[SI-LIST] Re: cPCI backplane signal traces - characteristic impedance
- From: steve weir <weirsi@xxxxxxxxxx>
- To: "Sexton, Brian M. \(US SSA\)" <brian.m.sexton@xxxxxxxxxxxxxx>
- Date: Mon, 23 Jun 2008 12:41:46 -0700
Brian, depending on the total lengths and distribution of slots
involved, with some effort you should be able to make this work. PCI is
pretty sloppy to begin with. The key to making timing is to damp out the
reflections fast enough. One thing that can help is to stick damping
resistors in the bus near the middle. Perhaps at the far end of the
flex would be a good place to start. If it gets really ugly, resistors
on either end of the flex should do it. Run this through simulation and
you should be able to zero in on a reliable solution.
Steve.
Sexton, Brian M. (US SSA) wrote:
> In reading the PCI specification for 66MHz cPCI backplanes, it states
> one should use 65 ohms for the bussed signals. I believe that COTS
> backplanes use a recipe like this with traditional strip-line
> construction. It also states the 66MHz backplanes should be 5 slots or
> less.
> =20
> We need to design a 5-slot cPCI rigid-flex-rigid backplane which also
> includes routing for lots of 100 ohm differential I/O pairs. It is only
> the I/O that crosses the flex. The cPCI section is the traditional 0.8"
> pitch using the normal hard metric connectors in one of the rigid
> sections. =20
> =20
> My problem is that it takes (4) double sided flex layers for all the I/O
> signals that cross between the hard boards. At 5 mils dielectric per
> flex, it is almost impossible to make the 65 ohms on those layers. If I
> use a traditional 65 ohm stripline for the PCI signals, and only use the
> flex layers for the I/O, my backplane will be greater than 0.200" thick.
>
> =20
> My question is how well a 66HMz cPCI would work running the signals at
> 50 ohms. Does anyone have any experience with this? I'm not sure how
> much margin is in the 65 recipe in the spec, and I don't have good
> models for the connectors and such to trust a simulation.
> =20
> My only other idea was to stackup the PCI area differently than the rest
> of the backplane and essentially make them asymmetrical striplines using
> a plane-void-signal-plane, where the I/O would be more of a plane-signal
> plane stackup. No signals would need to cross a stack up change.
> However, some people have been worried about the manufacturing yield of
> doing it.
>
> Thanks for your help.
> Brian
>
> ------------------------------------------------------------------
> 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 technical documents are available at:
> http://www.si-list.net
>
> 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
>
>
>
>
--
Steve Weir
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
California office
(866) 675-4630 Business
(707) 780-1951 Fax
Main office
(401) 284-1827 Business
(401) 284-1840 Fax
Oregon office
(503) 430-1065 Business
(503) 430-1285 Fax
http://www.teraspeed.com
This e-mail contains proprietary and confidential intellectual property of
Teraspeed Consulting Group LLC
------------------------------------------------------------------------------------------------------
Teraspeed(R) is the registered service mark of Teraspeed Consulting Group LLC
------------------------------------------------------------------
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 technical documents are available at:
http://www.si-list.net
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:
