[SI-LIST] Re: Split Ground Planes
- From: Chris Padilla <cpad@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 27 Oct 2004 08:27:03 -0700
Before the isolation transformers/chokes/etc. were placed inside the
cavities of double-stacked RJ45 (2x4, 2x6, 2x8, etc.) ethernet connectors,
the magnetics were discrete parts. We would often cut the gnd planes under
the row of magnetics which on a 48-port blade, would always span the
breadth of the board. Keep in mind with this technique that we didn't have
any traces directly crossing this split.
However, the PCB is often mounted on a metal carrying tray through
stand-offs and the RJ45s metal shield would also contact the tray since it
had to protrude slighly through the tray. This tray/standoffs was the only
other connection from one side of the split to the other.
Chris Padilla
EMC Engineer
Cisco Systems
San Jose, CA
At 10:14 PM 10/26/2004 -0700, Doug Smith wrote:
>Hi Chris,
>
>One of the places where cutting the planes can work is when passing
>through a balancing transformer which keeps the common mode current
>low. Also you may want to keep the ground plane from putting noise
>back into a balanced pair as well as potentially unbalance it if the
>capacitance is not kept precisely balanced (which most layouts I have
>seen don't).
>
>It is not possible to list all the variations, but care must be taken
>anytime the plane is split to account for all currents and where they
>go, including displacement currents through the air.
>
>Doug
>
>Chris Cheng wrote:
>
> > Hmmm hmmm, is there any ethernet standards guy around here to defend their
> > spec ?
> >
> > -----Original Message-----
> > From: Doug Smith [mailto:doug@xxxxxxxxxx]
> > Sent: Tuesday, October 26, 2004 3:04 PM
> > To: jose_moreira@xxxxxxxxxxx
> > Cc: si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Re: Split Ground Planes
> >
> >
> > Hi Jose and the group,
> >
> > I normally reply personally to people to avoid the glut of
> > "out-of-office" messages, but feel I needed to add to this discussion.
> >
> > Signal propagation is an important but small part of the trouble you
> > can get into with split planes if not implemented properly. Even if
> > you manage to get the signal propagation right, system reliability can
> > be severely compromised as it is easy to turn every cable in the
> > system into a great receiving array to pick up impulses you can easily
> > miss in the lab, but your customers won't!
> >
> > I worry also about the common mode voltage that can result across the
> > board and drive enclosures, cables, etc. This voltage will cause
> > radiation that depends on a lot more that the board itself. Also
> > common mode voltages can be developed from outside sources as
> > mentioned above.
> >
> > There are probably thousands of modes to get in trouble with split
> > planes of which the referenced paper covers a subset. You can cover
> > many of the problems by not ever allowing a trace to cross the slit.
> > Even DC signals like LEDs are generally too noisy to allow to cross a
> > slit in many cases.
> >
> > Most of the examples of splitting up ground planes I have seen were
> > not necessary and just caused problems (remember that at high
> > frequencies everything is connected together whether you want them to
> > be or not, lots of articles on my website about this effect). There
> > are a few justified cases though. Video can be one example where the
> > signal can be degraded by just a small amount of powerline current
> > (hum bars in the video).
> >
> > In general, I would not recommend splitting planes unless you can
> > state and MEASURE (to confirm) why a split plane is needed.
> >
> > Doug
> >
> > jose_moreira@xxxxxxxxxxx wrote:
> >
> >
> >>Nikhil,
> >>
> >>you can find the article on the IEEE xplore site (if you have acess) or =
> >>in a library.
> >>
> >>Details:
> >>
> >>Signal propagation and radiation of single and differential microstrip =
> >>traces over split image planes
> >>Orhanovic, N.; Raghuram, R.; Matsui, N.;
> >>Electromagnetic Compatibility, 2000. IEEE International Symposium on =
> >>,Volume: 1 , 21-25 Aug. 2000=20
> >>Pages:339 - 343 vol.1
> >>
> >>Best Regards
> >>Jose
> >>
> >>-----Original Message-----
> >>From: si-list-bounce@xxxxxxxxxxxxx
> >>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Nagel, Michael
> >>Sent: Dienstag, 26. Oktober 2004 16:39
> >>To: si-list@xxxxxxxxxxxxx
> >>Subject: [SI-LIST] Re: Split Ground Planes
> >>
> >>
> >>Nikhil,
> >>
> >>There is a quite instructive paper with the title:
> >>
> >>Signal Propagation and Radiation of Single and Differential
> >>Microstrip Traces Over Split Image Planes
> >>
> >>written by:
> >>N. Orhanovic, R. Raghuram, N. Matsui
> >>from: Applied Simulation Technology
> >>e-mail: { neven, raghu, matsui} @apsimtechcom
> >>
> >>Unfortunately I have no direct link to the document.
> >>
> >>Generally spoken - the split plane itself may not be
> >>harmful in terms of EMI unless you start routing=20
> >>traces across the gap!
> >>
> >>Best regards,
> >>Michael
> >>
> >>
> >>-----Original Message-----
> >>From: Nikhil Patel [mailto:npatel@xxxxxxxxxxxxxxxxxxx]
> >>Sent: Dienstag, 26. Oktober 2004 15:40
> >>To: si-list@xxxxxxxxxxxxx
> >>Subject: [SI-LIST] Split Ground Planes
> >>
> >>
> >>Hi all,
> >>=20
> >>
> >>I have been doing a lot of reading on Split ground planes and am a bit
> >>confused now. Do most people use just one solid ground in the schematic =
> >>and
> >>layout for EMI issues or do you prefer split ground planes? Any input is
> >>greatly appreciated.=20
> >>
> >>=20
> >>
> >>Thanks
> >>
> >>=20
> >>
> >>Nikhil=20
> >>
> >>=20
> >>
> >>
> >>
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> >
>
>--
>-------------------------------------------------------
> ___ _ Doug Smith
> \ / ) P.O. Box 1457
> ========= Los Gatos, CA 95031-1457
> _ / \ / \ _ TEL/FAX: 408-356-4186/358-3799
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