Regarding SP simulation for phase shifter (phase interpolator)

Dear all,

I would like to ask a question about S-Parameter (SP) simulation for phase shifter (phase interpolator) design. I am designing a 10GHz phase shifter using I/Q interpolation (so called, Cartesian phase shifter or phase rotator). In order to see the phase shift, I ran a transient simulation and it worked as expected.

However, when I ran a SP simulation, here is some simulation information,

1. There are two phase shifters: One’s output is reference and the other’s output is phase shifted.
2. The input port (port#1) generates an I signal, and the output ports (port#2 and port#3) are connected to output LC loads of two phase shifters. (Of course, I have one more input port for Q signal generation)
3. In order to see the phase difference of two phase shifter outputs, I checked the phases of S21 and S31. However, the phase difference between S21 and S31 is almost zero, which is not what I expected.

Hence, I am wondering if I cannot see the phase difference of the phase interpolation circuit with SP simulation or not. If it is possible, I am wondering what I did wrong.

For your information, when I designed a LC passive phase shifter, a SP simulation in the same manner worked fine.

Could you please advise?

Thank you so much!

Regards,

Jay