**From:** Ray Anderson (*raymonda@radium.eng.sun.com*)

**Date:** Tue Aug 08 2000 - 21:09:42 PDT

**Next message:**Ricchiuti Vittorio: "RE: [SI-LIST] : LVDS Skew"**Previous message:**Scott McMorrow: "Re: [SI-LIST] : RE : LVDS Skew"**In reply to:**Perry Qu: "[SI-LIST] : Tools for high-speed interconnect design"**Next in thread:**Charles Grasso: "Re: [SI-LIST] : Tools for high-speed interconnect design"**Reply:**Charles Grasso: "Re: [SI-LIST] : Tools for high-speed interconnect design"

Perry-

As you pointed out, digital designers like to see time domain waveforms

as the result of their simulations. A time domain simulator (a spice

variant, XTK, HP ADS or other time domain solve engine) is commonly

utilized. Inserting accurate models of discontinuities into such simulations

can sometimes be a challenging problem.

There are a number of ways such models can be developed:

Frequency Domain Methods:

_________________________

From field solvers or from frequency domain lab measurements s-parameters can

be determined. As you mentioned, by means of optimization, a lumped (or

distributed) topology can be fit to the synthesized or measured data to create

a discontinuity model that can be quite accurate over some defined bandwidth.

Alternatively, the same s-parameter data can be mathematically manipulated

(extract poles and zeroes) to form a rational function (i.e. a ratio of

polynomials). Pade' functions are one common example. The derived rational

function can be implemented in a spice simulation as polynomial controlled G

elements.

A third method is to directly mathematically extract circuit element

values from measured or simulated s-parameter data. The derived circuit

elements (usually R's, L's, and C's) can then be used in a spice simulation.

One "gotcha" of the frequency domain methods utilizing measured

characterization data is that some sort of de-embedding must be performed to

try and separate the desired discontinuity data from the undesired fixturing

data. Sometimes a non-trivial exercise.

Time Domain Methods:

______________________

Methods exist to utilize time-domain (TDR and TDT) data that is measured in

the lab to create models that are suitable for time domain simulations.

A nice thing about the time-domain methods is that you can take advantage

of time windowing to extract the data relevant to the discontinuity of

interest and to ignore responses caused by fixturing.

TDR and TDT measurements allow one to extract parasitics, equivalent

circuits, impedance and delay, frequency dependent Zo, and s-parameters.

Methods that may be employed include: Model optimization, Peeling algorithm,

time domain network analysis, and rational functions.

The model optimization method is somewhat similar to the freq. domain

optimization method except one optimizes element values in a time domain

simulation instead of a freq. domain simulation.

The peeling algorithm is implemented in the Tektronix IPA-310 and 510

systems, the tool offered by TDA systems as well as Agilent (HP).

Time domain network analysis is a mathematical method currently

under development.

The rational function method is a time-domain adaption of the frequency domain

method of the same name.

I won't get into any of the details here. (it can get quite involved), however,

my colleague Madhavan Swaminathan of the Packaging Research Center at Georgia

Tech and his students have done extensive work on both time domain and

frequency methods of modeling and have published much of the work and have

several new upcoming pubs in queue on the subject.

There are many variants on the methods I've outlined, and I'm sure many

other methods I haven't mentioned here. The point is, once you have either

measured or synthesized characterization data for a discontinuity, there

exist numerous methods for introducing the data into a time domain

spice (or spice-like) simulations. Each method has it's pros and cons.

Depending on exactly what you are doing some methods may be preferred

over others.

-Ray Anderson

Sun Microsystems

*> Hello, everyone:
*

*>
*

*> This topic has been brought up several times but I still want to have
*

*> some idea from you as to what tools you use and when you use them.
*

*>
*

*> For digital designers, one of the major concern is the waveform they get
*

*> at the devices pins. Such job is most efficiently done using traditional
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*> tools like HSPICE, XTK, etc., which can handle the active device models
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*> with non-linear characteristics.
*

*>
*

*> However, when the speed of signal get faster and faster, the
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*> discontinueties such as vias has to modelled properly. To get this
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*> done, full wave field solver such as Ansoft HFSS, which was
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*> traditionally aimed for RF/microwave/antenna design may have to be used.
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*> But the problem with field solver is that they usually does not support
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*> active device models (correct me if I'm wrong). My question is, how do
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*> you guys utilize results from field solver in your circuit simulation ?
*

*> As far as I know, one approach is to build an equivalent circuit for the
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*> interconnect under study, optimize the RLC values so that the S
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*> parameters from the equivalent circuit match that of the HFSS
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*> simulation. Is this approach the common one used by SI engineers ? Any
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*> other approachs that you use and the tools you used ? Please share with
*

*> me your experience.
*

*>
*

*> Thanks in advance.
*

*>
*

*> Perry
*

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**Next message:**Ricchiuti Vittorio: "RE: [SI-LIST] : LVDS Skew"**Previous message:**Scott McMorrow: "Re: [SI-LIST] : RE : LVDS Skew"**In reply to:**Perry Qu: "[SI-LIST] : Tools for high-speed interconnect design"**Next in thread:**Charles Grasso: "Re: [SI-LIST] : Tools for high-speed interconnect design"**Reply:**Charles Grasso: "Re: [SI-LIST] : Tools for high-speed interconnect design"

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