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

Eric Bogatin (eric@bogent.com)
Mon, 4 Oct 1999 11:31:33 -0500

Chris-

I don't have any first hand experience with the ApSIM RLCG tool. It's pretty
popular, and the folks at ApSim are sharp folks, so I would have confidence
in their solver. Perhaps Fred or Raj could offer some comments about their
tool?

--eric

Eric Bogatin
BOGATIN ENTERPRISES
Training for Signal Integrity and Interconnect Design
26235 W. 110th Terr.
Olathe, KS 66061
v: 913-393-1305
f: 913-393-1306
pager: 888-775-1138
e: eric@bogent.com
web: www.bogatinenterprises.com

> -----Original Message-----
> From: owner-si-list@silab.eng.sun.com
> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Chris Heard
> Sent: Monday, October 04, 1999 10:17 AM
> To: si-list@silab.eng.sun.com
> Subject: RE: [SI-LIST] : the old high-frequency return current model
>
>
> Eric, can you comment on the accuracy of Apsim RLGC?
>
> Chris
>
> -----Original Message-----
> From: owner-si-list@silab.eng.sun.com
> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Eric Bogatin
> Sent: Monday, October 04, 1999 10:54 AM
> To: si-list@silab.eng.sun.com
> Subject: RE: [SI-LIST] : the old high-frequency return current model
>
>
> Greg-
>
> A quick response about your comment on 2D field solvers-
>
> You are correct in that some 2D field solvers just solve the electrostatic
> equation (LaPlace's eq), and get the capacitance matrix. From this matrix,
> and assuming that the transmission mode is TEM, they will use the
> fact that
> the velocity of propagation is related to the effective
> dielectric constant
> and the speed of light to get the inductance matrix. So, for
> these solvers,
> the assumption is the current is only surface current and it is
> effectively
> skin depth limited. The return currents do what ever they need to achieve
> the TEM mode. The boundary element field solver from Hyperlynx,
> for example,
> uses this approach.
>
> Other field solvers, such as the Maxwell 2D Extractor, from Ansoft, will
> solve LaPlace's equation to get the capacitance matrix elements, and
> separately, Ampere's equation, to get the high frequency current
> distribution, and then the magnetic fields, and then the inductance matrix
> elements. The boundary condition it uses is that at high frequency, skin
> depth is so small, there is no field inside the conductor and it behaves
> like a perfect conductor, so the B field is only tangential at
> the surface,
> no normal component. I have compared the two results- Hyperlynx's and
> Ansoft's tool- and the agreement is within 2% across a wide range
> of aspect
> ratios.
>
> In addition, the Ansoft tool, for example, will separately, if requested,
> solve the Helmholtz equation (frequency domain solution for J and B) and
> extract the current distribution inside the conductor (using FEM), at a
> specified frequency. This will allow you to see the actual current
> distribution inside the conductor, as frequency is varied. The
> plots I sent
> around a few weeks ago were created using this tool. That's why the Ansoft
> tool is often referred to as the "Rolls Royce of field solvers".
>
> As with most things in life, not all 2D field solvers are created
> equal. Not
> only that, but in some "extraction tools" on the market, they aren't even
> field solvers, they are analytic approximations, with a very
> limited useful
> range, but the vendors don't tell you that. They rely on the
> notion that if
> you see three digits showing on the screen you will think the
> answer must be
> accurate to three digits! In general, its caveat emptor- and I think we as
> consumers of these tools, should be demanding of our vendors for
> full, open
> and easily obtained disclosure of what is the engine behind the
> curtain, and
> how have they confirmed the accuracy of their tool. This is one of the
> topics I will be covering in my next two "No Myths Allowed" columns on
> ChipCenter.
>
> --eric
>
>
>
> Eric Bogatin
> BOGATIN ENTERPRISES
> Training for Signal Integrity and Interconnect Design
> 26235 W. 110th Terr.
> Olathe, KS 66061
> v: 913-393-1305
> f: 913-393-1306
> pager: 888-775-1138
> e: eric@bogent.com
> web: www.bogatinenterprises.com
>
>
>
>
> > -----Original Message-----
> > From: owner-si-list@silab.eng.sun.com
> > [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of gedlund@us.ibm.com
> > Sent: Monday, October 04, 1999 8:06 AM
> > To: si-list@silab.eng.sun.com
> > Subject: RE: [SI-LIST] : the old high-frequency return current model
> >
> >
> > 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
> > Advisory Engineer, Critical Net Analysis
> > IBM
> > 3650 Hwy. 52 N, Dept. HDC
> > Rochester, MN 55901
> > gedlund@us.ibm.com
> >
> >
> > ---------------------- Forwarded by Gregory R
> > Edlund/Rochester/IBM on 10/04/99
> > 07:53 AM ---------------------------
> >
> > "Ingraham, Andrew" <Andrew.Ingraham@compaq.com> on 09/30/99 02:02:40 PM
> >
> > To: "'gedlund@us.ibm.com '"
> > <"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: gedlund@us.ibm.com [mailto:gedlund@us.ibm.com]
> > Sent: Thursday, 30 September, 1999 14:12
> > To: si-list@silab.eng.sun.com
> > 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
> > ask
> > them to compute C, L and Z for a given cross-section. Doesn't this also
> > conflict with the high-frequency current distribution picture?
> >
> > Eagerly awaiting your answers and hoping I have time to read them,
> >
> > Greg Edlund
> > Advisory Engineer, Critical Net Analysis
> > IBM
> > 3650 Hwy. 52 N, Dept. HDC
> > Rochester, MN 55901
> > gedlund@us.ibm.com
> >
> >
> >
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