I never heard of the 20H rule, So I did a search on it. I found some very
interesting articles on the subject, while I was searching I came across a
article on X2Y technology. Has anyone used any of these devices? If so do
they work as described and offer differential impedance within or close to
the X2Y component? 


-----Original Message-----
From: Brad Velander [mailto:[EMAIL PROTECTED]]
Sent: Thursday, July 12, 2001 7:55 PM
To: 'Protel EDA Forum'
Subject: Re: [PEDA] 20H rule, Planes etc (Ex: perimeter stitched ground
vi as question)

        this is the manner in which I understand this issue. If I am wrong
please correct me, I don't design antennas for a living and know very little
about such. I do know that I have read certain descriptions of these
problems and the proposed solutions but we all know there are bad solutions
backed up by specific experimentations that prove the solution.

This is the theory that I would apply to the situation.
        The manner that planes do not have noise fields between them is
achieved 'theoretically' because you have coupled your planes together to
near perfection through all of your properly selected, distributed and
positioned power supply bypass capacitors. Effectively reducing the
differential AC potentials between your planes to "0". Now that is the
perfect world which doesn't exist but the real world differences should be
relatively small if you have done your power distribution design correctly.

        Would the application of this to a trace (running parallel to your
plane edge) not be exactly the opposite circumstance? There you have
definite potential difference between the trace and the plane, much larger
differentials then between your two planes. And thus if differentials
between two planes with a little noise causes a dipole antenna effect, why
would a trace with significantly larger differential not cause the same
effect only stronger for the same relative length of the trace against the
plane edge?

        This seems very clear to me but then I don't design antennas so what
do I know. I only know that the article I read about traces and the 20H rule
to plane edges most definitely showed the desired effect in the results
arrived at with the field solvers.

Brad Velander,
Lead PCB Designer,
Norsat International Inc.,
#300 - 4401 Still Creek Dr.,
Burnaby, B.C., V5C 6G9.
Tel. (604) 292-9089 direct
Fax (604) 292-9010
website www.norsat.com

> -----Original Message-----
> From: Bruce Walter [mailto:[EMAIL PROTECTED]]
> Sent: Thursday, July 12, 2001 4:54 PM
> To: 'Protel EDA Forum'
> Subject: Re: [PEDA] 20H rule, Planes etc (Ex: perimeter 
> stitched ground
> vi as question)
> My understanding of the 20H rule, was with regard to planes.
> If the planes have noise fields between them (I don't know 
> how this can be
> avoided, regardless of component/trace/via placement), when 
> the planes end
> equally at the edge of the board, this makes a nice dipole 
> antenna, and the
> noise radiates off the edge of the board in a nice wide 
> pattern like you
> would want from a dipole.
> By applying the 20H rule, you distort the field between the 
> two 'poles', and
> the effective radiation is greatly reduced as this is a very 
> poor antenna.
> How well this applies to traces over a plane near the edge of 
> the board, I'm
> not sure, but I would imagine the effect would be local, and 
> would only be
> significant if there were a strong field between the two 
> conductors (poles).

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