Sigrity - Tip of the week: Coupling of Traces and Shapes in Coplanar Structure

The coplanar transmission-line structure has the signal trace and the return-path conductor on the same layer of the PCB. The signal trace, which is at the center, is surrounded by two adjacent outer ground planes.

This form of transmission lines is called “coplanar” because these four flat structures are on the same plane. The PCB dielectric is located underneath. Trace couplings to shapes next to a trace on the same layer constitute coplanar coupling.

To include coplanar coupling in impedance calculation, you need to turn on the coplanar option in the PowerSI tool.

In the PowerSI tool, go to Tools > Options > Edit Options > Simulation (Advanced) > Nets and Shapes and enable the Detect and model the coplanar traces coupling with field domains checkbox. Then, click OK.

When the coplanar detection option is checked, the edge of the coplanar shape will be converted into a fake trace with a max trace width of 1mm. It is one of the rules for coplanar checking in Cadence tools. If the width of a trace is beyond 1mm, its width will be reset to 1mm. The value of width is chosen as 1mm, as it is wide enough for modeling the coplanar effect.

As a result, if the trace has a 1mm width, you will not see any difference in the ERC result even if you convert trace to shape and turn ON Detect and model the coplanar traces coupling with field domains. You can only see the difference by changing the width of the trace to more than 1mm (say, 1.5mm) and then converting it to shape.

Here are two couples of traces (SIG2_N/ SIG2_P and SIG3_N/ SIG3_P) that are surrounded by two adjacent outer ground nets, and one couple of traces (SIG1_N/ SIG1_P) that is not surrounded by the ground net.

These are the simulated results of impedance when the traces are non-coplanar and coplanar.

There are differences in impedance results of  SIG2_N/ SIG2_P and SIG3_N/ SIG3_P nets when they are non-coplanar and coplanar due to the GND nets that are converted to shapes from the net. However, there is no difference in the results for SIG1_N/ SIG1_P, as there are no GND nets surrounding it and it is always non-coplanar.

To run a simulation of coplanar traces yourself and see impedance and coupling, click here to access the RAK on Coupling of Traces and Shapes in Coplanar Structure in PowerSI.

Team SimTech

Cadence Design Systems