[SI-LIST] Re: (no subject)
- From: "Raj Raghuram" <raghu@xxxxxxxxxxx>
- To: "Yu Liu" <yu_liu@xxxxxxxxxxxxx>
- Date: Wed, 11 Dec 2002 17:12:58 -0800
Yu,
Your point about verification was the one I wanted to make in the first
place. When you extract a model, you can run a time domain simulation and
verify through Fourier Transforms (as I originally described) that you are
able to get back your original S-parameters. When you directly do a time
domain simulation with various other circuit elements connected, there is no
way to ensure that the internal representation used in the simulator for the
s-parameters is accurate.
Best Regards,
Raj Raghuram
Sigrity, Inc.
"Achieve what others can't"
raghu@xxxxxxxxxxx
http://www.sigrity.com
4675 Stevens Creek Blvd. , Ste 130
Santa Clara, CA-95051
PH: 408-260-9344 x116
CELL: 408-390-7614
FAX: 408-260-9342
> -----Original Message-----
> From: Yu Liu [mailto:yu_liu@xxxxxxxxxxxxx]
> Sent: Wednesday, December 11, 2002 2:40 PM
> To: raghu@xxxxxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Subject: Re: [SI-LIST] Re: (no subject)
>
>
> Hi, Raj,
>
> Thanks for the explanation - I hope there is less than 400,000 elements
> needed to model a 2-port S-parameter network, just like Steve Corey's
> -good- compression tool does, although my original post does not mean
> for a 2-port.
>
> However, one can't help to raise the question about the rational
> function fitting techniques: When your data does not have a smooth
> variation, how many orders are needed for a certain accuracy? What
> is the complexity of the technique (order/state-variable vs "variation")?
>
> Also, assuming the fitting is performed in frequency domain, how is the
> frequency fitting going to affect the accuracy in time-domain? In other
> words, a fitting may look very "good" in frequency domain, how would one
> know it will be accurate for transient response?
>
> Best Regards,
>
> Yu
> ===
>
> Apache Design Solutions
> 1881 Landings Drive
> Mountain View, CA 94043
> Tel: (650)237-5410
> Fax: (650)969-4170
> Email: yu_liu@xxxxxxxxxxxxx
> web: www.apache-da.com
>
>
> Raj Raghuram wrote:
>
> > Yu,
> >
> > The calculation that 400,000 circuit elements are needed to
> represent 2-port
> > S-parameters is misleading and here is why. The number of
> circuit elements
> > needed to represent the frequency variation of one of the
> S-parameters is
> > usually not anywhere near the number of frequency points. The
> fit is done
> > using rational functions and the order of the rational function
> is far less
> > than the number of frequency points. If you have a smooth variation, a
> > relatively low order transfer function would do the job.
> Assuming 10th order
> > rational functions, the number is closer to 40 i.e you have 40 state
> > variables.
> >
> > Best Regards,
> >
> > Raj Raghuram
> > Sigrity, Inc.
> > "Achieve what others can't"
> > raghu@xxxxxxxxxxx
> > http://www.sigrity.com
> > 4675 Stevens Creek Blvd. , Ste 130
> > Santa Clara, CA-95051
> > PH: 408-260-9344 x116
> > CELL: 408-390-7614
> > FAX: 408-260-9342
> >
> >
> >
> >>-----Original Message-----
> >>From: si-list-bounce@xxxxxxxxxxxxx
> >>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Yu Liu
> >>Sent: Tuesday, December 10, 2002 7:21 PM
> >>To: si-list@xxxxxxxxxxxxx
> >>Subject: [SI-LIST] Re: (no subject)
> >>
> >>
> >>
> >>Hi,
> >>
> >>I agree with Raj that mixed-domain simulation is a challenging job.
> >>
> >>However, the fairly easy S-parameter checker as proposed only reflects
> >>an idealized linear situation. In real applications, people combine the
> >>nonlinear circuits (e.g. on chip Tx/Rx) with linear parts (e.g. off-chip
> >>packages/FR-4),(see the following figure), and perform transient
> >>simulation and generate eye-patterns. For Multi-Gbps applications, the
> >>integrated SPICE approach seems to be the only viable method.
> >>
> >> +------------+
> >> +----+ +---+ | BackPlane | +---+ +----+ ______
> >> PRBS>----| Tx |---|Pkg|---| |---|Pkg|---| Rx | / \
> >> >----| |---| |---| FR-4 |---| |---| | _\__/_
> >> +----+ +---+ | | +---+ +----+
> >> +------------+
> >>
> >>
> >>As for the first proposed approach, let's suppose you have a 2-port
> >>S-parameter data. For each frequency, there are four things you need to
> >>model: reflection at both ports, as well as forward/reverse
> >>transmission. Suppose you need one element for each parameter (there
> >>could be more), then there would be four elements. And suppose there are
> >>1000 frequency points, then the total number of elements will be 4000.
> >>
> >>Now suppose you need to model a 20-pin connector, with 1000 frequency
> >>points, you would need 400,000 elements (could be more) to describe
> >>the whole S-parameter data. So the computer resource is quite demanding.
> >>Yes, you can look at those elements. But how are you going to handle any
> >>deficiencies in program generated elements when they are beyond the
> >>users control?
> >>
> >>In the integrated SPICE approach, which is the (2) proposed from Raj's
> >>post, no matter how many ports you have, and how many frequency
> >>points there are, you just need one element in your netlist. You don't
> >>go through the lengthy model generation/curve fitting/optimization
> >>procedures. And the user retains full control of the DC
> >>S-parameter values.
> >>
> >>--
> >>
> >>Best Regards,
> >>
> >>Yu
> >>===
> >>
> >>Apache Design Solutions
> >>1881 Landings Drive
> >>Mountain View, CA 94043
> >>Tel: (650)237-5410
> >>Fax: (650)969-4170
> >>Email: yu_liu@xxxxxxxxxxxxx
> >>web: www.apache-da.com
> >>
> >>
> >>Raj Raghuram wrote:
> >>
> >>
> >>>Mick, Ege, Hassan, and others,
> >>>
> >>>As mentioned, there are primarily two approaches to do SPICE
> >>>
> >>type transient
> >>
> >>>simulations for components described in S-parameters.
> >>>
> >>>(1) Convert S-parameters to certain forms, either
> equivalent circuit
> >>>representations or certain table lookup format, from which
> >>>
> >>SPICE engines can
> >>
> >>>read and run.
> >>>
> >>>(2) Enable a SPICE solver to read S parameters directly. The
> >>>
> >>SPICE solver
> >>
> >>>will then internally do the things in (1), or do convolution
> >>>
> >>directly which
> >>
> >>>can be quite demanding for computer resources for large number of such
> >>>circuit components.
> >>>
> >>>While it may appear more attractive to have the time domain circuit
> >>>simulator directly read in and use the S-parameters,
> deficiencies in the
> >>>representation used are not then easily seen. Often the original
> >>>S-parameters or the circuit model representing them may not be stable,
> >>>causal, and passive. Also, extrapolation of the S-parameter
> >>>
> >>data to DC is
> >>
> >>>often a problem and separate DC values may be needed.
> >>>
> >>>It is technically very challenging to have accurate and
> >>>
> >>reliable transient
> >>
> >>>simulations from S-parameters of complicated responses. There
> >>>
> >>are quite a
> >>
> >>>number of tools out there. A tool that is claimed to have such a
> >>>capability, either through (1) or (2), does not necessarily
> >>>
> >>mean it can do a
> >>
> >>>good job.
> >>>
> >>>One of the major issues, in time domain simulations by a SPICE circuit
> >>>solver, is whether the circuit really behaves in the way it
> >>>
> >>should behave,
> >>
> >>>as characterized by its original S parameters.
> >>>
> >>>Here is a way to check whether the tool really does the job it
> >>>
> >>is supposed
> >>
> >>>to do. Assume you have a two-port circuit described by S parameters.
> >>>You have a SPICE equivalent circuit of the two-port network or
> >>>
> >>your solver
> >>
> >>>can directly read-in S parameters. Connect port one with a
> >>>
> >>time-varying
> >>
> >>>voltage source Vs(t) and a 50 ohm resistor, connect port 2 with a 50
> >>>resistor as shown in the following graph. Run the SPICE
> >>>
> >>engine to get the
> >>
> >>>transient voltages V1(t), V2(t) and Vs(t).
> >>>
> >>> 50 ohms --------------
> >>> |-------| | |
> >>> |-----| |------o| |------|
> >>> | |-------| + | | + |
> >>> --- | | ---
> >>> Vs(t)|+| V1(t) | |V2(t)| | 50 ohms
> >>> |-| | | | |
> >>> --- - | | - ---
> >>> |--------------------o| |------|
> >>> ----- | |
> >>> --- --------------
> >>> -
> >>>
> >>>Take Fourier transforms of Vs, V1 and V2; then the S parameters of the
> >>>two-port circuit can be extracted as follows:
> >>>
> >>> S11 = (V1(f)-Vs(f)/2)/(Vs(f)/2)
> >>> S21 = V2(f)/(Vs(f)/2)
> >>>
> >>>One can find S22 and S12 in a similar way by moving Vs to port 2.
> >>>
> >>>The S parameters extracted from the above procedure represent
> the actual
> >>>S-parameters of the two-port circuit in transient simulations;
> >>>
> >>the amount of
> >>
> >>>their deviation from the original S-parameters reflects how
> >>>
> >>accurate the job
> >>
> >>>is done.
> >>>
> >>>The above tests are fairly easy to do with any SPICE solvers.
> >>>
> >>I have some
> >>
> >>>netlist templates available and I would be happy to provide you
> >>>
> >>if you are
> >>
> >>>interested.
> >>>
> >>>
> >>>Best Regards,
> >>>
> >>>Raj Raghuram
> >>>Sigrity, Inc.
> >>>"Achieve what others can't"
> >>>raghu@xxxxxxxxxxx
> >>>http://www.sigrity.com
> >>>4675 Stevens Creek Blvd. , Ste 130
> >>>Santa Clara, CA-95051
> >>>PH: 408-260-9344 x116
> >>>CELL: 408-390-7614
> >>>FAX: 408-260-9342
> >>>
> >>>
> >>>
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> >>>
> >>
> >>
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