Ports with Negative Numbers

Ports may also have negative numbers as shown in the figure below. This feature can be used to redefine the ground reference of a port. Strictly speaking, em sums the total current going into all the positive ports with the same port number and sets that equal to the total current going out of all the ports with that same negative port number. For example, for a circuit with a +1 port and a -1 port, em sets current flowing into port +1 to be equal to the current flowing out of port -1. These are differential mode ports (sometimes called “balanced”, “push-pull” or “odd-mode” ports). An example is shown below.

Coplanar lines can be represented with differential ports. See below for an example of differential ports.