[SI-LIST] Re: Routing Signals Between PWB Layers - Part 2

Doug,

I concede that it is possible to create a condition where EMI is worse when
signals change layers as has been done with this demo.  However, the
conditions in this demo are not representative of what a good 4 layer
designs look like.  As some have pointed out, your demo does not include
any decoupling capacitors, which real designs have.  It does not contain
any active circuits, which real designs have.  It does not contain any
plane capacitance, which real designs have.

My fear is that some will get the impression that your demo applies in
these cases and lead them to invoke some terribly restricitve design rules
when they are not necessary.

Your tests would be more applicable if you did your tests on a real PCB
with real bypassing and such on it.  If you get the same result, then it is
valid to claim that layer changing is a detectable source of increased EMI
in a 4 layer PCB.  It does not follow that this is true in PCBs with more
than 4 layers, by the way.

Even then, the fact that there is a detectable increase is not evidence
that this is a problem that should be worried about.

There is a big difference between detectable and important and we should
make sure we don't cause worry just because something is detectable.  Next,
you need to show that this is important if this experiment is to go beyond
the academic.

After all, vias are detectable on a transmission line, but only in rare
cases are they a problem.  Another thing that worries engineers when it
shouldn't in almost all cases.


> [Original Message]
> From: Doug Smith <doug@xxxxxxxxxx>
> To: SI-List <si-list@xxxxxxxxxxxxx>
> Date: 5/3/2006 1:44:20 PM
> Subject: [SI-LIST] Routing Signals Between PWB Layers -  Part 2
>
> Hi All,
>
> I have been writing and recording again and have posted part 2 of my 
> article on signals that change layers in a PWB.
>
> Abstract: Printed wiring board, PWB, signal paths must often change 
> layers in a board stackup. Under some conditions this can cause 
> problems. An emissions example is used to illustrate the conditions 
> where changing layers can cause problems.
>
> The link to the article is the picture of the emissions plot at the 
> bottom of the home page at http://emcesd.com .
>
> There is also an audio discussion of this article on my podcast site: 
> http://emcesd-podcast.com where the direct link to the article is:
>
> http://emcesd-podcast.com/2006/may/2006-0503.mp3
>
> Can't download mp3 files? Download the following instead:
>
> http://emcesd-podcast.com/2006/may/2006-0503.dcs
>
> After download, change the extension from .dcs to .mp3 and the file 
> will then be able to play on most computers. Since the last HFNews 
> there have been two new podcasts posted.
>
> Doug
> -- 
> -------------------------------------------------------
>      ___          _       Doug Smith
>       \          / )      P.O. Box 1457
>        =========          Los Gatos, CA 95031-1457
>     _ / \     / \ _       TEL/FAX: 408-356-4186/358-3799
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> |  q-----( )  |  o  |    Email:   doug@xxxxxxxxxx
>   \ _ /    ]    \ _ /     Website: http://www.dsmith.org
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