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Microstrip Interdigital Capacitor, (No Steps at Ports) (EM Quasi-Static): MICAP1

Symbol

Summary

MICAP1 models a microstrip interdigital capacitor that connects to microstrip lines without width steps. Model takes advantage of EM Quasi-Static coupled lines approach to consider interaction between all fingers.

Topology

Parameters

Name Description Unit Type Default
ID Element ID Text M1
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
W1 Width of the feeding line at port 1 (Read-Only parameter) Length W[1]
W2 Width of the feeding line at port 2 (Read-Only parameter) Length W[1]
MSUB Substrate definition Text MSUB1[2]

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

[2] Modify only if schematic contains multiple substrates. See the “Using Elements With Model Blocks” for details.

Parameter Details

W1, W2. Parameters W1, W2 are read-only (output) parameters, represent the capacitor total width at ports 1 and 2 and are evaluated in the model as N(W + S)-S.

Parameter Restrictions and Recommendations

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

  2. Conductor thickness is set via substrate parameters.Model doesn't impose restrictions on thickness except requirement to be non-negative.

Implementation Details

EM Quasi Static technique allows user to model microstrip interdigital capacitor with wide range of conductor thicknesses.

This model accounts for the effect of phase shift along the microstrip line that connects fingers. It also includes the effect of the presence of width step at one port.

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

The model should be used for microstrip interdigital capacitors that connect to feeding microstrip line without width step. If this is not the case and the emphasized metallic strip that connects end of fingers at input ports is present at both sides (or at one side) it is recommended that model MICAP (or MICAP2) is used instead MICAP1. MICAP is geared toward interdigital capacitor that connects to microstrip lines having widths that are small comparing to capacitor width (less than 50% of capacitor width); MICAP2 has width step only at one port.

The model 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. It means that during the first evaluation of schematic the most time consuming intermediate parameters for each capacitor instance are being stored in memory cache. Each interdigital capacitor model checks this cache looking for its duplicate. Duplicate capacitors copy the appropriate parameters from memory cache saving substantially on their recalculation.

Layout recommendation: The MICAP1 layout cell does not draw any lines between the tips of fingers because it implies that the fingers protrude directly from the ends of lines attached to MICAP1 ports. You can evaluate the widths of these lines either using a simple formula (see "Parameter Details") or using the values of the MICAP1 output parameters W1 and W2.

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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