Coplaner waveguides (CPW) are widely used in packaging, high speed designs and on silicon. These structures are now supported in Allegro PCB SI.
The figure below shows a typical coplaner waveguide. The important distinction for a segment to be a coplaner waveguide is a segment "W" surrounded by two large shapes. In order to detect coplaner waveguide segments while traversing a net, there needs to be a Shape window. The existing Geometry Window is used as the shape window. For a given segment, if two shapes adjacent to the segment exist within the Geometry Window, that segment will be analyzed as a CPW structure.
CPW Detection Flow
- Enable CPW for the entire design using the Preferences form
- Disable any individual nets that you do not want to be handled as a coplaner structure. This is done by attaching the CPW_DISABLED property to these nets.
- Set the Geometry Window
- Only traces surrounded by shapes that are assigned a DC voltage will be modeled as CPW
- The EMS2D field solver is used to generate the model.
Global Setting to enable CPW
The InterconnectModels tab of the Signal Analysis Preferences form has been enhanced to add an option to enable/disable CPW.
One or both of the fields Enable CPW Extraction and EMS2D can be selected at a time. The effects on the field solver are as follows:
Only Enable CPW Extraction enabled:
With only this option enabled, the extraction code goes through a flow different from the regular (old) flow and tries to detect coplanar waveguides. The model generated is then passed to EMS2D to get a solution. Except for CPW, all other models are generated with BEM2D.
Only Field Solver EMS2D enabled:
With EMS2D enabled, the extraction code follows the normal (old) flow. The only difference is that instead of calling BEM2D to get the field solutions, the EMS2D field solver is called.
Both Enable CPW Extraction and EMS2D enabled:
With both options enabled, the extraction code uses the new flow, which tries to detect CPWs. All models are solved with the EMS2D field solver instead of BEM2D.
The Preferences button is only active when the EMS2D solver is selected. By default, EMS2D uses default frequencies (same as BEM2D).
- Frequency Settings
- Default Frequencies is used to create values in the same manner and with similar settings as BEM2D with respect to frequency data. If no cutoff frequency is set in the main Preferences dialog, both BEM2D and EMS2D will generate a DC and INF value. If a cutoff frequency is set, both will use a range of frequencies based on the cutoff value (usually DC to 50 GHz).
The other two radio buttons allow you to specify the exact Frequency range and/or Frequency Points.
- Frequency Point File
- Option to provide a frequency point file.
Example of a frequency points file:
- Mesh Order
- A Mesh Order setting of 1 will speed up the performance of the field solution without significantly sacrificing accuracy. A setting of 3 will provide the highest accuracy. EMS2D generates a mesh based on the wavelength of the highest frequency point. This setting impacts the order of the polynomials used to analyze each of these cells.
- Fast Frequency Sweep
- This setting should only be used when a large number of frequency points are being solved. Rather than generating a full solution for each point, this option allows for the solver to find acceptable ranges where values can be interpolated. This does not mean a degradation in accuracy, however, as the solver compares fully solved values to points where interpolation will be used and does so with a very tight tolerance.
- Output S-parameter Waveform
- Enables EMS2D to produce S-parameter touchstone file (.snp). The output file is named <modelname>.snp is written to the run directory. This is only enabled when the Frequency Settings are set to either specific Frequency Parameters.
Coplanar Waveguide Characterization
The following CPW structures are supported:
Single stripline CPW
Coupled stripline CPW
Single microstrip CPW
Coupled microstrip CPW
To support CPW structures, two new IML model types have been added to the SI library. These model types are:
- Single CPW - This describes a single trace with ground shapes on both sides
- Diff Pair CPW - This describes two diff pair traces with ground shapes on both sides
The following enhancements have been made to support these new model types:
- Coplanar Waveguide models are stored in the IML library in a section with a header of CPW. Each of these models is stored as a subsection within this section of the library.
- The IML model browser shows CPW models with the following model type keywords:
- The following new options have been added to the Model Type Filter on the IML model browser:
- Single CPW - shows only CPW Single models
- Diff Pair CPW - shows only CPW Diff Pair models
- Any CPW - shows all CPW models
- The Add Model button on the IML Model Browser includes an option for adding each of the new CPW models.
- The CloneSelection option on the Add Model button in the IML model browser handles cloning both types of CPW models
- When Edit or TextEdit are selected for a CPW model in the IML model browser, the model is displayed in a text editor
A CPW model can only be created if running from a product that supports the new EMS field solver. However, if you have an existing IML library that contains CPW models, you will still be able to see these models in the IML model browser regardless of the product that you are running. You must be running a product that supports EMS before one of these existing CPW models will be used for a simulation.
Disabling CPW extraction on individual nets
A new property, CPW_DISABLED, has been added to disable CPW extraction on individual nets. Attach this property to any nets that you want to be handled during analysis as non-CPW nets. If you have selected only the Ems2d Field Solver option (without Enable CPW Extraction), non-CPW nets will be generated with Bem2d.
As always, I'm interested in hearing how you employ these new features.