CST Design Studio can handle either Spice or Ibis models. Non linear models like yours should be taken care of in ADS or AWR, for instance. Maybe you can get it analyzed under  ADS/AWR (harmonic balance) and export the results (Spice Model or touchstone file) to CST.

I've successfully modeled these varactor diodes in ADS and HFSS.  Assuming you are operating strictly under reverse bias, you can replace the ideal varactor diode in that spice model with a capacitor.  Skyworks gives you the total capacitance of the diode for a range of reverse biases.  Calculate the variable capacitance in that model so that the total capacitance of the diode including the parasitic capacitance is as given by the datasheet. 

HFSS/ ADS are best options to simulate this design. Where CST can not provide the option to incorporate Active element. You may use AWR also. 

You can simulate non linear elements using CST by choosing  the circuit elements from the Block Selection Tree of CST Design Studio.

Kindly find design example for RLC circuit then you can apply same steps for your circuit.

Thanks everyone for your valuable suggestions.

I wanted to know about how do we get the equivalent spice model of the diode and also about the TRL Calibration method for de-embedding. Kindly suggest some good reading papers for the same.

Hi, How to choose variable capacitor in CST.I checked it in the circuit element, but I didn't find it. Need help in these regards.

Shahid Afridi

There is no option in the frame prperties to do, but you can write all values on which you want to sweep, as a vector, in CSV or TXT file that upload it using circuit

file option with the corresponding relative path,

Thank u so much.