[SI-LIST] Re: return current
- From: Paul Levin <levinpa@xxxxxxxxxxxxx>
- To: steve@xxxxxxxxxxxxxx
- Date: Wed, 10 Oct 2001 09:03:11 -0700
Dear Steve,
You are absolutely right; it was a bit late when I wrote last night's reply.
I meant frequency-dependent R's.
Sorry for any confusion.
Paul Levin
_______________________
Steve Corey wrote:
> Paul -- Is it safe to assume in your third paragraph below that you
> meant "frequency-dependent R's"? Voltage dependent resistance is
> nonlinear, e.g. diodes. Transmission lines may be nontrivial to model,
> but fortunately they are strongly linear in the GHz frequencies.
>
> Paul Levin wrote:
>
> > Dear Goutham,
> >
> > Inductance will force the high frequency components of the return
> > current to be shared equally by the two planes. The decoupling
> > capacitance in the vicinity of the driver *should* sort out the return
> > current and bring it to the correct voltage plane. Plane thickness is
> > not an issue: At these high frequencies, the skin depth is smaller than
> > any common copper thickness.
> >
> > The low frequency components will flow as one would expect from
> > voltage and load resistance.
> >
> > Intermediate frequency components will be somewhere in between.
> > The transition occurs in the region where the inductive reactance of
> > the transmission line overtakes the resistance (bulk or skin as appro-
> > priate) of the transmission line. If you feel up to some fun math, you
> > could model this as a four-winding transformer (two windings for the
> > trace and one for each shield) in series with voltage-dependent R's.
> >
> > Paul Levin
> > Senior Principal Engineer
> > Logic Innovations
> > _____________________________
> >
> > Goutham.S@xxxxxxxxxx wrote:
> >
> >
> >>Hi,
> >>
> >>In a stripline environment, with the signal trace equidistant from a ground
> >>plane and a power plane, what will the return current path be for a H-L and
> >>L-H
> >>transition be? Also what percentage of the return current will flow thro
> >>the
> >>ground and power planes, assuming the return current will be ditributed
> >>between
> >>the two ref planes. Also the two ref planes are assumed to have equal
> >>thickness.
> >>
> >>Any answers or comments........
> >>
> >>Regards
> >>Goutham Sabavat
> >>Force Computers india Pvt Ltd
> >>
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> --
> -------------------------------------------
> Steven D. Corey, Ph.D.
> Time Domain Analysis Systems, Inc.
> "The Interconnect Modeling Company."
> http://www.tdasystems.com
>
> email: steve@xxxxxxxxxxxxxx
> phone: (503) 246-2272
> fax: (503) 246-2282
> -------------------------------------------
>
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