Re: gEDA-user: Eliminate separate Vcc planes?

[Gene Heskett <gene.heskett@xxxxxxxxxxx>]

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.

>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.

>The
>frequencies in question are 400 MHz to 3 GHz.
>To me running Vcc traces all over the board is the surest way to raise
>inductance etc., and seems wrong to me.

It is.  But it is sometimes helpful to make those layers a bit like a star 
topology to help steer the noise properly and keep it from going willy-nilly 
anyplace it wants to go.

>Want to know what you thought of this consultants advice?

Rubbish.  Whats worse is that your boss probably _paid_ for that advice.

>Doesn't cover what happens in multi-rail systems either.

-- 
Cheers, Gene
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