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2 Asymmetric Coupled Coplanar Lines on Multilayer Substrate (EM Quasi-Static): GCPW2LNA



GCPW2LNA models a section of two asymmetric coupled coplanar waveguides (strip widths may be unequal and gaps between each strip conductor and closest lateral ground may have unequal width) on a multilayer dielectric substrate. The substrate can be suspended and/or shielded by the optional metallic cover. This model allows for an arbitrary metal thickness of signal conductors and lateral ground planes. A backing ground plane is inherent to this model. Approximate modeling of coplanar waveguide without a backing ground is also available by elevating the substrate above the backing ground.



Name Description Unit Type Default
ID Name Text TL1
W1 Conductor #1 width (nodes 1,3) Length W[1]
W2 Conductor #2 width (nodes 2,4) Length W[1]
S1 Gap width between conductor #1 and lateral ground Length W[1]
S2 Gap width between conductors #1 and #2 Length W[1]
S3 Gap width between conductor #2 and lateral ground Length W[1]
L Line length Length L[2]
CL Number of the substrate layer carrying conductors and lateral grounds   1
Acc Accuracy   1
GMSUB 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 GMSUB is present in the schematic, this substrate is automatically used. If multiple GMSUB substrate definitions are present, you must specify.

* indicates a secondary parameter

Parameter Details

GMSUB. Supplies parameters for multilayer dielectric substrate, conductor thickness, conductor metal properties, the presence/absence of metallic cover, and the cover height above the substrate. The GMSUB Cover parameter allows the addition/elimination of an infinite metallic plate acting as a cover/shield. Setting the Gnd parameter equal to "Suspended substrate" allows elevation of the dielectric stack above the backing ground. The elevation gap is filled with air and its height is specified by the HB parameter. A suspended substrate may be used for approximate modeling of CPW without a backing ground (see the "Recommendations for Use" section).

Note that the GMSUB T parameter is actually a vector, although GCPW2LNA uses only the first entry of this vector so you may enter the value of T as a scalar. T specifies the thickness of the signal conductors as well as both lateral grounds. It may be convenient to provide separate instance of GMSUB for GCPW2LNA.

Acc. The default value for Acc is 1. If Acc is less than 1 or greater than 10 it is auto-set to 2.

CL. Specifies the number of the dielectric layer that carries the signal conductor and lateral grounds. Layers are numbered from top to bottom. Note that if Cover is present the dielectric layer adjacent to the cover is excluded from the layer count. If the substrate is suspended, the air layer under the substrate is included in the layer count.

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. Lateral grounds are not PEC, they are assumed to be made of the same material and have the same thickness as signal conductors.

  2. 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.

  3. Modeling results are strongly affected by the substrate height and might differ substantially from modeling results obtained from models that implement the common conception (infinite thickness substrate) of a coplanar waveguide (for example, CPWLINE). A backing ground is inherent to this model (the impact of a backing ground may be substantially reduced by setting the GMSUB parameter Gnd equal to "Suspended substrate". See the "Recommendations for Use" section for details) .

  4. This model uses the GMCLIN model so some warning/error messages may originate from GMCLIN.

  5. To apply Method of Moments for analysis, a quasi-static model creates 1D mesh covering cross-sectional 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.


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 approximate modeling of the coplanar waveguide without backing ground, set the GMSUB parameter Gnd to "Suspended substrate" and set the HB parameter to a value equal to 2...3 of the total thickness of the multilayer dielectric stack.

  • 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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