RE: [SI-LIST] : Regarding plane splits

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From: David Kaiser (david.kaiser@mcdata.com)
Date: Tue Dec 05 2000 - 15:58:19 PST


If the frequency harmonics are too high for the 2 or 3 mil coupled
ground/power planes to provide good enough AC coupling even with filler on
other layers, where do you go from there? Does anyone know.
Maybe Ritchey Lee can answer this. Thanks.

David Kaiser
McDATA Corp.
310 Interlocken Pkwy.
Broomfield, CO 80021
(303) 460-4431
davidk@mcdata.com

-----Original Message-----
From: Ritchey Lee [mailto:leeritchey@earthlink.net]
Sent: Tuesday, December 05, 2000 4:28 PM
To: hirshtal@is.elta.co.il
Cc: Chan, Michael; Zabinski, Patrick J.; si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : Regarding plane splits

Properly done decoupling produces a very low impedance between Vcc and
ground at all
of the frequencies involved in switching. Since most of the edges are a
nanosecond
or less, this means a good plane capacitor. Very few, if any, application
notes even
acknowledge that this is needed. Therefore, most engineers don't get this
right and
have lots of high frequency noise on Vcc.

Lee

Itzhak Hirshtal wrote:

> Hello Mr. Lee,
>
> What do you mean by saying "the decoupling is not done well"? Can you
detail what
> is the good way to do it?
>
> Thanks
>
> Ritchey Lee wrote:
>
> > That is a DC view of an AC problem. When the decoupling is done well
enough
> > to provide the switching currents required to create the fast edges, the
two
> > sides of the split are at the same AC potential, namely that of the
underlying
> > plane. That's because you shorted them to this plane with the
capacitors.
> >
> > Clearly, if the decoupling is not done well, this is not true. Also, if
the
> > decoupling is not done well, there will be excessive noise on both VCC
> > planes. This will show up in many ways, one of them being potentially
high
> > EMI.
> >
> > Lee
> >
> > Chan, Michael wrote:
> >
> > > What happen if the split is for two different voltage planes?
> > >
> > > MChan
> > >
> > > -----Original Message-----
> > > From: Ritchey Lee [mailto:leeritchey@earthlink.net]
> > > Sent: Wednesday, November 22, 2000 11:39 AM
> > > To: Zabinski, Patrick J.
> > > Cc: si-list@silab.eng.sun.com
> > > Subject: Re: [SI-LIST] : Regarding plane splits
> > >
> > > All of these discussions fail to take into account the fact that the
> > > ground plane and the two power planes msut be well decoupled in order
to
> > > create a low impedance source for the switching currents that are
involved
> > > in those same swithcing edges. If this had been done well, the power
> > > planes will, of necessity, be shorted to the ground plane and that
will be
> > > the path around the split.
> > >
> > > In my experience, people who see the effects of a split have failed to
do
> > > a good job of power plane decoupling. Learn how to do this well and
> > > splits won't bother you.
> > >
> > > Lee
> > >
> > > Zabinski, Patrick J. wrote:
> > >
> > > > Aloke,
> > > >
> > > > As I mentioned in a recent posting, there are conditions in which
> > > > routing stripline in a configuration you describe sees no effects
> > > > from the split. More specifically, if your trace layer is "closer"
> > > > to the solid ground plane than the split power planes, then the
> > > > solid plane has "more" of an influence on the trace. As such, you
> > > > will "less" of a discontinuity from the split. I've tested this
> > > > in the lab under several conditions, and I believe this to be true.
> > > >
> > > > However, the reason I used "'s in the above statements is that I
> > > > haven't taken my experiments far along enough to be able to provide
> > > > any guidance as to how "close" is "close enough" to reduce the
> > > > discontinuity effects to the point where your system can tolerate
> > > > them.
> > > >
> > > > In what I call a 50/50 case where the stripline layer is centered
> > > > vertically in the stackup such that the distance to the solid ground
> > > > plane is the same as the distance to the split plane, you will
notice
> > > > the discontinuities.
> > > >
> > > > Pat
> > > >
> > > > >
> > > > > Hello all,
> > > > > I had a doubt regarding plane splits:
> > > > >
> > > > > In the stackup, if there is a power plane on one side of the
signal
> > > > > layer and a ground plane on the other side of the signal
> > > > > layer(symmetric
> > > > > stripline config), and if the ground plane is a solid ground plane
> > > > > having no discontinuities, then can I have splits in the
> > > > > power plane and
> > > > > run traces over the splits? Is the ground plane alone not
> > > > > sufficient to
> > > > > provide paths for return currents?
> > > > >
> > > > > With regards,
> > > > > Aloke
> > > > >
> > > > >
> > > > >
> > > > >
> > > >
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>
> --
> Itzhak Hirshtal
> Elta Electronics
> Ashdod
> POB 330
> Israel
> Tel: 972-8-8572841
> Fax: 972-8-8572978
> email: hirshtal@is.elta.co.il
>
> ------------------------------------------------------------------------
>
> Itzhak Hirshtal <hirshtal@is.elta.co.il>
> Eng
> Elta
>
> Itzhak Hirshtal
> Eng <hirshtal@is.elta.co.il>
> Elta
> Netscape Conference Address
> Netscape Conference DLS Server
> That's myself
> Additional Information:
> Last Name Hirshtal
> First Name Itzhak
> Version 2.1

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