S-parameter basic in Spectre RF

Hi

I want to understand how basically spectre calculates s-parameters in 50-Ohm setup. Say I use port 1 and port 2 both 50 Ohms at input and output.

So for s11 measurement there should be no reflections at port 2. Since port 2 has 50-Ohm impedance does spectre ensures that output impedance of circuit under measurement is also 50 Ohm for zero reflections?

How does Spectre neutralizes reflections at either ports, which is a fundamental requirement for measuring S-parameters ?

Regards

Vipul