[SI-LIST] Re: Differential traces over a split in the ground plane
- From: agathon <hreidmarkailen@xxxxxxxxx>
- To: tom_cip_11551@xxxxxxxxx
- Date: Thu, 21 Dec 2006 19:47:39 -0800
Tom,
oops... where did you get that idea?
There _is_ plane return current since the pair isn't 100% & perfectly
coupled.
Ideally, though, the two return currents cancel if your considered cross
section is big enough.
But they still need to get home via the paths to vdd or gnd.
-Agathon
On 12/21/06, tom_cip_11551 <tom_cip_11551@xxxxxxxxx> wrote:
>
> 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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