S-parameter files describe electrical n-port network models of physical devices as a tabular list of complex s-parameter values at specific frequencies. S-parameter files can model fixtures, cables, devices, or probes in your circuit. Formatted as ASCII text, s-parameter files are easily viewed or edited using any text-based file editor. S-parameter files can be generated with an 86100D, vector network analyzer, or circuit simulator. The InfiniiSim operators can import:

• 4-port or 2-port files in 4-Port Block Port Type mode
• 4-port, 2-port, and 1-port files in dual 2-Port or dual 2-Port Differential Block Port Type mode

By default for 4-port devices, the operators expect ports 1 and 3 to be on the left side and ports 2 and 4 to be on the right. However, this may not align with the way the data is formatted in your s-parameter file. You can correct for this when importing the file by using the Flip Model and 4 Port Numbering selections.

File Requirements

Two standard S-parameter file formats are supported: Touchstone (version 1.1) and CITIfile (version A.01.00 and A.01.01). Touchstone files are identified by the file extensions .s1p, .s2p, or .s4p. CITIfile files are identified by either .cti or .cit. Only CITIfiles which contain a single data package (each file may model only a single device) are supported.

Simulation operators require model information at DC (0 Hz). However, s-parameter files often do not contain DC values, especially if they originate from network analyzer measurements. When files do not contain DC values, the application, by default, copies the file's lowest frequency values to 0 Hz and set their phase components to zero. This techniques may not always yield appropriate DC s-parameter values. Ideally, you should measure or simulate the true DC values and manually add them to the s-parameter file with a text editor.

InfiniiSim operators require s-parameter values that are uniformly spaced in frequency with a spacing or resolution that is small enough to accurately represent the slowest time constants and time delays exhibited by the modeled device. However, this does not mean that all s-parameter files must contain uniformly spaced frequency data. Uniformly spaced file data is usually preferred, but not required if you are knowledgeable about how InfiniiSim interprets file data.

InfiniiSim expects s-parameter files to list their values in monotonically increasing frequency order and ignores those values at which frequencies are less than or equal to the frequencies of previously listed values. InfiniiSim also truncates all file data to a maximum of 100,000 frequency points.

InfiniiSim calculates transfer functions using only full 2 port (in 2 port analysis mode) or 4 port (in 4 port analysis mode). However, those blocks that model source impedance or load impedance (transmitter source, scope/probe, or scope input) can be defined by s-parameter files which contain only return loss information. In the absence of the special InfiniiSim keyword RETURN_LOSS_INTERPRETATION, InfiniiSim assumes that the return loss data models a simple passive shunt RLC circuit and calculates the insertion loss of the same circuit that would produce the file data.