I agree that when the via as not in the signal path it may look capacitive.
The reason it looks capacitive is that it is an open stub. There is a
reflection at the end of the line whenever you have an open stub
and it look capacitive. It should be modeled as an open stub.
Ron miller
Mike Degerstrom wrote:
> Ron makes a good point in that vias are more inductive than capacitive.
> I would suppose that if you had a very weak and/or slow driver one
> could treat the vias as lumped capacitance. Other examples where
> vias could be treated as capacitors is when they are used for test
> points and do not carry any real current. We studied many MCM
> technologies a few years ago and the vias were almost exclusively
> capacitive when hit with a 20ps risetime TDR. Even a MCM-L
> technology had capacitive vias and today's advanced pcb board
> are not that much different from a four year old MCM-L process.
> So I think that one would have to consider the technology and that
> vias within the same technology can different capacitive/inductive
> ratios depending on how many layers the signal tranversed.
>
> Mike
>
> On Jul 9, 9:50am, Ron Miller wrote:
> > Subject: Re: [SI-LIST] : Via Capacitances ...
> > 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
> > >
> > > **** To unsubscribe from si-list: send e-mail to
> [email protected]. In the BODY of message put: UNSUBSCRIBE si-list,
> for more help, put HELP. si-list archives are accessible at
> http://www.qsl.net/wb6tpu/si-list ****
> >
> > --
> > 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
> > [email protected], [email protected]
> >
> >
> >
> >
> > **** To unsubscribe from si-list: send e-mail to [email protected].
> In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP.
> si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****
> >
> >-- End of excerpt from Ron Miller
>
> --
> _______________________________________________________________
> Mike Degerstrom Email: [email protected]
> Mayo Clinic
> 200 1st Street SW
> Gugg. Bldg. RM 1042A Phone: (507) 284-3292
> Rochester, MN 55905 FAX: (507) 284-9171
> WWW: http://www.mayo.edu/sppdg/sppdg_home_page.html
> _______________________________________________________________
>
> **** To unsubscribe from si-list: send e-mail to [email protected]. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****
--
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
[email protected], [email protected]
**** To unsubscribe from si-list: send e-mail to [email protected]. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****