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

[Neeraj Pendse <cnepsc@xxxxxxxxxxxxxxx>]

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Sorry for spamming, folks ... I just realized my mailer is against me
today.
Arrrrghhh! Netscape !!
Thanks Gerald for pointing out.
Re-transmission follows:
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Paul,

I do not understand what you are trying to say at all.  I thought that
the driver, left to itself, will force the return current through one
plane. The purpose of the de-cap is to make it return on both the
planes, so that the thing looks like a stripline and you get all
advantages of it ... or for a microstrip to be able to carry both HL and
LH edges effectively with low loop inductance (in case of a S-P-P set
up)

Simply speaking, without a decoupling capacitor, won't we have one of
the  planes "out" of the loop path, because the CMOS driver will be
"off" for that loop?

Could you please elaborate?

- Neeraj.



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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-- 
g i g a b i t s @ m i l l i w a t t s
National Semiconductor
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