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Coplanar Line (EM Quasi-Static): CPW1LINE



CPW1LINE models a section of symmetric coplanar waveguide (gaps between strip conductor and lateral grounds have equal width) on a dielectric substrate. This model accounts for an optional metallic cover, an optional backing ground plane, and allows for arbitrary metal thickness of the signal conductor and lateral ground planes.



Name Description Unit Type Default
ID Element ID Text CP1
W Conductor width Length W[1]
S Gap width Length W[1]
L Conductor length Length L[1]
Acc Accuracy parameter   1
CPW_SUB Substrate Definition Text [2]

[1] User-modifiable default. Modify by editing under $DEFAULT_VALUES in the default.lpf file in the root installation directory.

[2] If only one CPW_SUB is present in the schematic, this substrate is automatically used. If multiple CPW_SUB substrate definitions are present, the user must specify.

Parameter Details

CPW_SUB. Supplies parameters for dielectric substrate, conductor thickness, conductor metal properties, the presence/absence of metallic cover/backing, and the cover height over the substrate. The Cover and Gnd parameters allow the addition/elimination of infinite metallic plates acting as a cover or grounded backing. The CPW1LINE model does not use the following CPW_SUB parameters: Hab, ER_Nom, H_Nom, Hcov_Nom, Hab_Nom, and T_Nom.

Acc. The accuracy parameter. The default value for Acc is 1. If Acc is less than 1 or greater than 10 it is set automatically to 2.

Parameter Restrictions and Recommendations

  1. The Acc parameter A is limited to 1 < Acc < 10. A larger value of Acc increases the density of mesh used in computations. The accuracy of model parameters may improve slightly by increasing Acc, at the expense of a noticeably longer computation time. Generally, a good trade-off between accuracy and computation time is to set Acc to 1.

  2. This model does not impose restrictions on the conductor thickness (it may be zero, positive, or negative). Negative thickness means that the conductor is recessed into the substrate.

Implementation Details

  1. Model implementation is based on the EM Quasi-Static technique described in [1]. It accounts for losses in metal and in the substrate dielectric. Dispersion is partly included.

  2. Setting Gnd=0 implies that the substrate is not backed by a perfect conductor, but is bounded by infinite air space. Modeling results are strongly affected by thin substrate heights and might differ substantially from modeling results obtained from models that implement the common conception of a coplanar waveguide (for example, CPWLINE). To model a classic coplanar waveguide featuring an infinitely thick substrate, set Gnd=0 and H >2(W+2S), where Gnd and H are parameters of CPW_SUB.

  3. To apply Method of Moments for analysis, a quasi-static model creates 1D mesh covering contours of all conductors. The mesh is made of linear segments (pulses) of varying length. The length of a pulse is relatively big at the conductor center; it decreases toward conductor edges to reveal the charge distribution across conductor. If the conductor width is too large it may cause the pulse size to approach zero for pulses close to edge. In these rare cases the model may display a “Length of pulse #nnn equal to zero” error message.

Circuit Model Synthesis

CPW1LINE supports synthesis of physical parameters based on electrical specifications using the Transmission Line Calculator. To open the Transmission Line Calculator, right-click a transmission line element in a schematic and choose Synthesize.

To perform transmission line synthesis:

  1. In the Electrical property grid, select Target check boxes for desired electrical parameters and enter a corresponding value.

  2. In the Physical property grid, update frequency and substrate parameters if needed, then select Synthesize check boxes for transmission line physical parameters to synthesize based on the targets.

  3. Click the Synthesize left arrow to run the synthesis program. The values in both property grids update with the synthesized results. An analysis is also performed with the final physical values. If synthesis cannot achieve the target values, it shows how close it came.

  4. Click OK to update the selected transmission line element with the synthesized physical parameters. Expressions are overwritten with the new, evaluated values. You can click the Undo button on the program toolbar to revert all parameters in the schematic document to their pre-synthesized state. Parameters from substrate elements are never updated since typically substrate elements are used by more than one transmission line element. Click Cancel to close the dialog box without setting the parameters into the element.


This element uses line types to determine its layout. By default, the layout uses the first line type defined in your Layout Process File (LPF). You can change the element to use any of the line types configured in your process:

  1. Select the item in the layout.

  2. Right-click and choose Shape Properties to display the Cell Options dialog box.

  3. Click the Layout tab and select a Line Type.

  4. Click OK to use the new line type in the layout.

See “Cell Options Dialog Box: Layout Tab ” for Cell Options dialog box Layout tab details.

CPW elements have special configurations for the defined line types. The center conductor geometry draws on all the layers defined in the line type. The spacing to the ground plane is then drawn on negative layers with the same name as all of the layers in the line type. You must then draw the same layers on the positive layer to complete CPW layout. See “Negative Layers ” for more information on setting up processes for positive and negative layers.

See “The Layout Process File (LPF)” for more information on editing Layout Process Files (LPFs) and to learn about adding or editing line types.

Recommendations for Use

NOTE: The implementation of EM Quasi-Static models heavily relies on the involved numerical algorithms. This may lead to a noticeable increase in simulation time for schematics that employ many such models.

If the thickness of any layer is too small in comparison with the thickness of another layer, simulation time may also noticeably grow.


[1] M.B. Bazdar, A.R. Djordjevic, R.F. Harrington, and T.K. Sarkar, "Evaluation of quasi-static matrix parameters for multiconductor transmission lines using Galerkin's method," IEEE Trans. Microwave Theory Tech., vol. MTT-42, July 1994, pp. 1223-1228.

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