[SI-LIST] Re: 3D-em simulation and terminations: macromodelling
- From: "Istvan Nagy" <buenos@xxxxxxxxxxx>
- To: "Yuriy Shlepnev" <shlepnev@xxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 8 Jan 2009 22:08:14 -0000
hi
thanks for the comprehensive answers.
i will think about these.
the part with the z-to-s conversion was quiet clear: dividing very big
numbers with very small numbers, with noise and finite number of digits... i
read about this somewhere (i.novak: frequency domain characterisation of
power...).
i was looking for books about s-parameters, but most of what i found just
spends 2 pages on it and costs more than 100$, they spend 95% of the content
for introduction to signal integrity.
maybe i will buy the "Microwave Differential Circuit Design Using Mixed Mode
S-Parameters" (but it has a very bad review on amazon.com), or one of them
adviced by you, Yuriy...
how does the characteristic impedance of a tline coming through a touchstone
file? to produce matching when connectied terminations in the system
simulator. is there some relationship between Z11/Z12/Z21/Z22 parameters and
Z0? (at what frequency value)
istvan
cct
----- Original Message -----
From: "Yuriy Shlepnev" <shlepnev@xxxxxxxxxxxxx>
To: "'Istvan Nagy'" <buenos@xxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
Sent: Tuesday, January 06, 2009 6:22 PM
Subject: [SI-LIST] Re: 3D-em simulation and terminations: macromodelling
> Hi Istvan,
>
> I see that you have got two answers on the S-parameters normalization:
> 1) J.X. Zheng: "It is independent of excitations and terminations."
> 2) G. Havermann: "...the port selection and geometry has a huge impact on
> the results."
> Both statements are correct in an appropriate context that may confuse you
> further.
> So, let me try to explain it starting from the first principles.
>
> Behavioral model of interconnect or power distribution network (PDN) is a
> black-box description of the system in form of a multiport. Each port of
> the
> multiport is defined by two terminals with a voltage V between them and a
> current I going in and out (both can be defined as special or modal
> variables). Multiport can be characterized by the impedance matrix Z as
> V=Z*I - defined with the open-circuit terminations. Short circuit
> termination produces admittance matrix Y description of a multiport -
> I=Y*V.
> For non-degenerate cases (always the case for non-ideal physical systems)
> two descriptions are equivalent and Z=Y^-1 and vice versa.
> Most of the frequency-domain electromagnetic solvers use immitance
> formulation and produce directly either admittance Y or impedance Z
> matrices
> (whatever is more convenient). These matrices can be written into a
> Touchstone file and used by a system level solver without conversion into
> S-parameters. Conversion to S-parameters is usually done as a
> post-processing step in that case. Note that most of the system-level
> solvers use the immitance formulation too (so called modified nodal
> analysis). Immitance description may be called "un-terminated" description
> because of no particular termination impedance is specified.
>
> S-parameters are convenient for measurements and can be introduced as a
> simple change of variables:
> a=0.5*(V/sqrt(Zo)+sqrt(Zo)*I) - incident waves, if t-line is attached
> b=0.5*(V/sqrt(Zo)-sqrt(Zo)*I) - reflected waves, if t-line is attached
> b=S*a;
> S=(Z/Zo-U)*(Z/Zo+U)^-1; or S=(U-Y*Zo)*(U+Y*Zo)^-1;
> Where Zo is the normalization impedance (assumed to be identical for all
> ports for simplicity), that is technically the termination impedance. Z is
> the impedance matrix and Y is the admittance matrix.
> As you can see the S-parameters describe multiport with a particular
> termination of all ports. If your electromagnetic solver requires setting
> up
> the termination impedances before the analysis - it may use the scattering
> formulation. Changing the termination impedances may change the
> S-parameters
> dramatically as was pointed out. But S-parameters with any termination can
> be easily transformed into "un-terminated" admittance form as
> Y=(1/Zo)*(U-S)*(U+S)^-1 or into impedance form or renormalized. In fact,
> practically all system-level solvers transform S-parameters back to
> "un-normalized" Y (there are exceptions) to perform the analysis of the
> multiport with the connected models.
> Thus, theoretically, all descriptions of multiports are completely
> identical
> for non-ideal physical systems. As a practical experiment, try to use
> Touchstone files with matrices of different type and with different
> normalization. The results of the system-level analysis of the terminated
> interconnects should be nearly the same (at least in frequency domain).
> Nearly, because of the numerical accuracy during the descriptors
> conversion
> - it may become an issue if the normalization impedance and diagonal
> elements of the impedance descriptor are different by multiple orders in
> magnitude, that may happen in case of PDN analysis for instance. A
> substantial difference may be observed in the time domain due to the
> reduced
> order modeling issues - the normalization or termination have to be
> carefully selected in that case (pointed out earlier by Vladimir
> Dmitriev-Zdorov).
>
> Finally, note that the "real" t-line model in your system-level solver is
> also a multiport described by admittance parameters of a multi-conductor
> transmission line with t-line p.u.l. parameters computed either with a
> formula or with a static field solver. You should expect good
> correspondence
> of such model with a model on the base S-parameters extracted with
> electromagnetic solver at lower frequencies (up to 1-3 GHz for typical PCB
> geometries).
>
> To learn further on multiports and S-parameters I would recommend the
> following books:
> B. Young, Digital signal integrity: Modeling and simulation with
> interconnects and packages, 2001
> J. Choma, Electrical networks: Theory and analysis, 1985
> H.J. Carlin, A.B. Giordano, Network theory: An introduction to reciprocal
> and non-reciprocal circuits, 1964
>
> Best regards,
> Yuriy Shlepnev
> www.simberian.com
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On
> Behalf Of Istvan Nagy
> Sent: Monday, January 05, 2009 1:34 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] 3D-em simulation and terminations: macromodelling
>
> hi
> if i simulate a pcb structure (a microstrip trace) with an 3d
> electromagnetics simulator to create a macromodell (s-parameter touchstone
> file), then do i have to connect terination resistors in the em simulation
> setup? or all the ports in all simulators are automatically terminations
> too? do i have to use the same term.resistor value in system simulation
> too
> (when using the macromodel. example in Agilent ADS or spice) to get
> accurate
> results?
> the problem is that in the em simulation we can not use the correct
> termination, since it comes with using IBIS buffer models and those are
> not
> compatible with any of the EM simulators, they can be connected in a
> circuit
> simulator only. so do macromodelling: first EM simulation, create
> macromodel, then system simulation with the other elements of the signal
> path.
>
> I was thinking:
> if the S parameter data ( S21(f) ) should depend on the actual
> termination?
> if we change the termination of a transmission line, then it changes the
> reflections (matched, unmatched...as we studied in the school). is it the
> same when we use an s-param (touchstone) macromodel instead of the real
> tline?
> if i dont terminate in the em simulator, then the strong reflections will
> cause strong loss and not-flat response, and that loss will go into the
> s-param file. then if i connect correct termination in the system level
> simulation, it will not to fix those reflections/loss anymore.
> am i right?
>
> so, how much usable is an s-parameter macromodell, and macromodelling
> anyway?
>
> Istvan Nagy
> CCT
>
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