Defining a Differential Port

source: Ports in Momentum

Defining a Differential Port

Differential ports should be used in situations where an electric field is likely to build up between two ports (odd modes propagate). This can occur when:

• The two ports are close together
• There is no ground plane in the circuit or the ground plane is relatively far away
• One port behaves (to a degree) like a ground to the other port, and polarity between the ports is developed.
• The ports are connected to objects that are on strip metallization layers.

The electric field that builds up between the two ports will have an effect on the circuit that should be taken into account during a simulation. To do this, use differential ports.

Differential ports have the following properties:

• They can be applied to objects on strip layers only.
• They are assigned in pairs, and each pair is assigned a single port number.
• Each of the two ports is excited with the same absolute potential, but with the opposite polarity. The voltages are opposite (180 degrees out of phase). The currents are equal but opposite in direction when the ports are on two symmetrical lines, and the current direction is approximated for other configurations.
• The two ports must be on the same reference plane.
  

  Note Port numbers for differential ports are treated in the following manner: On the layout, you will continue to see the port numbers (instance names) that were assigned to each port when they were added to the layout. Use the Momentum Port Editor dialog box to identify which pair of ports will be treated as a differential port. When Momentum simulates designs containing non-consecutive port numbers, the ports are remapped to consecutive numbers in the resulting data file. The lowest port number is remapped to 1, and remaining numbers are remapped in consecutive order. The port numbers are not changed in the design itself . A message in the Status window announces the change, and lists the mappings. For example, if you are simulating a design with ports numbered 1 and 3, the following status message informs you of the changes: Layout has non-consecutive port numbers. Output files will have consecutive port numbers. layout port -> output port 1 -> 1 3 -> 2 Also, when you view results, you will see S-parameters for the differential port numbers. In the example above, the layout would show p1, p2, p3, p4. The S-parameter results will be for combinations of the original P1 and P3 only.

  To define a differential port:

1. Choose Momentum > Port Editor .
2. Select the port that you want to assign this type to. Note the port number.
3. In the Port Editor dialog box, under Port Type, select Differential .
4. Under Polarity, make sure that Normal is selected.
5. Click Apply .
6. Select the second port.
7. In the Port Editor dialog box, under Port Type, select Differential .
8. Under Polarity, select Reversed .
9. Under Associate with port number, enter the number of the previously-selected port.
10. Click Apply .
11. Repeat these steps for other differential port pairs in the circuit.
12. Click OK to dismiss the dialog box.