N-Coupled Line Model

Most circuit design programs provide models for single and multiple-coupled transmission lines. However, it is often desirable to use EM simulated data in circuit design programs. These programs often provide transmission line models which utilize RLGC parameters. R, L, G, and C are the resistance, inductance, conductance and capacitance per meter of a transmission line. The RLGC parameters can be extracted from an EM simulation of a short section of the transmission line. They can then be used to model any length of line having the same cross-section.

Sonnet can export the RLGC parameters in a format compatible with the mtline component in Cadence Virtuoso Spectre.

Shown below is an example of a project composed of four transmission lines.

As in the circuit shown above, the input ports of your project must be numbered 1 through N and the output ports N+1 through 2N. The input of line M should be port M and its output should be port M+N. The software does not check for this condition, but issues a warning message if the number of ports is not an even number. The equivalent circuit is shown below for the four transmission lines shown above.

You may generate RLGC parameters automatically by setting up an output file in the project editor. To setup an output file in the project editor, select Circuit - Settings from the project editor main menu, then click on Output Files in the sidebar menu to display the Output Files page of the Circuit Settings dialog box. Click on the Add File button and select N-coupled Line Model from the drop list which appears. You may also generate an output file of RLGC parameters from previously generated response data in the response viewer. To do so, open your project in a Graph tab and select Output - N-coupled Line Model from the response viewer’s main menu. RLGC parameters are generated for each analysis frequency.

Shown below are the RLGC parameters in Spectre format for a two-line project:

After the Sonnet header, the format of the RLGC data is provided. This is followed by RLGC data and comments for each analysis frequency. Each comment line begins with the “;” character. Though the comment lines can be ignored, they contain useful modal information. Each value in the comment section has both a real and imaginary part. Characteristic impedance (Z0), propagation constant (γ), and the modal excitation vectors are provided for each mode.