how to simulate input impedance of a voltage doubler in ads

Hi,

I have designed a voltage doubler circuit (for RFID application) using ADS. I would like to find out what is the input impedance of the circuit at different input power and frequency so that I can proceed with the design of an impedance matching network which will be placed in between a 50 ohm antenna and the voltage doubler. May I know how can I do that in ADS? Could anyone please help? Thanks.

Use the s-parameter simulation.
Go to Simulate>smart simulation choose 2 port network.
Once you finish simulation you can check the s11 smith chart, use the marker to go to the required frequency, It gives you s11 as well as zin