varactor tuned microstrip filter in ads

Hi, i have designed a parallel coupled bandpass filter with center frequency of 1.7Ghz in ADS, i have tuned it by connecting capacitors at the open ends of stubs. Can any one tell me how can i connect and bias a high frequency varactor diode instead of these capacitors in ADS as i cant find a varactor diode in lumped components. If one can provide with a ADS documentation or example for this i shall be very thankful.

regards.