FreeLists / si-list / [SI-LIST] Re: return current

[Steve Corey <steve@xxxxxxxxxxxxxx>]

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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