[SI-LIST] Re: More About Plane Isolation
- From: Hirshtal Itzhak <ihirshtal@xxxxxxxxxx>
- To: steve weir <weirsi@xxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Thu, 14 Jul 2011 13:48:44 +0200
Hello Steve,
I'm not sure I understand: My 1st question relied on my recalling (from
University studies a very long time ago) that a metallic mesh with holes
smaller than half the wavelength doesn't allow the wave to go through
it. Does my memory betray me?
Now, if you mean the Bethe hole directional coupling that Yuriy Shlepnev
mentioned, then isn't it a narrow-band phenomenon that is irrelevant to
high-speed digital signals?
Regarding the trace distance from the hole - I surely will not allow it
to go over the hole! My intention is to keep it away as far as possible.
How far is it needed to be? Do you have a rule-of-thumb for that?
One more thing: Does the coupling depend on hole length? And how?
Thanks again,
Itzhak Hirshtal
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of steve weir
Sent: Tuesday, July 12, 2011 1:21 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: More About Plane Isolation
On 7/12/2011 12:01 AM, Hirshtal Itzhak wrote:
> Hello Yuriy,
> It took me a while, because I've lost my SI-LIST correspondence, but
> still, I would like to question your note about through-plane coupling
> via PCB holes, which I cite below (from the SI-LIST Archive).
> First, is it true that the coupling through a single hole will be very
> low at frequencies that correspond to wavelengths longer than twice
the
> hole diameter?
The amount of coupling depends on the geometry and the frequency, in
particular how many H the trace is away from the hole. In the limiting
case of 0H, we are trying to, and can generally be very successful
getting almost all of the signal energy to couple through.
> Second, what if the hole isn't really a hole, since it's filled with
> copper, or "filled" with a screw? This will turn the hole to a much
less
> coupling structure, isn't it?
No, see above.
There are two cases: A case where a via is attached to the plane, and
all others where there is antipad clearance. In the second case
dielectric in the antipad supports energy. Metal in the hole reflects
energy.
> Third, can you explain the issue of periodic hole pattern? Is it an
> issue at several GHz frequencies?
The frequencies that are relevant depend on the hole pattern. For
practical circuit boards 12" or less on the longest dimension quarter
wave or longer structures only exist for frequencies above 100MHz.
> Thanks
> Itzhak Hirshtal
>
> Your Note:
> Note, that these results are for the solid copper planes. If planes
have
> holes in vicinity of the traces, the coupling may substantially
increase
> at high frequencies even for much shorter segments. If small holes
form
> a periodic pattern, the structure may behave as very effective
> directional coupler with substantially larger coupling at microwave
> frequencies.
> As Scott already pointed out, you have to do numerical analysis for a
> structure that represents your particular design.
> Best regards,
> Yuriy
> www.simberian.com
>
>
>
>
>
>
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Steve Weir
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addressee, please be aware
that any use, disclosure, distribution and/or copying of this communication is
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If you receive this communication in error, please notify the sender
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