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

Symbol

Summary

GCPWBCGG models a section of two asymmetric broadside coupled coplanar waveguides (strip widths may be unequal, gaps between each strip conductor and closest lateral ground may have unequal width, and CPWs may also be misaligned) 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 waveguides without a backing ground is also available by elevating the substrate above the backing ground.

Topology

Parameters

Name Description Unit Type Default
ID Name Text TL1
WT Width of top CPW conductor (nodes 1,3) Length W[1]
WB Width of bottom CPW conductor (nodes 2,4) Length W[1]
ST1 Gap width between top CPW conductor and lateral ground Length W[1]
ST2 Gap width between top CPW conductor and lateral ground Length W[1]
SB1 Gap width between bottom CPW conductor and lateral ground Length W[1]
SB2 Gap width betweenbottom CPW conductor and lateral ground Length W[1]
Offs Offset of bottom CPW conductor from top CPW conductor Length W[1]
L Line length Length L[2]
CLT Number of the substrate layer carrying top CPW conductor and lateral grounds   1
CLB Number of the substrate layer carrying bottom CPW conductor and lateral grounds   2
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 CMSUB is present in the schematic, this substrate is automatically used. If multiple CMSUB 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 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 (conductor thicknesses) must be a vector of size>=2. GCPWBCGG uses only the two first entries of vector T (the first is Ttop, and the second is Tbot; see the "Topology" section). For each CPW, T specifies thicknesses of the signal conductors as well as lateral grounds. It may be convenient to provide separate instance of GMSUB for GCPWBCGG.

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

CLT. Specifies the number of the dielectric layer that carries the top 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.

CLB. Specifies the number of the dielectric layer that carries the bottom 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.

Offs. Allows a shift of the whole bottom CPW either to the left side (Offs<0) or to the right side (Offs>0) .

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 of respective CPW.

  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 Gnd parameter 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 large at the conductor center; it decreases toward the conductor edges to reveal the charge distribution across the conductor. If the conductor width is too large it may cause the pulse size to approach zero for pulses close to the edge. In these rare cases the model may display a “Length of pulse #nnn equal to zero” error message.

Recommendations for Use

  • To approximate modeling of the coplanar waveguide without backing ground, set the GMSUB Gnd parameter 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.

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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