[SI-LIST] Re: vias and current pathes
- From: Rick Collins <gnuarm.2006@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 20 Jul 2012 23:34:21 -0400
The OP asked about RF effects. I believe that for RF, unless you
have a very unusual board, overlapping ground planes will have near
zero impedance between them. So there does not need to be any
special vias located near the signal path. The return current will
flow directly between the two ground planes as if they were connected for RF.
This is also true of interruptions in the continuity of a single
plane, either ground or power. If a signal crosses a break in the
plane, but the plane on both sides of the break adequately overlap an
adjacent, not interrupted plane, the return path will couple from one
ground plane to an overlapping power or ground plane and back to the
other half of the interrupted ground plane. For RF all the
overlapping power and ground planes are like one.
Rick
At 05:28 PM 7/20/2012, Aaditya Kandibanda wrote:
>Herbert,
>Whenever a trace changes layers, the return current flows through the vias
>that stitch the adjacent ground planes of those two layers. If those two
>planes are not stitched, there would be a discontinuity for the return
>path.
>
>Thanks and regards
>Aaditya
>
>On Fri, Jul 20, 2012 at 2:19 PM, Herbert <afu@xxxxxxxxxxxxxxxxxx> wrote:
>
> > Hello to all,
> >
> > I am new here and I have searched about the real
> > current path for RF on a combination
> >
> > trace - via - trace
> >
> > On the trace there is current flow more or less identical
> > on both sides of the trace ( top - bottom ).
> > When flowing through the via , I would think, there is a more or
> > less equal corrent on inside surface and on outside surface.
> > That currents pass through the via and flow then on top and bottom
> > of the trace on the second side.
> > What about the interchnge of current between the sides and which influence
> > has the pad ring of the via ?
> > What about the idea of having less skinn effect when having
> > that situation with a ground plane on the second side ( means a grounding
> > via ) and using so called "thermal reliefs" ?
> > I have the opinion, that the thermal releafs have a negative effect
> > on the overall impedance of the via construction WITH thermal reliefs.
> >
> > Waiting for good ideas
> >
> > Herbert
> >
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