Guidelines for Inserting N-Ports

Before inserting an N-port in a project, review the following information:

• The S-parameter file for the N-port must be in one of the following formats:

  .szg	Planar EM and Ansys HFSS version 6 and later and Maxwell Strata from version 1.1.
  .s n p, .tou, .y*p, .z*p	Touchstone .snp, .tou, .y*p, and .z*p format files. For .snp files, the number of ports in the S-parameter file is indicated by n (e.g., a Touchstone file with one port has the extension *.s1p). If the S-parameter file has more than 10 ports, the *.tou extension is used.
  .flp	HFSS version 2014.
  .cit	CITIfile version 1

• N-ports are assumed to be non-radiative. The coupling between the N-ports and the rest of the circuitry is also ignored. Therefore, if you generate an output far-field or near-field file for a structure containing N-ports, the results are contain the radiation on the N-ports.
• Be aware the order in which you connect the model ports to the N-port ports affects the solution. When you select the ports to connect to the N-Port in the Layout window, the order in which you select them determines the order of port connection. It is very important to connect the right ports since most active device S-parameters are not symmetrical.
• If your N-port is an active or nonlinear device and you are inserting the device along a section of microstrip line, it is very important the dimensions of the microstrip are as precise (as close to the actual, physical model) as possible. This precision increases the solution’s accuracy when connecting the complex S-parameters of the N-port to the overall circuit.
• The simulation frequency must be in the range of frequency specified in the N-port S-parameter file. If the simulation frequency is out of range, an error message appears and the simulation is terminated. You need to either change the simulation frequency or update the N-port S-parameter file to include new frequency data.

If the simulation frequency does not coincide with any of the N-port frequencies, HFSS approximates the N-port S-parameters at the simulating frequency by interpolating them. The accuracy of the interpolation may deteriorate if the S-parameters contain sharp nulls or peaks. So for N-ports that resonate, ensure there are enough data points in the N-port S-parameters near the resonance.

• N-ports cannot be connected to coupled mode, slotline, or CPW ports.
• If you modify the S-parameter file of an N-port, you must update the N-port in the structure by re-importing the file to use the updated data.