Differential port S-parameter calculation

hello all,
i am tring to find s-parameters of pcb traces using ADS-momemtum. it has total 24 ports out of which 8 ports are differential one . when i associate the the differential ports to each other using port editor option, then after simulation it gives 20 port result. but i have to use s-parameter file further with 24 ports. Then how can i find out s-parameters of differential ports. and why we make the impedance of associated ports 100 ohm while the single ended are having 50 ohms.

Thanks for help in advance.....

You can simulate with 24 standard ports. Then, the differential and common mode response of lines is included in your results, so this makes sense.

To have a look at specific differential mode results from that 24port file file, you can always set up a test bench with ADS ports connected differentially between two ports of the S-parameter data set.

The 100 ohm of a differential port is how the programmers implemented this (two 50 ohm ports in series). In other software like Sonnet, this is implemented differently. Both is valid, as long as it is properly documented.