Subject: Re[2]: connector models

From: [email-masked]
Date: Mon, 15 Dec 1997 06:44:04 -0330

Greetings All...
     
     Just for clarification...
     
     The connector companies main issue with IBIS is the lack of coupled 
     line modeling.
     
     The connector companies suggested the use of a Maxwell matrix for the 
     connector models a couple of years ago.  I think a partial option has 
     been installed in the V.3.0 IBIS specification.
     
     As far as "first pass"....  I beleive that IBIS in V.2.0 can support 
     series L, parallel C, and series R.  This is the simplest solution 
     with the most probablility of inaccurate results.

     ~~~~~~~
     Now I think the other significant problem is going to become 
     apparent...  The fact the IBIS simulators have different methods of 
     "circuit solving".  When a connector model is provided in matrix 
     format, the results will depend on the simulators solution algorithms 
     (not an issue with Berkeley SPICE).  
     
     I am concerned that connector models will perform differently on 
     different simulators. Is this a valid concern??
     ~~~~~~~~

     A part of the problem might be... (besides the above concern)
     
     * Semiconductor manufactures need a way to provide models that do not 
     provide insighth to proprietary fabrication methods.  Connector 
     companies already have this... SPICE.
     
     *  Some SPICE simulators can accept some versions of IBIS models some 
     can not support IBIS at all.  Are there any IBIS simulators that 
     support SPICE?  In this case, it all depends on were the "energy" is 
     assigned.  Most connector companies do not sell simulation software.  
     As such connector companies might not be the best source for driving 
     this issue.

     Gus Panella
     Molex, Incorporated.

______________________________ Reply Separator _________________________________
Subject: Re: connector models
Author:  Kellee Crisafulli <[email-masked]> at INTERNET
Date:    97/12/12 11:08 AM

Hi John,
     
At 09:28 AM 12/12/97 +0100, you wrote:
>Are the following statements reasonably truthful? 
>   - Many attempts have been made to adapt IBIS to 
>     the creation of connector and cable models. No
>     agreed synthax has been found, due mainly to the 
>     complex nature of coupling. 
The problem is just the lack of energy. 
We were very close to agreement at last years
Design Super Con.  The major problem I saw is that the participants 
from the connector companies have not provide enough energy to
get resolution.  If one or more connector companies put in even a 
small fraction of the energy that an Intel or National Semi puts into 
IBIS this would have been completed long ago.
     
>   - Limited connector and cable models are possible with 
>     IBIS 3.0, using the expanded package model and the new 
>     series element.
yes
     
>   - There is no medium-term prospect of a better IBIS synthax 
I would agree there is no short term prospect.  Medium term is 
always possible just needs energy from a few good people.
Long term is highly probable
     
>   - Most simulator companies have developed their own 
>     connector model synthax.
Or they use one of the formats developed by a connector company.  However 
there is alot to be desired in any format I am currently aware of.  This is 
one thing that has made it so difficult to accomplish an IBIS standard for 
connector
modeling.  It is easy to agreement on a simple standard for connector models. 
It was
difficult to get acceptance of that being good enough for a first pass.  So 
rather than
go forward with something basic we got nothing!
     
Summary:
We let those that must have everything or some special interest prevent the 
creation
of simple starting point.  This could have been overcome with a small push from 
one or two people after last years Design Super Con.  Anyone with some energy to
provide could make the difference on this one.  It will take a few months to 
revive it
though.

     
------------------------------------------------------------------------- 
Have a great day...
Kellee Crisafulli at HyperLynx
[email-masked]    http://www.hyperlynx.com 
-------------------------------------------------------------------------

---

From: Eric Bogatin <[email-masked]>
Date: Mon, 15 Dec 1997 10:12:54 -0600

Please forgive my ignorance if I repeat what has already been said, as I  only periodically follow the IBIS forum.

I wanted to add my vote of support to what Gus Panella said about connector models. I've caught parts of threads from previous discussions about how to model a connector in such a way that the model can be used by SPICE for signal integrity analysis of such effects as reflection noise, cross talk and ground bounce. Then there's the question of how do you model it so that you can do arbitrary assignment of ground connections after the fact. The last issue is how to take this model and integrate it in a formalism that an IBIS simulation can use.

I think the modeling of connectors, as well as leaded IC packages, and most other physically small structures with multiple leads, is straight forward and unambiguous. The equivalent circuit model includes the series elements, R, L and C to ground for each lead. In addition, there are the coupling terms between every pair of leads. There is the capacitance between each pair of leads and the mutual inductance between each pair of leads. When you have more than two leads, it gets complicated just keeping track of all the terms, so it is convenient to use a matrix formalism to describe the different terms. However, keep in mind that the matrices are just a short hand to track the terms that exist in the equivalent circuit model.

With this circuit model, SPICE can model all the signal integrity effects. However, there is a limitation to the bandwidth of such a model. The shortest rise time that can accurately be simulated with this lumped circuit model is roughly reached when the time of flight through the connector is greater than about 1/3 the rise time. For a 150 psec time delay connector, this lumped circuit model is accurate for rise times greater than 450 psec, for example.  Otherwise, absolutely all the electrical effects (except EMI) of this connector can be described by this circuit model. Why don't we adopt this circuit model as the simplest starting place?

The concept of coupling capacitance is right out of SPICE. If you can draw the equivalent circuit, each of the capacitances you draw has an unambiguous value in SPICE. You just have to use engineering experience to extract these SPICE values from either a measurement or a field solver calculation.  

Some of the confusion arises because field solvers also report results in terms of capacitance matrices. However, the field solver definition of capacitance matrix is not the same definition of the matrix as used by SPICE. As long as you use the term 'SPICE' matrix or 'Maxwell' matrix (named for James Clerk maxwell, not an Ansoft term), there is no ambiguity. Most field solver tools will automatically convert between the two forms. I don't think there is any ambiguity in either of these uses of a capacitance matrix. Either one has a complete electrical description of the coupling capacitance between a collection of leads.

The inductance terms are a bit more confusing. Some folks use the term loop inductance or effective inductance to describe the inductance of a lead or pair of leads. There are unambiguous definitions for each of these terms, though they are not often used correctly.  What is confusing is that SPICE does not use either term. The SPICE definition of inductance is based on the concept of partial inductance. There is a lot of confusion and debate in the industry about this concept, but the fact remains that it is in popular use and works just fine.

There are two kinds of inductances, partial self inductance and partial mutual inductance. If you have ever used the terms self or mutual inductance, you were really referring to partial inductances whether you knew it or not. When you draw a circuit diagram with inductances in it, and then write a SPICE deck for it, you are really using partial inductances. In a collection of leads, each lead has a partial self inductance and between every pair of leads there is a partial mutual inductance. Again, to keep track of them, it is useful to use a matrix. The diagonal elements are the self inductances, the off diagonal terms are the mutual inductances.

The output of some field solvers is the same partial inductance matrix as is used with SPICE. There is no ambiguity about a circuit diagram and a partial inductance matrix. We have just gotten in the habit of calling it simply an inductance matrix, so it gets confused with loop and effective inductance. The value of this matrix is that you can now decompose any loop, which is what is measured, in terms of combinations of self and mutual inductances. 

The matrix values are purely a geometrical effect. The actual currents in the leads does not affect the value of the inductance matrix elements. Once you have the matrix, you can construct the equivalent circuit diagram and calculate any loop that might be measured. It is more difficult to go the other way- from measurements, extract the self and mutual inductances. It is possible, but requires some tricks.

This means that if you have the (partial) inductance matrix for a connector, you can use it directly, with the circuit topology, in a SPICE situation and assign return paths after the fact in the circuit diagram. This model will allow the accurate calculation of ground bounce, as the return path is changed in the circuit diagram.

My apologies if I have gone on too long on a topic that is obvious to some of you. If you would like more information, Gus and I wrote a paper on using partial inductance to model connectors that was in the 30th Annual Connector and Interconnection Symposium, 1997, Anaheim, CA. Contact me off line and I can get you a copy.

--eric

>Author:  Kellee Crisafulli <<[email-masked]> at INTERNET

**********************************************************************

Eric Bogatin

Product Manager, Signal Integrity Products

Ansoft Corporation

26235 W. 110th Terr.

Olathe, KS  66061

voice:  913-393-1305

fax:      913-393-1306

email: [email-masked]

pager: 888-775-1138

web site: <color><param>0000,0000,ffff</param>www.ansoft.com 

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"In God we trust, all others, show your data"

**********************************************************************

---

From: Kellee Crisafulli <[email-masked]>
Date: Mon, 15 Dec 1997 09:49:01 -0800

Hi Gus,IBIS

>     been installed in the V.3.0 IBIS specification.
Close but it is not used and it isn't set up for connectors.

>     with the most probablility of inaccurate results.
It doesn't work for connectors.  Connectors need an input and an output
defined.
The pin definitions in IBIS 2.0 don't provide that.  The Version 3.0 sytax is
much closer with the additions that Steven Peters made.

>     (not an issue with Berkeley SPICE).  
Not true.
It is the same issue as for Spice, the problem is a standard.  Just
a coupling matrix is not a connector model.  It could be; but just as it could
be in Spice the format must be agree upon and then the simulators all work.

>     different simulators. Is this a valid concern??
Yes, but it should not be a problem if the format is well defined.

>     companies already have this... SPICE.
Perhaps but it is not in any sort of standard format.  Each connector company
uses different syntax.  Also not all Spice packages support all connector
formats.  Also most connector companies do not provide their models for open
download over the web.  It requires an NDA or even extra fees.

> Some SPICE simulators can accept some versions of IBIS models some 
> assigned.
IBIS is not a simulator.  IBIS is a description format, as has been
pointed out by several people on this forum.
Many Spice packages support IBIS.  All SI simulators I know
of support IBIS.  Spice syntax has evolved in 30 separate directions over
the years.  It is now to the point that it is very difficult to exchange Spice
files between two different types of Spice simulators and have it work.
With IBIS we are attempting to maintain a standard syntax that all simulators
can use including Spice.

> Most connector companies do not sell simulation software.  
This I believe is the real issue, I feel the connector company's
participation is critical.  As I work at a simulator company I
would very much like to see all the connector companies participate
in developing a single standard that all companies could use.  It
is certainly not required that the connector company have a simulator.
In fact I think it is a big disadvantage because then they become
biased toward their simulator.

Perhaps we need a multi-day focused session on connector modeling to
hammer out a syntax.  I would very much like to see all the connector
company's represented at the Jan. Design Con IBIS meeting and
start to hammer out a solution.  Perhaps we do a Phase 1, Phase 2, Phase 3.

---

From: [email-masked]
Date: Mon, 15 Dec 1997 14:44:41 -0330

Greetings...

     First Kellee,  Thanks for the insight into what V2 and V3 support (as 
     far as connectors are concerned).  It has been my experience that some 
     of our customers beleive that IBIS can be used for even the most basic 
     connector models.  Please correct me if I misinterpretted the V2 and 
     V3 statements.

     In regards to Connector model format..
     
     *  All SPICE simulators I know of can read Berkeley SPICE format 
     lists.  I think there is a similiar goal in the IBIS committee / 
     specification...   that all IBIS simulators can read all IBIS format 
     models.  For this reason, we have taken a position as a company to 
     only support a Berkeley compatible format.  From your comments, I've 
     learned that this is not the case for all connector manufacturers.  
     Per comments below, is there a SPICE simulator that can not accept 
     Berkeley SPICE that is known to the IBIS group?

     The following might be seen as an agressive statement, that is not my 
     intent.  Is it reasonable to suggest that the IBIS specification  
     support standard Berkely SPICE formats for 3e1.  I do not know the 
     extended impact of this suggestion on the rest of the IBIS world. I 
     apologize in advance for stepping on anyone's toes.  But, I think the 
     connector companies might find this an acceptable standard format.  It 
     gets around the matrix issue and model confirmation issues (i.e. a 
     model only needs to be confirmed to empiricals in Berkely SPICE, not 
     in every SI simulator in the market)
     
     Comments???

     * SI simulators...  What is the definition of a SI simulator.  I don't 
     think Berkely SPICE supports the IBIS format and it may be considered 
     a SI simulator.  I don't have an answer here....  But I would like to 
     refine a common definition.
     Comments?

     In regards to providing open downloads over the web...
     **  Model complexity is a big issue here.  Many times end users need 
     assistance in dealing with a connector SPICE model.  As such, support 
     is generally needed.  We distribute SPICE models on a one on one basis 
     in order to assure that we are able to provide effective support
     **  In regards to NDA's...  Yes, they do exist...   mainly it is to 
     provide protection for the newer designs.  Remember,  in a connector 
     companies case...  a signal integrity solution is a portion of our 
     product, as such it is something that we (connector companies) would 
     like to generally keep from our competition.

     As far as..  Meeting to hammering out a syntax...
        I would like to invite the IBIS group to evaluate the use of SPICE 
     component syntax as outlined in SPICE 3e1  Users manual, April 1, 
     1991, for support in the IBIS syntax.
     
     Is this an acceptable addition to the IBIS syntax?
     If not.... why?  
     
     This way we (as an IBIS group) might document a discussion matrix 
     before we start in a meeting.  Comments??

     Gus Panella
     Molex, Inc.
     PH:630-527-4617

______________________________ Reply Separator _________________________________
Subject: Re: Re[2]: connector models
Author:  Kellee Crisafulli <[email-masked]> at INTERNET
Date:    97/12/15 9:49 AM

Hi Gus,IBIS

     
Not true.