Parameterized Design - Differential Lines

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Parameterized Design - Differential Microstrip Transmission Lines



Abstract:This examples shows a typical parameterized differential microstrip transmission line design in EMPro.A differential transmission line is commonly used in high speed digital applications because of its outstanding noise performance. Differential pairs can be found in various applications such as printed circuit boards, connectors, and cables. Due to non-conventional signaling compared to commonly used single ended configurations, it is easy to make mistakes while setting up the design for EM simulations.

Figure 1: Differential Microstrip Transmission Line

Design Challenges

Setting up EM ports can be challenging. EM ports include parasitics and also interact with the design. With differential signaling, the port setup can be even more difficult. With multi-conductor applications like the differential pairs, it is recommended to use "modal" rather than "nodal" analysis. By downloading the example project from the link below, you'll see how proper port setup with a differential microstrip pair can be achieved, along with complete parameterization of the design. The example project also includes a Python script to convert single ended s-parameters to mixed mode s-parameters.

Figure 2 shows the parameters used in the differential pair.

Figure 2: Parameters for the differential pair

Figure 3 shows the Python script that converts single ended s-parameters to mixed mode.

Figure 3: Mixed mode conversion script in EMPro


The differential pair was analyzed using the FEM simulator in EMPro. The results below indicate good performance up to 20GHz.

Figure 4 shows full port single ended s-parameters.

Figure 4: Single Ended S-parameter Plot

Figure 5 shows mixed mode s-parameters.

Figure 5: Differential to Differential (SDD) Plot

Figure 6 shows FEM mesh on the copper and substrate. In this example, edge mesh was enabled only for the copper structure.

Figure 6: FEM Mesh

Also, note in the animation above, the best way to view the fields in the Advanced Visualization window is to select "Multiple Mode Excitation" and add a weighting of -1 to "Port 1 Mode 2".


In this example, a typical differential microstrip pair design was illustrated. The link below provides an example project, complete with proper port setup, parameterization, scripting etc., to easily characterize, optimize, and analyze the design.

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