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Asymmetric Coplanar Line (EM Quasi-Static): CPWALINE

Symbol

Summary

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

Topology

Parameters

Name Description Unit Type Default
ID Name Text CP1
W Conductor width Length W[1]
S1 Left gap width Length W[1]
S2 Right gap width Length W[1]
L Conductor length Length L[2]
Acc Accuracy parameter   1
CPW_SUB Substrate definition Text [3]

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

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

[3] 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.

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

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.

References

[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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