[SI-LIST] Re: (no subject)
- From: "C. Kumar" <kumarchi@xxxxxxxxx>
- To: raghu@xxxxxxxxxxx, Yu Liu <yu_liu@xxxxxxxxxxxxx>
- Date: Thu, 12 Dec 2002 06:15:05 -0800 (PST)
it looks like the arguement is going in circles.
Any reduced model implemenation (freq domain or otherwise) will involve
errors. I think Raj's stuff is very usefull because it lets standard spice
simulators into the play. Besides the technology can always evolve. Nspice can
have its domain too with large circuits. In other words there is room for
everyone here.
But guys it was enjoyable discussion
Raj Raghuram <raghu@xxxxxxxxxxx> wrote:
Yu,
Actually we are in agreement. Simulation of s-parameters in the time domain
can be tricky and all the more reason for the kind of verification I
proposed.
The check is only the first step towards a more complex simulation with
on-chip and off-chip elements. If we fail the simple check, we cannot trust
the results of the more complex simulation.
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 6:14 PM
> To: raghu@xxxxxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Subject: Re: [SI-LIST] Re: (no subject)
>
>
> Hi, Raj,
>
> Let me rephrase the question: Suppose you use the average error criteria
> in the curve fitting, then there could be large errors at local parts
> even though the overall error is small. If the transient simulation
> happens to use just the large error part in the frequency domain (one
> worst-case would be a single frequency signal at the large error point),
> the results would generate large errors in time-domain. And in addition,
> maybe you could give us a rough idea about the complexity of the
> technique (number of states/elements vs frequency viriations)?
>
> The fairly easy S-parameter check is a good way to check the
> implementation. However, it is not the goal of what people want. In the
> end, any technique is rendered useful only when it can perform close to
> real-case simulations. In this case, it means combining the on-chip and
> off-chip circuits and generating close-to-measurement eye-patterns.
>
>
> 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,
> >
> > 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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