Coupled Transmission Lines

Sometimes, your circuit may contain multiple parallel feedlines as shown in the circuits below:

There are two issues in the above examples:

1.  The ports are close to each other. When ports are close to each other, their fringing fields can couple to each other. If the ports are not calibrated at the same time, this would result in errors. Fortunately, Sonnet uses coupled lines for the calibration standards (by default), so all coupling between the fringing fields of the ports is removed in the de-embedding process (see the figure in Shared Reference Planes.)
2.  The feedlines are close to each other. When feedlines are close to each other, they can couple to each other. If you are using reference planes, this coupling should be removed. Since calibration standards are coupled lines, this coupling is removed by default.

Both of these issues are resolved by using “shared” reference planes. When the reference planes are set to “shared,” coupled lines are used for calibration standards, thus removing the coupling between the ports and the feedlines. The alternative to shared reference planes is “independent” reference planes. The table below summarizes which types of reference planes, shared or independent, are used for which type of ports:

The topics below discuss the implications of using shared or independent reference planes.

Shared Reference Planes

Independent Reference Planes