[SI-LIST] Re: risetime effects of plane breaks
- From: steve weir <weirsp@xxxxxxxxxx>
- To: "Brad Crowell" <brad.crowell@xxxxxxxxxx>, "Si-List" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 21 Jan 2005 05:32:10 -0800
Brad, I tend to think that idea would help, but I don't know how much
without constructing a model.
Regards,
Steve
At 09:22 AM 1/21/2005 -0400, Brad Crowell wrote:
>Yes, that is what I am suggesting. This would probably be a closer analogy
>to a board to board connector than IC bond wires, as the physical scale
>would be closer to the connector situation.
>
>Brad
>
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx
> > [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir
> > Sent: January 21, 2005 9:06 AM
> > To: brad.crowell@xxxxxxxxxx; Si-List
> > Subject: [SI-LIST] Re: risetime effects of plane breaks
> >
> >
> > Brad, are you proposing parallel tracks on the signal layer
> > interdigitated
> > 1:N signal traces that extend completely across an island?
> >
> > Steve
> > At 08:50 AM 1/21/2005 -0400, Brad Crowell wrote:
> > >Chris's analogy to wire bonding makes me wonder if this would be
> > helpful in
> > >crossing splits. Assuming one can't avoid it and has to cross a
> > plane split
> > >with a bus, would including several traces from one part of the
> > >plane, across the split, to another area of the plane be helpful
> > in reducing
> > >the negative effects of the split? In theory, this would provide a
> > >continuous path for the return currents on that plane that is
> > more closely
> > >coupled to the signals than would otherwise result. This should
> > then provide
> > >some benefit.
> > >
> > >Brad
> > >****************************
> > >Brad Crowell, P. Eng.
> > >Senior Hardware Designer
> > >AMIRIX Systems
> > >Phone: 902-450-1700 Ext 287
> > >Fax: 902-450-1704
> > >www.amirix.com
> > >****************************
> > > > -----Original Message-----
> > > > From: si-list-bounce@xxxxxxxxxxxxx
> > > > [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Chris Cheng
> > > > Sent: January 20, 2005 8:14 PM
> > > > Cc: Si-List
> > > > Subject: [SI-LIST] Re: risetime effects of plane breaks
> > > >
> > > >
> > > > Scott,
> > > > Excellent summary. That was my concern on striplines crossing
> > with a bus
> > > > rather than individual signals. In a way, it is like wire bond
> > > > signal leads
> > > > without the ground leads mixed among them. The signals start
> > referencing
> > > > each other instead. Or you can see it as a trade-off between adding
> > > > shielding layers or spreading the bus spacing (decreasing routing
> > > > channels)
> > > > in a high density/performance design. My own rule of thumb is
> > > > space them at
> > > > least equal or larger than the gap itself when crossing. That's
> > > > is at least
> > > > a 3x decrease in routing channels so it is quite costly and has
> > > > to be weight
> > > > against adding shielding layers. Sometimes its worth it,
> > > > sometimes its not.
> > > > As for EMI, if you dig back some discussion I had with Steve, I always
> > > > prefer solid ground planes referencing microstrips on top and
> > > > bottom of PCB
> > > > and then stitch the edges with ground vias. Hopefully any of
> > those excited
> > > > noise on the cut power planes will be trapped inside.
> > > >
> > > > -----Original Message-----
> > > > From: Scott McMorrow [mailto:scott@xxxxxxxxxxxxx]
> > > > Sent: Thursday, January 20, 2005 2:39 PM
> > > > Cc: Si-List
> > > > Subject: [SI-LIST] Re: risetime effects of plane breaks
> > > >
> > > >
> > > > When this thread started I was on vacation. However, I found this
> > > > interesting enough to resurrect some previous simulations I'd
> > performed
> > > > in CST Microwave Studio. After much playing, twiddling and generally
> > > > having fun I can say several things:
> > > > 1) It's pretty easy to confirm Doug's results using 3D fullwave
> > > > simulation. In fact, in about 30 minutes I can replicate his case and
> > > > create a design that can be easily modified for many other
> > > > possibilites. The microstrip split plane crossing is a no-brainer.
> > > > Just don't do it and expect anything approaching an EMI
> > "clean" system.
> > > >
> > > > 2) Chris and Steve ... and eventually myself, wanted to know
> > more about
> > > > the various different stripline plane crossing configurations, so I
> > > > setup a simulation with a VDD island not unlike what might be
> > found in a
> > > > memory system, and performed multiple simulations with dual
> > asymmeteric
> > > > stripline crossing the plane twice on it's way to the memory
> > module. Not
> > > > surprisingly the following is true:
> > > >
> > > > It is best not to cross a split plane ... even with stripline.
> > > > If you do, it is better to cross a split that is adjacent to a
> > > > ground plane
> > > > It is even better if you cross a split adjacent to a
> > ground plane on
> > > > the stripline layer furthest away from the split plane
> > (i.e. next to
> > > > a ground plane)
> > > > It is worst to cross a split plane that has no adjacent ground.
> > > > The width of the gap in the plane makes very little
> > difference until
> > > > it becomes really small or really big.
> > > > Crosstalk scales almost linearly with the number of aggressors
> > > > crossing the split. (i.e. - it can get really bad!)
> > > > Bypass of the split power island helps for frequencies below 500
> > > > MHz, provides no help for frequencies higher than 500 MHz, and as
> > > > such has no benefit to most of the noise and crosstalk created by
> > > > high speed signals crossing onto and off of the island.
> > > >
> > > > The energy released into the power/ground plane cavities by high speed
> > > > signal split plane crossings is huge and essentially cannot be
> > > > suppressed with bypass capacitors. Any attempt at supprerssion with
> > > > capacitors exhibits what I call a "Whack-A-Mole" property. You can
> > > > never get rid of those pesky little moles. All you can do is to move
> > > > them around by thumping them. Given that all this energy is rattling
> > > > around the PCB power planes from split plane crossings, it will
> > > > eventually go somewhere. Since it's really easy to develop
> > all sorts of
> > > > resonant power island cavities that have primary resonant
> > frequencies in
> > > > the 500 MHz to several GHz range, it is not at all unlikely that any
> > > > split plane crossing has an extremely strong potential to excite a
> > > > resonance in a frequency range that will cause most systems
> > to fail EMC
> > > > compliance testing About all you can do is to shield the
> > cavity patches
> > > > using ground layers. This should reduce the radiated energy
> > > > significantly, but will not totally eliminate it, since eventually it
> > > > will find it's way to all those pesky device and package leads.
> > > >
> > > >
> > > > best regards,
> > > >
> > > > Scott
> > > >
> > > > --
> > > > Scott McMorrow
> > > > Teraspeed Consulting Group LLC
> > > > 121 North River Drive
> > > > Narragansett, RI 02882
> > > > (401) 284-1827 Business
> > > > (401) 284-1840 Fax
> > > >
> > > > http://www.teraspeed.com
> > > >
> > > > Teraspeed is the registered service mark of
> > > > Teraspeed Consulting Group LLC
> > > >
> > > >
> > > >
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