how to set up vco simulation in cds?

Since i wanna simulate a diff vco, how to connect the fudemental freq output port and the reference port?
I have referred to the diff vco example in the spectreRF user's guide & connected one current pulse to the reference port,but the fundemental freq i have got differ from the freq i designed.If i changed the rise time of the current pulse,different fundemental freq i got,so how can i?
who can tell me how to set up the right scheme vco simulation in cds?thx

can anyone help me to set up pss simulations of vco in cds?

help me!