> >At  the 1999 PCB east conference I learned about the 20H rule.  This rule
> >simply put states that EMI can be substantially reduced by keeping the
> >power planes back from the edge of the ground planes by 20 time the
> >dielectric thickness between the two planes.   This would be a lot easier
> >to implement than vias .
>
> This rule of thumb appears to be highly controversial among experts. To my
<snip>

I don't claim to be an expert, but here's my thoughts anyway:

What if your ground plane is as noisy as your power plane?  If the ground
plane and power plane cover the same areas they can (almost) completely
nullify each other's noise IFF the noise currents in each plane are
correlated with the other plane.  An instance where the noise currents would
NOT correlate would be for non-differential (i.e. ground-referenced I/O,
power feeds, etc.) signals entering/exiting the board.  But for local
(on-board) circuitry, the noise in each plane should correlate.

You might say that the ground plane being as noisy as the power plane is
ridiculous.  Try an easy experiment:  use a working digital circuit that has
inherently noisy circuitry, such as a computer board.  Connect it to a power
supply that has an isolated negative terminal (typical bench power supply).
Hook up your scope probe and measure the noise on your power plane.  Now
connect your scope ground wire to the power plane and the probe tip to the
ground plane.  The ground looks just as noisy, doesn't it?  That's because
your ground reference is now the power plane.  The fact is, RF noise
radiation doesn't care what your ground reference is, because radiated
emissions are (by definition) radiated, NOT conducted.

I suppose the idea of trimming the power plane back is to reduce the fringe
fields present at the board edges.  But you may end up radiating more with
the uncovered ground plane areas.  The problem is, there is no way to know
which effect would be greater without doing lots of simulation, or actually
building the prototype both ways and taking measurements.  Pick one and go
with it.  I'd pick the matching planes.

Best regards,
Ivan Baggett
Bagotronix Inc.
website:  www.bagotronix.com


----- Original Message -----
From: "Abd ul-Rahman Lomax" <[EMAIL PROTECTED]>
To: "Protel EDA Forum" <[EMAIL PROTECTED]>
Sent: Wednesday, July 11, 2001 6:17 PM
Subject: Re: [PEDA] perimeter stitched ground vias question


> At 10:09 AM 7/11/01 -0500, Mark E Witherite wrote:
> >At  the 1999 PCB east conference I learned about the 20H rule.  This rule
> >simply put states that EMI can be substantially reduced by keeping the
> >power planes back from the edge of the ground planes by 20 time the
> >dielectric thickness between the two planes.   This would be a lot easier
> >to implement than vias .
>
> This rule of thumb appears to be highly controversial among experts. To my
> knowledge it has never been tested in a controlled experiment. One of its
> effects might be the change in the direction of peak radiation, so a study
> would need to be fairly sophisticated and not just make a measurement in a
> single direction.
>
> Vias are easy to implement. If they are spaced closely enough, which
> depends on frequency, they will essentially complete a ground shield, but
> this may have little effect on radiation except at certain angles. In
> general, where there are multiple ground planes, an increased number of
> vias will be helpful, but one would want these vias not only at edges but
> also close to all potential noise sources.
>
> I simply don't have sufficient experience to know or even make a solid
> guess, and I have never seen a study or even a sufficiently sophisticated
> analysis with a field solver. Perhaps someone else has and my
understanding
> is out of date.
>
> Attempts have been made to track back the source of the 20H rule, and it
> does not seem to be much more than a bright idea of one writer; that, of
> course, does not make it wrong.
>
> [EMAIL PROTECTED]
> Abdulrahman Lomax
> P.O. Box 690
> El Verano, CA 95433
>

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