Discrepancy between s-param and z-param in spectre

I am trying to characterize a 2-port network using spectre. Both the input and output impedances of this network is supposed to be 200ohm in the frequency band of interest.

So I made two identical instances of this 2-port network. For the first instance, I connect PORT1 to the input and PORT2 to the output. For the second instance, I connect PORT3 to the input and a 200ohm resistor to the output. PORT1, PORT2, and PORT3 are all set to 200ohm.

After I ran SP simulation, I plotted S11, S33, and the real parts of Z11 and Z33. The curves for S11 and S33 are identical.(They overlap) However, the curves for Z11 and Z33 vary significantly. Z33 ranges from 140~260ohm, which makes sense. However, Z11 varies from 60~1100ohm. And the shape Z11 does not resemble that of Z33.

How can this be? If S11 and S33 are identical, shouldn't Z11 and Z33 be identical too?

That's because of the definitions of S parameter and Z parameter. In a two port, S11 means the input reflection coefficient when the output is matched; and Z11 means the input impedance when the output is open.

In your case, for the first instance, you connect TERM at both port, which means to do simulation for a two-port. As defined, the consequent Z11 means the input impedance when output is open; But when you connect a ture resistor at the output, the simulator will deal with a single-port(The output is terminated), the Z11 means the input impedance when the output is connected to a 200ohm resister.

With respect to S parameter, both instance means the input reflection coefficient when output is connected to a 200ohm resister(identical with the reference impedance).

That's right. Thanks a lot.