in the real case...
I would make a 3D simulation of the real case to get a
reasonable answer to this question
Hope this help
Giancarlo
-----Messaggio originale-----
Da: si-list-bounce@xxxxxxxxxxxxx per conto di tom_cip_11551
Inviato: ven 22/12/2006 0.46
A: si-list@xxxxxxxxxxxxx
Oggetto: [SI-LIST] Differential traces over a split in the ground plane
Hi,
Consider, first, the ideal case.
Considering edge coupled differential traces over a plane(I suppose
that one could generalize to any kind of differential pair over a
plane). If the legs of the pair, in the ideal case, were identical
in length, width, etc, I would expect that there would be no
differential to common mode conversion and there would be no return
current through the ground plane.
If there is no return current in the ground plane then I would
expect that if the ground plane was "choked" with ferrites (these
are ideal ferrites that take up no physical space) at some point in
between the two ends of the differential pair, then the odd mode
impedance would not be affected. I would expect that the even mode
impedance would increase as a result of the ferrites.
Now consider the real world.
In a PCB, in order to "choke" the ground plane underneath the
differential pair I would first have to physically cut the plane,
bring vias to the surface and then use a ferrite chip to connect the
two sections of plane.
My question is, does that physical cut cause an increase in the odd
mode impedance (and, hence, a discontinuity) of the differential
pair, at the point of the cut in the plane? I would expect the
answer to be yes.
Thank You
Tom
tom_cip_11551@xxxxxxxxxxx
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