[SI-LIST] Re: risetime effects of plane breaks
- From: steve weir <weirsp@xxxxxxxxxx>
- To: ron@xxxxxxxxxxx, Chris.Cheng@xxxxxxxxxxxx
- Date: Sat, 22 Jan 2005 15:18:34 -0800
Ron,
Do you really mean to imply that the cross-talk falls substantially if the
slot gap is increased to 2H or more? That is a new and very
counterintuitive notion to me. I would be very interested in seeing any
A/B model that could demonstrate such a phenomena.
Regards,
Steve.
At 10:13 PM 1/21/2005 -0800, ron@xxxxxxxxxxx wrote:
>A few years ago Intel discovered that when a trace crosses a split it
>can excite a transmission
>line mode into the slot between the panes called "slot line" strangely
>enough. If the gap is small
>it works quite well and all the traces crossing it become cross-talk for
>one another.
>
>To avoid it make the gap at least 2 or 3 times the thickness of the
>dielectric.
>
>ADS (Agilent) has a model for slot lines with the other transmission lines.
>
>Ron
>
>Chris Cheng wrote:
>
> >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
> >
> >
> >
>
>
>--
>Ronald Miller
>Ghz Data, Signal Integrity Consulting
>7721 Sunset Ave.
>Newark CA 94560
>tel 510-793-4744
>cell 510-377-9380
>fax 510-742-6686
>www.ghzdata.com
>
>------------------------------------------------------------------
>To unsubscribe from si-list:
>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
>or to administer your membership from a web page, go to:
>//www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
>List technical documents are available at:
> http://www.si-list.org
>
>List archives are viewable at:
> //www.freelists.org/archives/si-list
>or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
>Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
