Dispersive Material Calculator – XFdtd

The dispersive material calculator finds Debye-Drude parameters that fit measured data and constant loss tangent by using optimization.

Calculator Settings

The Calculator group in the upper-left corner of the window defines input values for computing a new set of Debye-Drude parameters. The Debye-Drude Model Parameters group in the lower-left portion of the window shows the initial values for the Debye-Drude parameters and then the new fitted set, which are computed from the input values. The graphs on the right side of the window compare the input data with the Debye-Drude parameters.

The two buttons (, ) and checkbox below the graphs toggle the visibility of the legends, allow editing of the graph axes, and toggle connecting the input data points with a line, left to right, respectively. The graphs can also be panned and zoomed with the mouse in the same manner as other XF graphs.

Workflow

Open the calculator from the right-click menu of the Materials branch of the Project Tree or click the Dispersive Material Calculator button in the Debye-Drude material editor.

The following workflow guides users through the editor:

Setup the input data.

Fit Type: controls how the material properties are input through the Input Data drop-down menu.
Upper and Lower Frequency range: covers the entire input material data or a subset.
Input Data: specifies the discrete material properties.

Enter number of poles and solve.

Number of Poles: specifies how many poles will be included in the Debye-Drude material that is created.
Solver Settings: modifies the optimization settings.
Solve: uses a Particle Swarm Optimization (PSO) to find the best Debye-Drude material parameters for the input data.

Finalize parameters.

Debye-Drude Model Parameters: reflects the optimization result.

Create the new material.

Create Material or OK: pushes the Debye-Drude Material Parameters to an XF material for use in the project.

Optimization completed in the second step updates the graphs and reports a fitness level between 0 and 1, 0 indicating a perfect match between the new dispersive material and input data.

Users can try the following possible solutions if the fit is not acceptable:

Increase the Number of Poles for more flexibility when finding a solution.
Click Solve again so that PSO's utilization of random numbers can potentially find a different solution.
Modify the Solver Settings to control how the PSO searches the solution space.
Edit the Debye-Drude Model Parameters directly. This becomes more difficult as the number of poles increases.

When Create Material or OK pushes the new parameters to the XF material, it also saves the input data with the material for future reference or recomputation. It is accessible by clicking the Dispersive Material Calculator button on the material editor.

Input Data

The calculator computes Debye-Drude parameters for two types of input data: loss tangent and sampled. Both types are entered as discrete data points over a frequency range and the Input Data fields change depending on the Fit Type selection.

The Constant Loss Tangent type generates Debye-Drude parameters such that the loss tangent is constant over the frequency range. Constant Relative Permittivity and Constant Loss Tangent are discretized over the frequency range based on the Number of Samples.

The Sampled Data type can be specified as Conductivity and Permittivity, Real and Imaginary Permittivity, or Loss Tangent and Permittivity. Enter this data by either typing it into the table or importing it from a comma separated version (CSV) file using the button. The CSV import expects three columns, the first of which is frequency in Hz, and the latter two are the sampled material data.

Clicking the or button allows the user to add or remove data points within the table.