Input impedance calculation using DFT for Radio Frequency

Hi

I am trying to find the input imepdance of an RF rectifier circuit by using FFT in cadence. The equation I'm using is 

                      Zin = DFT(Voltage)/DFT(Current) at the required frequency.

However, after evaluation cadence gives me only the magnitude of the impedance Zin.

How can I get both the real and imaginary parts after the above computation?

Thanks