I tried to measure the input impedance of differential input stage of mixer using s2p analysis in Cadence Spectre RF,

I got same Z11 in following cases

1) When the Port connected deferentially
2) Using AnalogLib ideal_balun
3) Using RFlib balun (with Balanced output impedance set to 25 Ohms),

with balanced output impedance of RFlib balun set to 50 ohms I get Z11 value half of that in cases 1) and 2) and for balanced output impedance set to 100 ohms I get Z11 value 1/4th of that is cases 1) and 2).

Could someone please let me know which is the correct test bench for measuring input impedance of differential LNA, I am trying to setup a test bench for differential LNA for simulating all it's performance parameters.

Thanks much in advance.