From: Raymond Y. Chen (email@example.com)
Date: Fri Aug 04 2000 - 10:59:49 PDT
I agree with you on most of the things that you pointed out in your email.
But there is one thing that either you don't know about it or you don't want
to mention it, which is: there is a commercial tool that can effectively and
promptly solve very-complex packaging structures involving power delivery
system and signal delivery system, and the interactions between them, hence,
the return path.
You are right, modeling multi-layer power/ground plane structures with tens
of thousands of power/ground vias is a very challenging job because of its
distributive-circuit nature. That is why the traditional 2-step approach on
power/ground modeling is not realistic on real world problems.
Step 1, use EM field solvers to extract a circuit network to represent the
power delivery system;
Step 2, then combine the power/ground model with the driver/receiver into a
SPICE stack, and ask SPICE type of circuit solve to do the simulation.
As you know, even if the EM extraction can handle the complexity, the
extraction itself takes 0 second, and the extracted circuit model is
accurate enough to represent all the wave-propagation phenomena inside the
multi-layer structure, we still can not finish step 2 due to the size of the
circuit matrix need to be inversed.
So is there a good approach to model/simulate the power/ground return path,
as you wondered? I believe, technically, there is a very good way to do
that - a hybrid methodology that uses EM field solver to calculate the EM
fields inside the packaging structure, while uses circuit solvers to compute
the voltages and currents in the non-linear/linear circuit components, and
the field solver and circuit solvers are linked together and run
simultaneously, hence changing the traditional 2-step approach to a 1-step
What is the benefit of this approach on power/ground/signal integrity
analysis? Check out the demo of our software tool SPEED97 at,
http://www.sigrity.com/download/demo/spd_demo.exe, the very first example
provided in the user's manual, is the exact structure that Cosmin asked XTK
and SPECCTRAQuest to solve. You will find, Todd, the simulation time of our
tool in this case is only a few second (not hours). And you can also see the
animated waveform of power/ground noise propagation, return path
discontinuity, and signal transmission, while the simulation is running.
BTW, the SPEED2000 demo version will be available for download in 2-weeks.
Raymond Y. Chen
Vice President, Products and Services
* Sigrity, Inc. Tel: 408.377.2180
* 2105 Hamilton Ave. Suite 310 Fax: 408.377.2565
* Santa Clara, CA 95125 Web: www.sigrity.com
>[mailto:firstname.lastname@example.org]On Behalf Of Todd Westerhoff
>Sent: Friday, August 04, 2000 9:02 AM
>Subject: Re: [SI-LIST] : Via model
>You've got a good question, and a valid concern, but the answer is
>long and involved. Let me see if I can give the short answer coherently.
>The return path - the thing you're ultimately concerned about - is
>a function of a lot more than just the via model. As a matter of
>fact. although the via model certainly has something to do with
>it, the real issue is the presence or absence of high-frequency
>decoupling between the signal's reference planes in the vicinity
>of the via. The via model itself could not predict this, and
>thus, wouldn't give you what you're after.
>Analyzing return paths in the time domain requires a tool that
>properly models the power supply, decoupling capacitors and power
>planes, in addition to the trace being studied. That requires
>that the planes be modeled as a mesh with their associated
>non-ideal characteristics, such that the localized effects of
>power demand (i.e. switching current), decoupling and the
>transition of a signal between reference planes can be
>That also means that the modeling / analysis time would jump from
>seconds/minutes to hours/days, depending on the complexity of the
>model. As far as I know, there are no current commercial
>analytical solutions to the problem you propose - and that's not
>for lack of desire. It's because of the complexity of the problem.
>The biggest question - and the biggest challenge - with simulation
>tools is understanding the limits of their accuracy, and in that
>light, the correct way to apply them to a problem, and the
>conclusions that can/cannot be drawn from the results. Most
>commercial SI tools make the assumption that reference planes
>behave ideally, and that trace behavior can be predicted from the
>2-D cross-sectional characteristics of the trace and its distance
>from its respective reference planes. That assumption, and a few
>others, makes it possible to simplify the modeling problem to the
>point where circuit extraction and simulation can take place as an
>interactive process - point to a trace, and get a result.
>If you want to get a more accurate answer, you can, but the cost
>is a substantial increase in the complexity of the circuit
>extraction, modeling and analysis processes. Ask anyone who's
>using a 3-D field solver in conjunction with SPICE what their
>process is, and how long it takes to generate a model and run
>analyses. It ain't seconds, and it ain't minutes.
>So - the real question is - which approach do you use, and when?
>I would argue that you're best off using the interactive tools to
>study the problem and hone-in on a potential solution to a
>problem, keeping in mind the limitations of the tools, their
>applicability to the problem at hand, and the likely amount of
>error you expect in the analysis. Once you're close, you can use
>a more detailed, and lengthy analysis process to close in on the
>ultimate answer, should you need to do so.
>In the particular case of return path analysis, I'm not sure that
>you'll find any commercial that satisfy your need. The
>alternative is to use an expert-based rules system to examine the
>design, checking against a known set of design rules. This isn't
>an analytical approach, but it serves to automatically and
>highlight areas that deserve thought (and possibly) detailed
>analysis using some other method. A common example is the case
>where a trace runs across a split plane with no localized
>decoupling (between the planes). The additional loop inductance
>caused by the discontinuity in the return path degrades the
>signal's edges and frequently causes the circuit to fail. The
>case you propose is similar - the transition between layers
>changes the signal's reference plane, and will have the same
>effect - increasing the loop inductance. There are commercial
>tools available that deliver the expert-based rules checking
>capability, which (at least) provide a fast way to highlight
>problem areas, even if they don't analytically predict the effect
>on the circuit.
> Anyway, that's my $0.02. Hope you found it useful.
>At 10:02 AM 8/3/2000 -0700, Cosmin Iorga wrote:
>>Does anybody know a field solver that can extract and simulate
>>a via model from a board database? The via model should include
>>both signal and return path if the transmission line changes the
>>reference plane. The following picture tries to illustrate it:
>>|______________________________| <-- trace 1 (top layer)
>> ___________________________ || __________________________________
>>|___________________________ || ________________ground plane 1____|
>> || ||
>> signal via --> || || <-- ground via (return path)
>> || ||
>> ___________________________ || ___||_____________________________
>>|___________________________ || ________________ground plane 2____|
>> trace 2 (bottom layer)--> |___________________________________|
>> The signal propagates through trace 1, which is a microstrip
>> line referenced to ground plane 1, then through the via and
>> trace 2, which is a microstrip line referenced to ground plane 2.
>> The return path for the current consists of ground plane 2,
>> ground via, and ground plane 1. The transition from the top
>> to the bottom layer will look more inductive if the distance
>> between the signal and ground vias increases. So far I've tried
>> XTK and SPECCTRAQuest but none of them could extract correctly
>> this via model.
>> Best Regards,
>> Cosmin Iorga
>> (818) 874-7149
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> Todd Westerhoff
> Product Marketing Director | High Speed Systems Design |
> Cadence Design Systems | 270 Billerica Road | Chelmsford, MA 01824
> ph: (978) 262-6327
> fx: (978) 446-6798
> email: firstname.lastname@example.org
> internal information website: http://www-ma.cadence.com/~toddw
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