I am a senior student in electrical engineering and for my thesis i am designing a pll. I have started with the design of the vco. However i am stuck because i can't get the same frequency when i use varactors (always applying the voltage that gives them the right capacitance) as when i design the oscillator at a steady frequency( 5Ghz). I've tried a lot of things, however i get frequencies much higher than expected. In order to achieve a 5Ghz frequency, i have to set large values to the varactors and the inductance(nH!!). I know that they are large enough as i have a frequeny of 5Ghz with values much lower (pF, pH). I attach a screenshot of the vco (only the varactors and mosfets). I suspect i have done something wrong with the wiring but i am not sure.
Does anyone have any idea?

I tried the swapping you mention, but i get no oscillation then, just noise.

hello,
When you swap also it should oscilate at least at some frequency. may be you forgot to add some intial conditions for start-up, or when you swap the capacitance will change probably it will increase and there is lot of loss at the lower frequency so hence no oscilations.

So one check is swap and increase the current, you will see oscilations.  
as rfidea told you should always find the capacitance  and resistance by using as AC simulations, add this to tank loss. However many times you will get more frequency in simulation compared to 1/(sqrt(LC)).

Thanks.

Actually for the last week no, because i am studying for exams; maybe from next week while studying for the GRE as well