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What's Good About Coplanar Waveguide Support in PCB SI? It's now in SPB16.2!

Comments(12)Filed under: PCB design, SPB 16.2, coplanar, CPW Extraction

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.

EMS2D Preferences

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
Single CPW
Coupled CPW


Library Enhancements

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:
    • SingleCPW
    • DiffPairCPW
  • 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.

Jerry GenPart


By redwire on March 13, 2009
What assumptions above the CPW are made? Soldermask?  Metal height (for example, a grounded metal case)?  
Oh, it's "coplanar" not "coplaner" :)

By Jerry GenPart on March 13, 2009
Hi redwire - Yep, typo in the first line of my post. The stackup in the board is used to determine the structure. If there is a soldermask defined, then this will be taken into consideration. Anything outside the stackup is not used.

By bala r on January 31, 2011
What will be impact on impedance if we use ground trace instead of big shape.How we can check it out?Will the width of the ground trace play any role?

By Jerry GenPart on February 1, 2011
Hi bala,
I checked with our PCB SI expert AE, and she says -
"Ground traces are not taken into consideration, so the impedance will not change. It is only shapes that will affect the impedance when next to a trace."

By bala r on February 1, 2011
Thanks Jerry.Again,For example if i use 10 mil wide ground trace and then 10 mil wide signal trace and then 10 mil wide ground trace,( the airgap among these traces will be 10 mil),then assume the simulated impedance is 50 ohm .If i change the airgap between signal trace and ground trace(from 10mil to 20mil),what will be impact on impedance.Can i check it out in PCB SI.?

By Jerry GenPart on February 2, 2011
Hi bala,
Here's a reply from our PCB SI AE expert -
If you use a ground trace (which is a cline, or connect line), you will not see any difference in the impedance, regardless of whether or not you have enabled CPW extraction. Our tools do not support CPW extraction for ground traces surrounding a signal trace.
The only time you will see a change in the impedance is if you use a shape to surround the signal trace. The width of the ground shape that is surrounding the signal trace does not matter. The tool just looks for the existence of a shape associated with a DC voltage (within the geometry window) to determine if the trace will be analyzed as a CPW structure.
To see the change in the impedance, extract the net into SigXp from PCB SI (Analyze > SI/EMI Sim > Probe > View Topology) and note the impedance on the trace in SigXp. Change the spacing from the shape to the trace and extract again. You will see a change in the impedance in the CPW trace in SigXp.
Jerry G.

By oscar miguelino on November 1, 2011
CoPlanar waveguide information

By Yannick FAVRE on November 10, 2011
Are meshed ground planes (e.g. used for flex circuits) with track width, hole width and orientation taken into account in the solver ?

Same question for a 'small' ground plane which is not infinite regarding the Cline, i.e. edge of ground plane is closed to edge of line

By Jerry GenPart on November 10, 2011
Hi Yannick,

1. For the meshed ground planes, based on our current 2D field solver, (bem2d or ems2d), we can factor the mesh plane effects into account for the impedance value and reported in the IML model as additional data in the comment section.

2. For the small ground plane, our SPB16.6 version of the EMS3D field solver will be able to solve this type of structures rigorously and provide an S parameter model for it. For our current SPB16.5 release, the solver can handle it as a shape. The difficulty is how our GUI side and the geometry extraction part can get the information. So, could you please provide us either an example or a design (with the interested net specified). We can take a look and see how it's supported in our SI flow. I would suggest you contact our Customer Support team (file a new Service Request at - http://support.cadence.com) and a PCB SI AE expert will be able to work with you on this.

Jerry G.

By Waveguides on November 27, 2013
The shape under show a classic coplanar waveguide. The main difference for a part to be a coplanar waveguide.

By Cindy20 on February 27, 2014
I am testing my post, just wanted to make sure it post correctly before I start posting & asking questions. Thanks            

By Jerry GenPart on February 27, 2014
HI Cindy - yep, your post made it!
Jerry G.

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