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Microstrip Interdigital Capacitor on Multilayer Substrate (Aggregate): MMICAP

Symbol

Summary

MMICAP models a multilayer microstrip interdigital capacitor that has an emphasized transverse metallic strip connecting fingers at both input ports. All conductors making up the capacitor may be placed on any dielectric layer from substrate stackup. MMICAP takes advantage of the EM Quasi-Static coupled lines approach to considering interaction between all fingers.

MMICAP$ is the corresponding intelligent cell (iCell). An iCell model is identical to its non-iCell equivalent with the following exception: Certain dimension-related parameters are not explicitly user-specified; rather, they are automatically and dynamically determined by the dimensions of the attached elements. See “Intelligent Cells (iCells)” for a detailed discussion of how to use iCells, their advantages, and their limitations.

Topology

Parameters

Name Description Unit Type Default
ID Element ID Text MI1
W Finger width Length W[1]
S Spacing between adjacent fingers Length S[1]
G End gap width Length S[1]
L Length of the overlap region of the fingers Length L[1]
N Number of fingers   4
WP Width of the fingers transverse interconnect Length W[1]
W1 Width of the feeding line at port 1 (for layout cell only) Length W[1]
W2 Width of the feeding line at port 2 (for layout cell only) Length W[1]
NL Number of dielectric layer carrying conductors   1
GMSUB Substrate definition Text GMSUB1[1]

[1] User-modifiable default. Modify by editing under $DEFAULT_VALUES in the default.lpf file in the root installation directory. See the AWR Microwave Office Layout Guide for details.

Parameter Details

W1. This parameter is secondary for the MICAP$ iCell model.

W2. This parameter is secondary for the MICAP$ iCell model.

GMSUB. Supplies parameters for multilayer dielectric substrate: The number of layers Nlayer, conductor thickness T, conductor metal properties, the presence/absence of metallic cover, and the cover height above the substrate. The GMSUB 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 the HB parameter specifies its height. Note that the GMSUB T parameter is a vector. Although MMICAP uses only one entry of T for all conductors, all Nlayer entries must be filled (Nlayer+1 in case of Gnd set to "Suspended substrate").

Parameter Restrictions and Recommendations

  1. The number of fingers N must be 2 ≤N≤ 16.

  2. Conductor thickness is set via substrate parameters. MMICAP does not impose restrictions on thickness except for the requirement to be non-negative.

  3. Parameters W1 and/or W2 have no effect on simulation results, they are provided for layout drawing only. However, neither of them should exceed capacitor width, namely N*(W+S)-S.

  4. The number of dielectric layer carrying capacitor conductors NL must comply with the rules of numbering GMSUB layers. Parameter NL sets the number of the dielectric layer (specified at GMSUB) that carry capacitor conductors on top. If the GMSUB Cover parameter is set to "Metallic Cover" and the capacitor is on top of the dielectric stack, then NL=1 (the dielectric layer just under the cover is not included in the count). If the GMSUB GND parameter is set to "Suspended Substrate" and the capacitor is on the bottom of the dielectric stack, then NL=N+1, where N is the number of layers in the dielectric stack specified by the GMSUB N parameter.

Implementation Details

The EM Quasi-Static technique allows you to model a microstrip interdigital capacitor on multilayer substrate with a wide range of conductor thicknesses. Capacitor position within the substrate layers is specified by layer number NL.

MMICAP accounts for the effect of phase shift along the microstrip line that connects fingers. It also includes the effect of the presence of width steps at ports.

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 the conductor edges to reveal the charge distribution across the conductor. If the conductor width is prohibitively 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.

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.

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

MMICAP accounts for losses in metal and in substrate dielectric. Dispersion is partly included.

To decrease the calculation time for schematics that contain several microstrip interdigital capacitors, cache is implemented for this model. This means that during the first evaluation of a schematic the most time-consuming intermediate parameters for each capacitor instance are stored in memory cache. Each interdigital capacitor model checks this cache looking for its duplicate. Duplicate capacitors copy the appropriate parameters from the memory cache, saving substantially on their recalculation.

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

References

[1] 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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