Go to www.awrcorp.com
Back to search page Click to download printable version of this guide.

2 RFIC Coupled Microstrip Lines (FEM Quasi-Static): FM2CLIN



FM2CLIN models a section of two edge-coupled microstrip lines situated either atop or within insulating layers located on top of a conducting substrate. This stack is typical for many RFIC manufacturing technologies. A backing ground plane is always present. Optional perfectly conducting straps may be placed at both sides of the line at an arbitrary level to model lateral ground. FM2CLIN can account for the presence of a conducting (high-doped) layer between insulating layers and the conducting substrate.



Name Description Unit Type Default
ID Element ID Text TL1
W1 Width of conductor #1 Length W[1]
W2 Width of conductor #2 Length W[1]
S Spacing between conductors Length W[1]
L Conductor length Length L[1]
MRFSUB Substrate definition Text MRFSUB1[2]
*IsGndStrap Ground straps presence (Switch No/Yes)   No
*GndGap Distance between ground strap and inductance edge Length 10
*GndWid Width of ground strap Length 10
*GndLevel Height of ground strap above substrate Length 10
*IsHDLl High-doped layer presence atop of substrate (Switch No/Yes)   No
*H_HDL Bulk conductance of high-doped layer Siemens/m 700
*Cond_HDL Height of high-doped layer Length 2 um
*SaveToFile Switch "Save to txt file"=Yes/No   "No"
*FileName Name of text file with computed model parameters String Same as model name

[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 “Using Elements With Model Blocks” for details.

* indicates a secondary parameter

Parameter Details

MRFSUB. See the MRFSUB documentation. Note that notations H1, H2, H3, Er1, Er2, Er3, Tand1, Tand2, Tand3, Sig3 on the cross-sectional view above are MRFSUB respective parameters.

SaveToFile. The parameter is hidden by default and set to No. You can toggle this parameter to Yes and No. When set to Yes, the model creates a text file (the default is "model_name.txt") at the current project location. This text file contains characteristic impedances, propagation constants, effective dielectric constants, and values of RLGC line parameters at each project frequency. Each row contains respective values computed at the frequency specified in the first column (frequency in GHz, R in ohms/m, L in H/m, G in S/m, C in F/m).

This model implies that in the general case (W1 ≠W2), two modes, namely, C and P exist in a modeled structure (see [2]). Note that in a symmetrical structure (W1=W2) C-mode corresponds to even mode and P-mode corresponds to odd mode.

FM2CLIN outputs complex characteristic impedances Re(Zc1), Im(Zc1), Re(Zp1), Im(Zp1), Re(Zc2), Im(Zc2), Re(Zp2), Im(Zp2), complex effective dielectric constants Re(EeffC), Im(EeffC), Re(EeffP), Im(EeffP) (see traditional effective dielectric constants in columns 38, 39), and losses for C and P modes LossC (dB/m), LossP(dB/m) to columns 2-15. Entries of R, L, G, and C matrices are distributed among columns 16-31 in respective order. Columns 32-37 contain Re(Rc), Im(Rc), Re(Rp), Im(Rp), BetaC, BetaP where Rc is the ratio of C - mode voltage in the second line to C - mode voltage in the first line; Rp is the same ratio in P-mode case (see details in [3], section 4.3.1); BetaC and BetaP are propagation constants of C- and P-modes in Rad/m. Note that columns 38 and 39 contain traditional effective dielectric constants ErC_Eff and ErP_Eff (they do not account for losses). The total number of columns in the text file is 39.

The text file that is created might be linked or imported to a project as a data file and frequency behavior of any previously mentioned parameter may be viewed using the proper data measurement. Note that the first column (frequency) is always in GHz so these measurements might be incompatible with other Cadence® AWR® Microwave Office® software measurements placed on the same graph; thus, you may prefer to place the aforementioned data measurements on a separate dedicated graph.

FileName. By default this parameter is hidden and is set to FM2CLIN.TXT. You can change the file name for each model instance to an arbitrary name with a length not exceeding 64 symbols.

Parameter Restrictions and Recommendations

  1. There are no limits on T except possible aggravations due to an overly thin T (see "Implementation Details"). Note that the conductor must stay within the passivation layer so the conductor thickness T must be less than 0.95*H1.

  2. To exclude passivation, set MRFSUB parameters Er1=1, Tand1=0. Assign a value to H1 that well exceeds T to avoid an overly thin air layer between the top of the conductor and the top of the mock "passivation" layer (see "Implementation Details").

Implementation Details

The 2D Finite Element Method (FEM) in conjunction with the quasi-static problem formulation provides a very stable solution for RFIC's most common dimensions and frequency range. The FEM engine is partially based on the FEMM solver ([1]); it comprises a mesher ([2]) and two independent solvers: an electrical solver and a magnetic solver. The mesher generates a mesh that covers the entire cross-section including a cross-section of conductors. Usually, this method consumes a reasonable amount of computing resources; however, in certain situations memory consumption and run time may escalate because specified dimensions force the mesher to generate overly dense mesh. Commonly, this happens due to an error in the parameter specification that results in creation of extremely narrow layers.

The following figure demonstrates the typical distribution of an electric current across a conductor cross-section and contours of magnetic potential (conductors are at 1A of impressed current each). Note that this model provides mesh fine enough (and large enough) to reveal small details of skin effect and current crowding.

This figure demonstrates the distribution of an electric field and equipotential lines (conductors are at potentials 1 and 0). Note how the closely located ground strap affects field distribution.


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

Computation time substantially grows with mesh size. Mesh inside conductors may heavily contribute into total mesh size because size of a mesh cell inside conductor is governed by value of skin depth at the highest simulation frequency. Mesh does not vary with frequency so the same fine mesh works at lower frequencies. Frequency-independent mesh results in improved stability and consistency but with some sacrifice in solution time.

NOTE: The implementation of FEM Quasi-Static models relies on temporary intermediate text files. This model creates these files temporarily in the project directory and subsequently deletes them. There may be several dozen files and they may use up to 100 MB of disk space, so ensure that your hard drive has sufficient free space.


[1] FEMM (by David Meeker) home page: https://www.femm.info/wiki/HomePage

[2] Jonathan Richard Shewchuk. Triangle. A Two-Dimensional Quality Mesh Generator and Delaunay Triangulator. Follow this link for information and download: http://cs.cmu.edu/~quake/triangle.html

[3]R. Mongia, I. Bahl, and P. Bhartia, RF and Microwave Coupled-Line Circuits, Artech House, Norwood, MA, 1999.

Legal and Trademark Notice