Branch Line Coupler - CST2013 MWS Examples

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General Description

This example shows the extraction of a SPICE compatible network model based on the S-parameter calculation of a branch line coupler. The device consists of four strip line ports, coupled by two transversal bridges with each other. The energy excited at one port is coupled almost in equal shares to the two opposite ports.

Structure Generation

The strip lines as well as the substrate is modeled by defining basic bricks, whereby some parts are mirrored due to the symmetry of the structure. Using the local coordinate system offers the possibility to create the shapes relative to each other. The metallic ground plane of the device is represented by the electric boundary condition. Some outer space is added above the structure by defining a surrounding background material.

Solver Setup

The four microstrip lines are terminated by two multipin waveguide ports which propagate the fields into and out of the structure.

In order to obtain the SPICE model for the complete device all port modes are excited consecutively, however, due to the symmetry of the structure the definition of the corresponding S-Parameter symmetry condition reduces the calculation time by factor two.

The excitation signal is a Gaussian shaped pulse referring to the frequency range from 0 to 10 GHz.

Post Processing

The S-Parameters are available in the folder 1D Results in the navigation tree. Based on these S-parameters a SPICE compatible network model is derived by applying the MOR (Model Order Reduction) method. The extraction can be accessed via Post Processing: Signal Post Processing Network Parameters Model Order Reduction... In order to ensure an overall passivity of the model the corresponding check button is activated in the dialog.

A netlist is created and the calculated S-Parameters can be found in the folder Network Extraction in the navigation tree. The netlist is then used in CST DESIGN STUDIO as a "SPICE import block" and a SPICE calculation is performed. The resulting S-Parameters can be accessed via the S-Parameters entry in the navigation tree.