From: Perry Qu (email@example.com)
Date: Mon Jul 17 2000 - 12:32:06 PDT
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 ?
Hassan Ali wrote:
> > > 2.For a via through several power and ground planes, does the SI
> > > consider the effects of those planes when doing the extraction?
> > >
> > For this I believe you need a full wave solver such as their HFSS.
> > rate (frequency content) & geometry really are the factors. If you
> > rate is slow compared to the geometry, then the complication of an
> > additional solver MAY not be necessary. But, since we MAY not know
> > those rules of thumb & guidelines, take no chances & use a full wave
> > you probably do have edge rates which are "fast". Your investment
> > 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?
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