[SI-LIST] Re: enforcing passivity on s-parameter
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: nitin.chhabra@xxxxxx
- Date: Thu, 16 May 2013 08:06:47 -0400
I suggest a different approach. Passivity violations are due to the errors
in either an EM field solver method or measurement. In transient
simulation in Hspice, if you cascade multiple elements, each element's
passivity violation can result in convergence problems, since the error
indicates gain in a passive element. "Fixing" the passivity violation
involves an algorithmic process to try and "guess" what the value should
be, or to at least choose new values for those points that are
well-behaved. This itself can introduce errors. Remember, it's just a
guess, no matter how educated the tool vendor says it is.
An alternative approach is to realize that the passivity violation is
usually extremely small with respect to the actual signal amplitude passing
through the model. The simplest fix is to turn down the gain, that is, add
enough loss so that the total gain is less than 1. But that's what
interconnect channels do. They have attenuation that is much larger than
the passivity violations in all but the worst constructed S-parameter
models or measurements. In that case, the best approach to "fixing" the bad
s-parameter is to concatenate it with other lossy s-parameters in the
frequency domain, and then use the resulting composite model section
transient analysis.
In my process, I create full channel s-parameter cascade models for all of
the interconnect parameters that I want to sweep and test. The full
frequency domain response can then be evaluated, and the models can be used
in fast transient analysis much more efficiently. I trade off disk storage
space for time and efficiency in simulation. With Terabyte drives this
seems like a good trade to me.
It's a simple method, adds no additional sources of error, and is what
nature does. Oh, and it's faster in transient simulation, too.
best regards,
Scott
On Thu, May 16, 2013 at 3:29 AM, Nitin Kumar CHHABRA
<nitin.chhabra@xxxxxx>wrote:
> Hi Amit,
>
> Enforcing the passivity do changes the S-parameters.
> You can enforce passivity , but before enforcing the passivity you need to
> check 2 things :
> 1. What is the algorithm tool is using to enforce the passivity.
> 2. Check the frequencies where the violation is occurring and make sure
> that is not lying in the zone of interest.
>
> After checking the above two things, you can enforce the passivity and do
> the transient simulation.
>
> With regards,
> Nitin
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Amit Kumar
> Sent: Thursday, May 16, 2013 7:41 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] enforcing passivity on s-parameter
>
> Hello Experts,
> Many a times I face convergence issues in simulation because of
> s-parameter.
> I then enforce passivity on s-parameter and it does work most of the times.
> The question I have is : Can we trust the results we get after enforcing
> passivity on s-parameter or enforcing passivity spoils the s-parameter?
>
> Regards
> Amit
>
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