[SI-LIST] Re: [SI-LIST]: Which tool is the best - LINPARdisc ussion

Chris,

You know that traditional IBIS doesn't model the entire buffer circuit.  It 
wasn't meant to do that and for the targeted application it didn't need to.  
Now with IBIS 4.1 we can add in whatever level of modeling that we need.  I 
have an IBIS 4.1 model of the Altera Stratix GX receiver with built-in 
equalization.  I can measure the signal at the output of the receiver and still 
have the convenience of using an IBIS model in my board level signal integrity 
simulator.  The underlying model can be either SPICE or VHDL-AMS.  For a lot of 
people, that is the correct trade-off of simulation detail and required 
expertise.  You personally might prefer the option of more detail that requires 
more expertise, but some other people don't have that much time and experience. 
 They need to get an answer that's really good without a lot of setup and 
analysis time.

Happy simulating,
Weston



-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of Chris Cheng
Sent: Friday, April 23, 2004 5:36 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: [SI-LIST]: Which tool is the best - LINPARdisc ussion

The problem is not just what the differential waveform looks like at the 
receiver after the interconnect. There are very subtle common mode variation at 
the receiver that can impact the OUTPUT of these multi-Gb/s receivers some of 
which have very limited common mode range.
Those who know me should know how anal I am to define I/O timing at the OUTPUT 
of the receiver rather than the input. Try that on your IBIS model.
To add more fun, try that with those built-in multi-stage equalizing receivers.
Any smart IBIS proponent out there can show me how to model that ?

-----Original Message-----
From: George Tang [mailto:gtang@xxxxxxxx]
Sent: Friday, April 23, 2004 5:22 PM
To: Chris.Cheng@xxxxxxxxxxxx; steven.corey@xxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: [SI-LIST]: Which tool is the best - LINPARdisc ussion


Chris,

You are exactly right.  In a simulation, there are certain parts of your system 
which dominates the simulation time, i.e.., the transistor models.
But there are also certain parts of your system that dominates the system 
performance.  For example, after you finish your system simulation with the 
actual drivers, you can replace the transistor driver model with an ideal 
driver with a fixed rise-time and output impedance.  If you vary the rise-time 
from 30ps to 50ps to 70ps, you may find that after 35 inches in
FR4 and a few connectors, the resulting waveforms are almost the same ( > 1ns 
rise-time).  Only in this case, you can choose to use a simplified driver in 
place of the transistor model.  But you are right.  In general, transistor 
level models are always preferred for better accuracy.

Kind regards,

George



-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Chris Cheng
Sent: Friday, April 23, 2004 3:08 PM
To: 'steven.corey@xxxxxxxxxxxxxx'; 'si-list@xxxxxxxxxxxxx'
Subject: [SI-LIST] Re: [SI-LIST]: Which tool is the best - LINPARdisc ussion


Totally agree.

The key is cross check the model with measurement and trusted field solver 
results.

Like I said before, my personal favorite tool is ADS which has all the 
distributed, lumped and S-param models you want. However, once in a while I 
have encrypted HSPICE model from vendors that forces me to switch back to 
HSPICE. As mentioned in another thread, I have zero faith in the field solver W 
element in HSPICE. That leaves me with either paying a few $$K for an expensive 
field solver tool or using Linpar. I'll take the later anyday.
But I can always calibrate the interconnect models with ADS before using it in 
full circuit simulations.

While there are very smart people here that claim they can simulate multi-Gb/s 
I/O's with IBIS. I am just too dumb to do so and I only trust full transistor 
level SPICE I/O models. With that, you can pretty much ignore any impact of 
your interconnect models on your simulation time.

-----Original Message-----
From: Steve Corey [mailto:steven.corey@xxxxxxxxxxxxxx]
Sent: Friday, April 23, 2004 2:14 PM
To: 'si-list@xxxxxxxxxxxxx'
Subject: [SI-LIST] Re: [SI-LIST]: Which tool is the best - LINPARdisc ussion


Chris -- I break "bad" approximations into roughly two categories:

1.  inaccurate fit to the measured data
2.  non-passivity

#1 can be quantified prior to running system simulations by comparing against 
measurements or trusted field-solver results.  If a rational approximation 
algorithm breaks down, it will show up as #1.

#2 tends to result in more catastrophic failure, such as oscillation or 
exponential growth.  However, the oscillations may not be crazy enough to cross 
your personal threshold, and those are probably the most insidious.  The best 
approach is to guarantee #2 mathematically prior to simulation, especially if 
you're a model provider.

If you've convinced yourself that interconnects aren't contributing heavily to 
your simulation time, you're probably right.  It definitely depends on the 
nature of the system you're simulating.  In the question of lumped (i.e., 
rational approximation) vs. distributed models, we aren't apologists for any 
particular approach.  We provide the ability to extract both, and let the user 
determine which is more appropriate based on the application.

   -- Steve

-------------------------------------------
Steven D. Corey, Ph.D.
Time Domain Analysis Systems, Inc.
"The Interconnect Analysis Company."
http://www.tdasystems.com

email: steven.corey@xxxxxxxxxxxxxx
phone: (503) 246-2272
fax:   (503) 246-2282
-------------------------------------------



Chris Cheng wrote:
> Steve,
> My question is, if the approximation is bad, what does it exhibit 
> itself
in
> the simulation ?
> If it results in "internal time step too small" or crazy oscillation, 
> I
know
> immediately the model has problem and move on. If however, it 
> generates
the
> wrong waveform with incorrect delay, I will be very worry.
> Interconnect model impact on simulation memory and time is not a 
> concern
for
> me, I believe my I/O circuits and package models dominates that by far.
>
> -----Original Message-----
> From: Steve Corey [mailto:steven.corey@xxxxxxxxxxxxxx]
> Sent: Friday, April 23, 2004 8:43 AM
> To: 'si-list@xxxxxxxxxxxxx'
> Subject: [SI-LIST] Re: [SI-LIST]: Which tool is the best - LINPARdisc 
> ussion
>
>
> Ray -- although your statement is correct, it is numerically difficult 
> to achieve.  Not all algorithms are robust enough to fit long, nearly 
> lossless delays.  Furthermore, simulations of electrically large, 
> nearly lossless systems take longer as well.  How to extract delay and 
> treat it separately when fitting such data is a current area of research.
>
> Of course, this phenomena isn't isolated to rational approximations, 
> but is a general problem with stiff systems -- those which have both 
> microscopic and macroscopic behaviors excited simultaneously.  A long 
> cable with little loss driven with a fast-risetime signal is a good 
> example.  As a broad generalization, the wider the range of behaviors 
> being simulated, the more memory and time that will be required to 
> simulate it.
>
> Different simulation and modeling techniques are optimized for 
> different types of systems.  For example, good distributed 
> transmission line models are often better for simulating long 
> transmission lines at fast risetimes than are straight rational 
> approximations.
>
>    -- Steve
>
> -------------------------------------------
> Steven D. Corey, Ph.D.
> Time Domain Analysis Systems, Inc.
> "The Interconnect Analysis Company."
> http://www.tdasystems.com
>
> email: steven.corey@xxxxxxxxxxxxxx
> phone: (503) 246-2272
> fax:   (503) 246-2282
> -------------------------------------------
>
>
> Raymond Anderson wrote:
>
>>Raj Raghuram wrote:
>>
>>
>>
>>>>I am not sure modeling with S-params would work for long 
>>>>transmission lines i.e. metres in length. Most s-parameter 
>>>>simulators use a rational fit which in the end is a lumped model. Maybe you 
>>>>can comment on this.
>>>>
>>>>
>>>
>>
>>The delay information required to model an electrically long structure 
>>such as a transmission line is contained in the phase information of 
>>the complex s-parameters.
>>
>>Most instruments output the phase of s-parameters in the range of +180 
>>to -180 degrees.  Plotted wrt frequency it resembles a sawtooth. This 
>>modulo 360 phase info needs to be unwrapped into a linear phase 
>>progression to interpret it as delay.
>>
>>When a rational polynomial approximation is fitted to the unwrapped 
>>s-parameter data, if the phase part of the approximation is the same 
>>as the phase of the original s-parameter then I'd expect the resultant 
>>macromodel to exhibit the same delay characteristics.
>>
>>Comments ???
>>
>>
>>-Ray Anderson
>>
>>Sun Microsystems Inc.
>>
>>------------------------------------------------------------------
>>To unsubscribe from si-list:
>>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>>
>>or to administer your membership from a web page, go to:
>>http://www.freelists.org/webpage/si-list
>>
>>For help:
>>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>>
>>List FAQ wiki page is located at:
>>                http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>>
>>List technical documents are available at:
>>                http://www.si-list.org
>>
>>List archives are viewable at:
>>              http://www.freelists.org/archives/si-list
>>or at our remote archives:
>>              http://groups.yahoo.com/group/si-list/messages
>>Old (prior to June 6, 2001) list archives are viewable at:
>>              http://www.qsl.net/wb6tpu
>>
>>
>>
>>
>
>
>


--

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List FAQ wiki page is located at:
                http://si-list.org/wiki/wiki.pl?Si-List_FAQ

List technical documents are available at:
                http://www.si-list.org

List archives are viewable at:
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List FAQ wiki page is located at:
                http://si-list.org/wiki/wiki.pl?Si-List_FAQ

List technical documents are available at:
                http://www.si-list.org

List archives are viewable at:
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List FAQ wiki page is located at:
                http://si-list.org/wiki/wiki.pl?Si-List_FAQ

List technical documents are available at:
                http://www.si-list.org

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List FAQ wiki page is located at:
                http://si-list.org/wiki/wiki.pl?Si-List_FAQ

List technical documents are available at:
                http://www.si-list.org

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

Other related posts: