S-Parameter Variables
This module automatically generates variables for the S-parameters. The port names (use numbers for sweeps to work correctly) determine the variable names. If, for example, there are two ports with the numbers 1 and 2 and Port 1 is the inport, the software generates the variables S11 and S21. S11 is the S-parameter for the reflected wave and S21 is the S-parameter for the transmitted wave. For convenience, two variables for the S-parameters on a dB scale, S11dB and S21dB, are also defined using the following relation:
The model and physics interface names also appear in front of the variable names so they can vary. The S-parameter variables are added to the predefined quantities in appropriate plot lists.
In addition to the S-parameter variables, variables are also added for voltage standing wave ratio (VSWR) when using a single port, two-port ABCD-parameters when using two ports, and reflection coefficient with multiple excitation when all ports are excited, as shown in the table below:
Name
Description
emw.VSWR_N
VSWR, port N
emw.ABCD
ABCD-parameter, full 2-by-2 matrix
emw.ABCD_A
ABCD-parameter, A
emw.ABCD_B
ABCD-parameter, B
emw.ABCD_C
ABCD-parameter, C
emw.ABCD_D
ABCD-parameter, D
emw.Gamma_N
Reflection coefficient with multiple excitation, port N
In the table above, emw is the tag for the physics interface. This tag can be different for different physics interface instances. N is the name of the port (a number).
The VSWR variable is generated when a model uses and excites a single port.
The ABCD-parameters are produced when two ports are employed. When a port sweep is conducted, the full 2-by-2 S-parameter matrix is transformed into the ABCD-parameters using a 50 Ω reference impedance. If not, the network is assumed to be reciprocal, with S21 = S12 and S11 = S22. The hypothetical full S-parameters are then converted to ABCD-parameters accordingly.
The reflection coefficient with multiple excitation variables are useful for examining the inflow back to the excitation port, especially when multiple ports are excited simultaneously, as seen in phased antenna array applications.