**From:** Lynne Green (*lgreen@mail.hyperlynx.com*)

**Date:** Mon Jul 17 2000 - 13:34:44 PDT

**Next message:**Chris Cheng: "RE: [SI-LIST] : inductance extracted by ansoft SI3D"**Previous message:**Doug McKean: "Re: [SI-LIST] : inductance extracted by ansoft SI3D"**In reply to:**Perry Qu: "Re: [SI-LIST] : inductance extracted by ansoft SI3D"**Next in thread:**Hassan Ali: "RE: [SI-LIST] : inductance extracted by ansoft SI3D"

Few simulation packages support both S-parameters

and IBIS models. (I can only think of a couple off

the top of my head.)

Digital circuits are strongly non-linear. S-parameters were

originally developed for "linear" small-signal circuits.

Funny how the two keep coming back together again!

- Lynne

"All the world's an analog stage,

whereon digital plays bit parts."

-----Original Message-----

From: owner-si-list@silab.eng.sun.com

[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Perry Qu

Sent: Monday, July 17, 2000 12:32 PM

Cc: si-list

Subject: Re: [SI-LIST] : inductance extracted by ansoft SI3D

Hassan:

I was wondering that for critical nets with very fast rise time, we can

extract the S parameters using field solver such as HFSS based on whatever

3D geometry that you have and then import the S parameters directly into

your circuit simulator such as MDS or S-SPICE. In such a way, the inductive

or capacitive effects of any discontinuety will be taken into account and we

don't have to bother about extracting the lumped parasitics value of vias

etc.

BTW, does any one know whether MDS/ADS support IBIS model ?

Regards

Perry

SI Specialist

Alcatel CID

Kanata, Canada.

Hassan Ali wrote:

* > > 2.For a via through several power and ground planes, does the SI 3D
*

* > > consider the effects of those planes when doing the extraction?
*

* > >
*

* >
*

* > For this I believe you need a full wave solver such as their HFSS.
*

Edge

* > rate (frequency content) & geometry really are the factors. If you
*

edge

* > rate is slow compared to the geometry, then the complication of an
*

* > additional solver MAY not be necessary. But, since we MAY not know
*

all

* > those rules of thumb & guidelines, take no chances & use a full wave
*

because

* > you probably do have edge rates which are "fast". Your investment in
*

* > understanding a refined full wave solver will be worth it.
*

As to the original question, AFAIK (as far as I know) Ansoft SI Q3D is

NOT capable of computing via parasitics in consideration to individual

planes. You see, SI Q3D considers the entire via structure comprising of the

signal traces connected to the via, the via barrel (the plating on the via

hole), and all the pads at different layers, as ONE conductor. ALL the

ground planes are considered connected hence they make ONE conductor. In

that case, the self L and R values computed for the via structure are for

the ENTIRE via structure as mentioned above (i.e. not just for the via

barrel), and the capacitance to ground is with reference to ALL the ground

planes (i.e. you don't get separate values for capacitance with reference to

EACH individual ground plane). That information is not very useful for

critical SI analysis. And unfortunately, I don't know of any software tool

that can accurately compute separate parasitics. Any suggestions?

To illustrate further the problem in question, suppose I want to include

via parasitics for a signal that goes from the top pcb layer to an inner

signal layer, then I need to include via parasitics of only that portion of

the via that gets into the path of my signal i.e. not the parasitics of the

entire via structure. Any body knows how to do that with the presently

available tools?

As to the capabilities of HFSS, I think many people make wrong

assumptions on how full-wave field solvers can help us (SI engineers). First

of all HFSS would NOT spit out via parasitics! Using your various signal

traces as "ports", HFSS can accurately compute scattering (S) parameter

matrix for all the ports. These S-parameters are computed for each

propagation mode of interest (e.g. TEM mode) and indeed takes into account

the electromagnetic (EM) field interactions of all the structures in the

geometry of the problem (e.g. for a via, all the conductors, power and

ground planes).

That is well and good, but the problem is that you CANNOT (easily)

separate individual interactions in terms of R, L, and C parasitics. The

only method I know of is to find a lumped-element equivalent circuit (which

may not be unique) and use a microwave circuit simulator (like Touchstone,

Libra, ADS, MDS, Ensemble, SuperCompact, APLAC, etc.) to optimize the R, L,

and C, values to make the equivalent circuit have the same S-parameters as

the original 3D structure. This is a painful process and at best not

accurate and reliable. This is because, at high frequencies, all the

parasitics are distributed and therefore cannot (easily) have an accurate

lumped-element equivalent. Am I too much of a pessimist here? Any ideas of

what works best?

Regards.

Hassan.

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**Next message:**Chris Cheng: "RE: [SI-LIST] : inductance extracted by ansoft SI3D"**Previous message:**Doug McKean: "Re: [SI-LIST] : inductance extracted by ansoft SI3D"**In reply to:**Perry Qu: "Re: [SI-LIST] : inductance extracted by ansoft SI3D"**Next in thread:**Hassan Ali: "RE: [SI-LIST] : inductance extracted by ansoft SI3D"

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