For long structures, the phase changes quickly as a function of frequency due to the group delay.
The group delay causes a linear change in the phase with frequency.
This can make it difficult to keep track of intervals of 2pi from the phase data in the S-parameter files
without using a large number of data points in the frequency domain using the standard S-parameter data file format that we showed for the Optical N Port S-Parameter element.
In these cases, an alternative method for formatting the S-parameter file data can be used, where we isolate the group delay, or linear contribution to the phase.
The group delay is specified as an additional parameter in the header section.
The third column of the S parameter data,which is the phase, then requires that the linear phase associated with the group delay be subtracted from the total phase.
The linear phase is 2 pi times the group delay times f minus f0, where f0 is the center frequency of the S-parameter frequency data.
For more details, see the example linked below this video.