I'm doing a parametric AC simulation (mmsim 11.1). I need a lot of steps, but the single simulations are fast.
When I add an S-parameter block the parametric simulations slows down because a rational fitting process is required at each simulation step ("Starting rational fitting process." message appears in the output log form).
Is it possible to save the rational model of the S-parameter block? I would like spectre could calculate the rational model only at the beginning of the sweep and then use it on the following sweep steps.
First of all, we normally don't recommend using rational (see Tawna's latest version of her nport app note) but if you really did want to, the ROM data file field on the nport component allows you to specify the file in which it is saved.
In reply to Andrew Beckett:
Hi Andrew,After my post "Enforcing causality of S parameter in file..." (Thu, Dec 1 2011 2:20 AM)==================================================================="Hello, I'm using harmonic balance analysis (mmsim 11.1) with a schematic containing anS-parameter model. In the NPORT component I set Causality Correction = no During thesimulation I get the following message: Notice from spectre during periodic steadystate analysis `hb'. Enforcing causality of S parameter in file Is it possible toavoid this time-consuming step? Thank you very much Aldo"===================================================================I opened a service request with customer support.It was suggested to me to userational fitting with a low order like 3 or 4 (I used 4).
However now, with AC, I see no problem to use SPLINE, but with HB I should continue touse RATIONAL for the issue explained in my previous post.
Do you see any drawback (I've not read yet the application note on NPORT)?
In reply to Aldo2:
Using the rational model is often even more time consuming than the causality correction...
The main problem with the rational method is that it really has not had much attention over the last few years - the focus has been on the convolution-based approaches (spline and linear). So I generally would advise against it, especially for larger number of ports or higher order systems. I have used it to check results in certain specific situations, but it would not be my first choice. Hopefully the specific issue you were having was followed up with R&D to help ensure that the s-parameter modelling is improved to handle the s-parameter data you were having problems with. Although of course we'd need to have your data to do that - not sure (I've not checked the service request).
I'm sure Tawna will chime in when she sees this.
Sr. Staff Support AE, Global Customer Support
Cadence Design Systems, Inc.