# RE: [SI-LIST] : the old high-frequency return current model

Dr. John L. Prince ([email protected])
Tue, 05 Oct 1999 08:49:46 -0700

At 08:05 AM 10/4/99 -0500, you wrote:
>Thanks everybody for writing back. I'm hearing some common themes about
return
>current distribution:
>
>1) Inter-plane capacitance is the first source of ac return current.
>
>2) "The shorter the rise time, the closer the via or de-cap." This
suggests to
>me that the return current distribution vs. frequency is really a
continuum from
>using the whole plane at dc to using a small swath under the trace at
microwave
>frequencies. If this adage is true, then there still must be a significant
>amount of return current at one rise time away from a signal via at most
of the
>frequencies we're interested in as signal integrity engineers.
>
>3) 2D field solvers don't assume anything about return current. I guess if I
>had thought long enough, this should have been clear. A 2D field solver
>essentially solves an electrostatic problem for capacitance, often
assuming an
>infinite plane, and then computes inductance using the telegraphers
equations.
>The magnetostatic problem is never solved. Nevertheless, they still provide
>accurate solutions from a return current perspective.
>
>Greg Edlund
>IBM
>3650 Hwy. 52 N, Dept. HDC
>Rochester, MN 55901
>[email protected]
>
>
>---------------------- Forwarded by Gregory R Edlund/Rochester/IBM on
10/04/99
>07:53 AM ---------------------------
>
>"Ingraham, Andrew" <[email protected]> on 09/30/99 02:02:40 PM
>
>To: "'[email protected] '"
> <"IMCEAMAILTO-gedlund+40us+2Eibm+2Ecom"@compaq.com>
>cc:
>Subject: RE: [SI-LIST] : the old high-frequency return current model
>
>
>
>
>Greg,
>
>Maybe the answer is that, even though the current distribution looks a lot
>different when the nearest decoupling cap is more than a few trace widths
>away, its effect on the impedance is just not that much to worry about?
>
>Kind of like the argument about using chamfered corners on trace bends.
>Yes, in theory you get an impedance discontinuity if you don't, but I think
>Ed Sayre says you'll never see it unless you operate well above 1GHz.
>
>If the cap is half an inch away from the via, that's around 80ps away. So
>unless the risetimes are of that order of magnitude or faster, then the
>discontinuity might be insignificant.
>
>But then at those kinds of speeds, our discrete capacitors are pretty much
>ineffective anyway. By that point we are relying on the intrinsic
>capacitance between layers. The higher you go in frequency, the better that
>intrinsic capacitance looks (and works).
>
>Regards,
>Andy Ingraham
>
>
>
>-----Original Message-----
>From: [email protected] [mailto:[email protected]]
>Sent: Thursday, 30 September, 1999 14:12
>To: [email protected]
>Subject: [SI-LIST] : the old high-frequency return current model
>
>
>Shoot! I was out of town and missed one of the most interesting discussions
>of
>the year! (Plane-jumping return currents) So at the risk of re-opening this
>thread, filling all your mailboxes again, and being branded an outcast, here
>goes. (Remember, that delete button is only a few inches away...)
>
>You're all familiar with this picture of high-frequency return current
>bunching
>up under the signal trace, right? According to the picture, it dies off
>pretty
>quickly as you move along the x-axis away from the trace. Well, I've been
>considering rules for the area density of ground vias and decoupling
>capacitors,
>and it occurs to me that if this picture were true, then the only place for
>a
>ground via or capacitor is within 2-3 trace widths of the signal via in
>question. (Which is, for most of our applications, absurd.) Otherwise I'd
>be
>forcing the return current out of that very tight loop, increasing the
>inductance, adding a discontinuity, generating plane noise, emissions, and
>all
>those nasty things. Now, I know that boards work quite well up to a few
>hundred
>MHz with considerably less than 100 de-caps per square inch! So where's the
>discrepancy? Is there a hole in my fairly simplistic, qualitative analysis?
>Or
>is this just like everything else: knowing how some parameter varies
>between
>the end cases is much harder than analyzing the end cases?
>
>On another tangent, I believe 2-D field solvers make the assumption that the
>return current is evenly distributed across the surface of a plane when you
>them to compute C, L and Z for a given cross-section. Doesn't this also
>conflict with the high-frequency current distribution picture?
>
>
>Greg Edlund
>IBM
>3650 Hwy. 52 N, Dept. HDC
>Rochester, MN 55901
>[email protected]
>
>
>
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>
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>
>

I don't want to be too picky, but you assume that all 2D field solvers are
electrostatic. We (and I am sure many others) have developed years ago
(1990) a 2D magnetostatic solver which we use frequently and which gives us
all sorts of interesting results including current distributions, all in a
"New York minute". Although I must admit that for very high frequencies,
where the skin depth is small compared to the dimensions of the conductors,
the New York minute stretches out to perhaps a Texas minute (as a native
(rare bird!!) I am familiar with Texas minutes....sometimes they are long,
sometimes quite short).
John L. Prince, PhD
Professor
Director,Center for Electronic Packaging Research
Department of Electrical and Computer Engineering
University of Arizona
Tucson, AZ 85721
[email protected]
520-621-6187
520-621-2999(FAX)

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