Re: [SI-LIST] : Via Capacitances ...

Ron Miller (rmiller@Brocade.COM)
Fri, 09 Jul 1999 13:43:54 -0700

I take issue with My Sayres statement that "vias for whatever
reason turn out to be capacitive."

This infers that the primary effect is capacitive when in fact
any line has both capacitance and inductance.

My point is that the capacitance as a via traverses is very small
compared to the series inductance.
via
_
- - - - - - - - - - - | | - - - - - top ground
----------trace------| |
_ _____________________ | | _________ pwr/gnd
_______________________ | | _________ pwr/gnd
| |------ trace
_______________________ | | __________ pwr/gnd
| |
The capacitance as the traces traverse the board from the
top trace to the bottom trace is very small and this section
looks inductive.

The stubs top and bottom look capacitive due to the reflections.

So, keep the stubs short by putting the traces near the top and
bottom surfaces.

Ron Miller, no title needed.

"Dr. Edward P. Sayre" wrote:

> Folks:
>
> It may not be convenient and may stress your personnel or financial
> assests, but this is why you make SI measurements to confirm your results.
> Almost anybody who has made the measurement knows that vias for whatever
> reason turn out to be capacitive. If one checks with Dr. Johnson's book,
> you will find that vias are modeled by both capacitors and inductorance.
> By both measurement and SPICE simulation, NESA has shown this to be true.
> (See papers on our web site.)
>
> (The next sentence should be read in the context that I was one of the
> original students involved in the development of the Method of Moments
> field simulators and know a lot about old and new simulation software.)
> There is ample evidence to show that many field simulator derived SPICE
> models (especially those from 2-D simulators) are often unreliable with
> respect to the component values but usually have the correct topology.
> That is to say, if you make a measurement and then adjust the SPICE
> component values and re-simulating one or twice, you can get satisfactory
> comparisons with the measurements.
>
> Otherwise, there is no way to verify the correctness of field solver
> derived SPICE models for interconnect physical features like vias or
> connectors.
>
> ed sayre
> ============
> At 09:50 AM 7/9/99 -0700, you wrote:
> >Doug
> >
> >I modeled a via some time back with ground planes and
> >clearance holes through it. I had intended to use the capacitance
> >to make up for an inductance and needed 300 ff.
> >
> >I used an OEA field solver Metal and to my amazement there was
> >hardly any capacitance. I surmised that this was due to the small
> >area of the ground planes in tha horizontal direction. Since I needed
> >capacitance I added some big fat ground vias all around the signal
> >via, and got the 300 ff.
> >
> >Also, HP ADS which is the cadillac of RF simulators models vias
> >as inductor, as does Touchstone before EEsof got swallowed up by
> >HP.
> >
> >Ron
> >
> >Douglas McKean wrote:
> >
> >> Could someone give rough estimate
> >> of via capacitance?
> >>
> >> I'm thinking the orientation of the traces
> >> connected to the via have a major impact.
> >>
> >> For instance, an .062 board with a via
> >> from one side to the other, different
> >> capacitances would be had with the two
> >> following constructions ...
> >>
> >> I. Top Trace
> >> -------------
> >> |via
> >> -------------
> >> Bottom Trace
> >>
> >> II. Top Trace
> >> -------------
> >> |via
> >> -------------
> >> Bottom Trace
> >>
> >> Ideas? Comments?
> >>
> >> Regards, Doug McKean
> >>
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> >
> >--
> >Ronald B. Miller _\\|//_ Signal Integrity Engineer
> >(408)487-8017 (' 0-0 ') fax(408)487-8017
> > ==========0000-(_)0000===========
> >Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA 95131
> >rmiller@brocade.com, rbmiller@sjm.infi.net
> >
> >
> >
> >
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> >
>
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
> | NORTH EAST SYSTEMS ASSOCIATES, INC. |
> | ------------------------------------- |
> | "High Performance Engineering & Design" |
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
> | Dr. Ed Sayre e-mail: esayre@nesa.com |
> | NESA, Inc. http://www.nesa.com/ |
> | 636 Great Road Tel +1.978.897-8787 |
> | Stow, MA 01775 USA Fax +1.978.897-5359 |
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
>
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--
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
     ==========0000-(_)0000===========
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
rmiller@brocade.com,  rbmiller@sjm.infi.net

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