[SI-LIST] Re: Ground clearance at connector vias
- From: Jim G Roberts <jgroberts@xxxxxxxxxx>
- To: scott@xxxxxxxxxxxxx
- Date: Thu, 27 Mar 2003 17:26:22 +0100
Hi Scott,
Much of our reseach was joint with FCI using there 10GHz test bed and
simulators.
If I understand correctly, you are indicating that the genaration of the
parallel plate-mode is due to the anti-pad shape.
My experience is that the major source of paralle-plate mode comes from bending
the
transmission path. This will occur at the backplane connector since it is
orthognal.
The best way of reducing it is to ensure that the planes are shorted as close to
the pin/via as possible.
An added problem for press-fit which we use, is that dependent on the layer in
use,
you will end up with a stub due to the length of the pin.
Mostly this can be seen as additional capacitance but it is also an antennae
which
can launch other modes.
One of the solutions is to back drill, the other is to ensure that you exit the
connector at the bottom layer minimising the length of the stub.
--
Regards, __________ James G Roberts
/___ ____ | jrobert@xxxxxxxxxxxxxxxxxxxxx
Jim __ / /___/ / jgroberts@xxxxxxxxxx
/ /_/ /---| | Room: BE436, Hilversum
\____/ /_/ Tel: +31 35 687 4308 Fax: 5976
Scott McMorrow wrote:
> Jim,
>
> I'm not exactly sure how your research was performed. However, I do
> agree that one must take into account the via spacing and the antipad
> sizing. The reason for placing an oval antipad (or two circular antipads
> connected by a rectangular region which works better in CAD systems) is
> to eliminate any parallel plate mode power injection into the plane
> cavities. Yes, it is possible to create a narrow band impedance match
> with some coupling to the planes, but at the sacrifice of impedance
> mismatch at higher frequencies. It is generally better to eliminate the
> planar power injection with larger antipads that encompass the entire
> differential via. This will reduce return loss over a very large
> frequency range. An optimization can then be performed with a 3D full
> wave field solver that uses the via spacing and antipad size as
> variables and seeks to minimize return loss. An optimized design with
> an antipad region encircling both vias can be designed to have better
> broadband return loss than an optimized design with individual antipad
> regions around each differential via. Now, for press fit connectors,
> the PTH via spacing cannot be altered, but for surface mount
> connectors, there is a great latitude of spacing that is possible.
>
> regards,
>
> scott
>
> --
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 2926 SE Yamhill St.
> Portland, OR 97214
> (503) 239-5536
> http://www.teraspeed.com
>
> Jim G Roberts wrote:
>
> >Hi Scott,
> > When you say it is "much better" are you saying that you can achieve
> > a
> >larger reduction in capacitance than with two circular.
> > Along the same lines would it not also be attained by a rectangular
> > cut
> >out which maybe easier to attain in CAD than an oval which will cost many
> >segments.
> > I would also like to add that it is only necessary to reduce the
> >parastic capacitance in line with the inductance of the pin/via to achieve
> >the
> >correct impedance. Research in Backplane conducted at Lucent with upward of
> >30
> >layers has shown this to be acheivable with standard antipads for 50 ohm (100
> >ohm balanced) but not for 75 ohm.
> >
> >--
> >Regards, __________ James G Roberts
> > /___ ____ | jrobert@xxxxxxxxxxxxxxxxxxxxx
> > Jim __ / /___/ / jgroberts@xxxxxxxxxx
> > / /_/ /---| | Room: BE436, Hilversum
> > \____/ /_/ Tel: +31 35 687 4308 Fax: 5976
> >
> >Scott McMorrow wrote:
> >
> >
> >
> >>Christian,
> >>
> >>The recommendation is for an oval antipad that encompasses both vias of
> >>a differential pair, not just one. And, yes, it is much better than two
> >>circular antipads around each via.
> >>
> >>best regards,
> >>
> >>scott
> >>
> >>--
> >>Scott McMorrow
> >>Teraspeed Consulting Group LLC
> >>2926 SE Yamhill St.
> >>Portland, OR 97214
> >>(503) 239-5536
> >>http://www.teraspeed.com
> >>
> >>Christian Schuster wrote:
> >>
> >>
> >>
> >>>Fabrizio and all the others:
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>>I have a question regarding ground clearances for signal vias in a
> >>>>backplane connector, like the Molex/Teradyne HSD. Connector and
> >>>>semiconductor manufacturers recommend using an oval ground clearance
> >>>>around the differential pairs, to minimize reflections.
> >>>>
> >>>>
> >>>>
> >>>>
> >>>I completely agree that increasing antipad sizes is one way to reduce
> >>>parasitic via capacitiance. What makes me wonder is the fact that
> >>>oval shapes are proposed. Is that a mechanical/space requirement?
> >>>
> >>>If not: has anybody ever seen a difference in via transmission between a
> >>>circular and an oval antipad of the same area?
> >>>
> >>>
> >>>Regards,
> >>>
> >>>Christian Schuster
> >>>IBM
> >>>
> >>>
> >>>
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