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Coplanar Line (Closed Form): CPWLINE

Symbol

Summary

CPWLINE models a section of coplanar waveguide on an infinitely thick dielectric substrate. This model implies that the coplanar waveguide is ungrounded (the backing ground plane, if present, is way below the signal strip and does not affect the modeling results).

Topology

Parameters

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]
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, and conductor metal properties. The CPWLINE model does not use the following CPW_SUB parameters: Hcover, Cover, Gnd, Hab, ER_Nom, H_Nom, Hcov_Nom, Hab_Nom, and T_Nom. The H parameter is used only for validation purposes (see "Parameter Restrictions and Recommendations").

Parameter Restrictions and Recommendations

  1. This model issues a warning if the following condition is violated: H < 2(W+2S) (H, W, and S are parameters of CPW_SUB). Actually, this approach implies that H is large compared to W+S, so the warning alerts you that the substrate height is too small, initial model assumptions are not met, and the accuracy may degrade.

  2. The CPW1LINE model can be used as a (slower) alternative to CPWLINE, however, note that setting the substrate parameter Gnd=0 for CPW1LINE implies that a substrate is not backed by a perfect conductor, but is bounded by infinite air space. In this case modeling results are strongly affected by thin substrate heights and might differ substantially from the modeling results obtained from CPWLINE, that implements a common conception of coplanar waveguide. To match the results of these two models, set Gnd=0 and H > 2(W+2S).

Implementation Details

This model implements a technique developed in [1]; this approach implies that H is large, hence, actual modeling results do not depend on H.

Layout

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

To account for finite H, the impact of a grounded backing plane and/or metallic cover, EM Quasi-Static model CPW1LINE should be used instead of CPWLINE.

References

[1] W. Heinrich, "Quasi-TEM Description of MMIC Coplanar Lines Including Conductor-Loss Effects," IEEE Trans. Microwave Theory Tech., vol. MTT-41, January 1993, pp. 45-52.

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