Spectre Tech Tips: Spectre S-Parameter Quality Checking and Fitting Tool

In Spectre, the nport element allows users to include S-Parameter data for time and frequency domain circuit simulation. At times, Spectre users face convergence or performance problems when the S-Parameter data has poor quality.

The typical S-Parameter issues that might cause simulation problems in transient analysis are:

• Missing or incorrect DC operating point
• Insufficient lowest frequency point
• Highest S-parameter frequency point lower than the circuit operation frequency or less than 1/rise_time in Hz
• Too few frequency points per decade
• Discontinuities in real/imaginary or phase
• Non-causality
• Non-passive behavior

In Spectre 23.1 ISR5, we have released a new tool, Spectre S-Parameter Quality Checking and Fitting tool, which allows users to check and improve the quality of their S-Parameter data prior to the Spectre simulation.

The tool contains two components:

• S-Parameter Quality Checking tool
• S-Parameter Fitting tool

The S-parameter Quality Checking tool can be started using the following command line.

spectre +sparam <Sparam_filename> +checking -o <SparamToolOutput>

The tool checks the S-Parameter data and creates a report in the optional <SparamToolOutput> directory as shown in the following image:

The identified problems are marked Failed. There are two solutions for addressing the Failed problems:

• Fix the problems indicated by the Quality Checking Tool by re-extracting the S-Parameter file
• Use the fitting tool to create a rational fitted model (RFM).

The Fitting tool addresses the problems reported by the S-Parameter Quality Checking tool. It will generate an RFM, which is passive and causal by construction. The more failures that need to be fixed, the more likely the RFM will deviate from the original S-Parameter file. 

The fitting tool can be started using the following command line.

spectre +sparam <Sparam_filename> +fitting=rfm -o <SparamToolOutput>

The fitting tool reports fitting errors and RFM quality information, helping users to identify the inaccuracies in the fitted model as compared to the original model. The Rational Fitted Model Status category reports an overall status indicating the quality of the fit: Good, Moderate, and Poor. When the quality is reported to be poor, we strongly recommend the regeneration of the S-Parameter data.

A detailed description and illustration of the Spectre S-Parameter Quality Checking and Fitting tool can be found in Article 20512255 on support.cadence.com. Additionally, you can view the related information using spectre -h sparam on the Linux prompt.  

Related Resources

• Spectre Classic Simulator, Spectre Accelerated Parallel Simulator (APS), and Spectre Extensive Partitioning Simulator (XPS) User Guide
• SPECTRE® S-Parameter Quality Checking and Fitting Tool (Article 20512255)
• 7 Habits of Highly Successful S-Parameters (Article 20500094) 

You may also contact your Cadence support AEs for guidance.

For more information on Cadence products and services, visit www.cadence.com.

About Spectre Tech Tips

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