The reference impedance specified in touchstone file is the termination condition under which S-parameters are measured. It doesn't mean the S-parameter block has to be terminated by the exact same impedance in the actual circuit. It's perfectly fine to connect a S-parameter block measured at 50Ohm to a 75Ohm termination or any other network. Different terminations lead to different reflections and different responses. So in my opinion the definition of Rx circuit with s4p touchstone in BIRD119 is incomplete. It's like you have a capacitor in the circuit but you don't know how one of its two terminals is connected. Regards, Fangyi