From: Lynne Green ([email protected])
Date: Mon Jul 17 2000 - 13:34:44 PDT
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: [email protected]
[mailto:[email protected]]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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