Why TDR impedance varies for my Transmission line and corresponding s-parameter file

Hi,

I have created a differential channel with microstrip transmission line with CILD. I have also extracted the s-parameter file (.s4p) file for the corresponding transmission line. I actually will need this .s4p file to use in cadence. I was checking the TDR impedance of the transmission line and the of the extracted s-parameter file of my transmission line. Somehow it doesn't match in the beginning.

My s-aparameter touchstone file format generated in ADS using conventional method of 4 port of 50 ohm each.

My test bench to compare the TDR impedance varies for my Transmission line and corresponding s-parameter file:

S-parameter port mapping:

The resulting TDR impedance plot of both:

You can see, till 4ns, the impedance is not same, but I used the same equation to get the impedance. Can anyone help me to know why did this happened and what to do to fix it?

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