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Axiem Rat Race Coupler Example
A rat race coupler is a common coupler circuit based on a circular structure with ports spaced along the circle. This example is taken from a publication in IEEE Microwave and Wireless Component Letters, vol. 17, no. 1, January 2007, pgs. 46-48, titled "Miniaturized Rat Race Coupler With Suppression of Spurious Passband", by Jen-Tsai Kuo, Juo-Shiuan Wu, and Yi-Chyun Chiou. This rat race coupler has flanges on the inner side of the circle to enhance the performance of the circuit.
AXIEM Simulation Details
The EM structure in this example suits an off-grid EM simulator like AXIEM well due to the angled shapes and the combination of large and small geometries. The layout for this structure was generated directly in the EM layout using Boolean operations to cut out the shapes on the inner side of the circle.
Auto Ports are used at each port location. Notice the text "A" next to the port. Double click on a port and see that the Explicit Ground Reference is set to Auto. This means that AXIEM will analyze the geometry around the port to determine the best port setup. Right click on the EM document and select Preview Geometry. A new view of the EM structure will open and notice how the ports are now displaying. There are two differences.
1. An extension plane was added to each port since there was enough room to add these extensions for deembedding accuracy.
2. The ground type of the port was set to Connect to Lower. You can tell this by the arrow next to the port pointing down. You can also double click on the port and the Explicit Ground Reference will be set to Connect to Lower.
Probably the most important setting for AXIEM is the Grid_X and Grid_Y set to 2 mils (double click on the Enclosure node under the EM structure and see the Enclosure tab). This gives the narrow lines a good edge mesh along the lines. Each of the 6 inner flange shapes has a mesh density set to low. This is done by selecting the shapes, right-clicking, selecting Shape Properties, clicking on the Mesh tab, and setting the Meshing Density to low. These settings were chosen after viewing the mesh and realizing the flanges could potentially be meshed coarser and still get a good answer. This changed the number of unknowns from approximately 4200 to 2500, so a significant change to speed up the simulation yet made very little different in the results. You can change the mesh setting for these shapes back to Normal to see that the answers are nearly identical.
This structure also uses AFS to give an accurate answer from 0 to 5 GHz with steps of 0.1 GHz in less than 20 frequencies simulated.
The inner flange shapes of the coupler have been parameterized to adjust their lengths. An edge length shape modifier has been added to each flange and the length has been assigned to the "L" variable that is visible in the EM document. You can change the L variable in the layout to adjust all of the flange lengths. This example has been setup to sweep the flange lengths. To enable the sweeps, open the EM schematic by first making the EM structure the active window, then select the View EM Schematic button from the toolbars. Enable the SWPVAR block. Before simulating, right click on the EM structure and select Preview Geometry and use the dialog to preview all the geometries to be simulated. Finally, simulate the project to see all the sweep points simulate.
The graphs in this project show various aspects of the coupler. The graphs starting with "InPhase" show the coupling magnitude and angle and the isolation magnitude using the coupler to split the signal in phase ( 0 phase shift).
The graphs starting with "OutPhase" show the same characteristics but using the coupler to split the signal out of phase (180 phase shift).
The "Match" graphs show the return loss at each port. The "Phase Delta" graph shows the difference in the phase for both situations, not that a measurement can do this directly, no need for equations.
The graph "Coupling using Measurement Variables" and the Output Equation "Measurement Variable Doc" demonstrate one way that measurement variables can be used.
You can easily change which S-Parameter is displayed on a graph by using measurement variables to define the To and From ports for an S-Parameter measurement and then tuning on those variables. Measurement variables must be defined in an Output Equations Document, and to use them in a measurement click the Use Vars button in the Measurement dialog box.