Sonnet User's Guide - Sonnet Software

Chapter 21 SPICE Model Synthesis
using a coarser resolution to produce less data points but still more than
50 data points. You may change the resolution of an adaptive sweep in
the Advanced Options dialog box in the project editor. Select Analysis
⇒ Setup, then click on the Advanced button in the Analysis Setup
dialog box which appears.
• Increase the number of data points in the critical frequency band in
which you are concerned and decrease the number of data points in
frequency ranges which are not as important.
• You may be able to increase the accuracy of your model by using the
Stability factor. Please see "Broadband Spice Extractor Stability Factor"
in the next section.
Broadband Spice Extractor Stability Factor
Although the extracted model may be a good fit to the S-parameters, a transient
analysis which uses the model may be unstable. To help with this problem, Sonnet
provides a control which pushes the poles away from the unstable region. However,
pushing the poles too far could result in a less accurate fit usually by causing
a decrease in the Q of the circuit. Therefore, you should only use this control if
you are having stability problems with the model.
The Broadband Spice stability factor allows you to control the amount by which
the model fitting forces the poles of your model away from the unstable region.
The factor represents a magnitude ratio for which the real part of a pole cannot be
less than the magnitude of the pole.
The higher the stability factor the greater the effect on the model. Reasonable values
for the stability factor are between 0.5 to 1.0e-5; the default value is 1.0e-3. If
you are having stability problems, raising this value may result in a stable model.
The forcing done using the stability factor could result in a decrease in Q such that
strongly resonant structures need a lower value, but unstable structures need a
higher value to maintain their stability.
You input the stability factor in the Advanced Broadband Model Options dialog
box, opened when you click on the Advanced button in the Broadband Model Entry
dialog box in the project editor or response viewer. You enter the stability factor
in the Additional Options text entry box using the following format:
-Stability
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