Re: gEDA-user: Eliminate separate Vcc planes?

[Gabriel Paubert <paubert@xxxxxxx>]

On Mon, Oct 19, 2009 at 10:50:49AM -0400, Gene Heskett wrote:
> On Monday 19 October 2009, Bob Paddock wrote:
> >Boss just sent around something he got from a consultant on
> >doing "proper" EMI design (which I've been doing for years already,
> >I thought until consultant came up with this):
> >
> >"Eliminate separate Vcc planes.
> 
> What's he/she smoking, it must be great stuff and I want a sample.

Keep some for me also. Thanks.

> 
> >This ancient practice is long overdue for an overhaul.  Years ago, the
> >leaded capacitors were not able to provide a good enough short at VHF
> >and above, so the reasoning was that the parallel plates of Vcc and
> >ground made a good UHF capacitor.   The problem with this is twofold:
> >it takes away one or more ground planes, and more importantly doesn’t
> >allow the designer to control where the noise current goes.   Noise
> >follows the path of least impedance, which may be anywhere on the PCB
> >after you punch holes in the Vcc plane for vias and to route traces
> >that have no other room to go.  The best way to control noise is to
> >use a separate trace for Vcc, and apply series and shunt elements to
> >control the noise currents."
> >
> >There is no attribution as to were that advice comes from.
> 
> And as a C.E.T. with 60 years of electronics experience, troubleshooting to 
> the part level, I sure as heck would not want my name attached to such 
> advice.

Well, the assumption is that there are traces that have no other
romm to go than the VCC plane. This is something that I would never
do (either you have a plane only interrupted by the unvaoidable vias
or you don't have a plane). I sometimes do an exception for double
layer boards, but of course never in an area of high frequency signals...

	Gabriel


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