redhat said:

here is the complete circuit; it is a vco

http://www.geocities.com/aezzat3/vco1.jpg
i don't know the capacitance of the varactor because it depends on (

the input voltage + Vcc-IR3) ,so how to know the resonant frequency

equation?

If you don't know the varactor capacitance, there is no way to

calculate a resonance frequency. The capacitance (as a function

of input voltage) would be found on a spec sheet, if it can be

found anywhere. Do you have a spec sheet for it? If not, have

you looked on the manufacturer's web site? Or searched with Google?

It might be helpful to know if an simplifiying approximation is

valid. Is the input signal small enough so that the capacitance

is largely determined by Vcc? Then you can use THAT capacitance

(as determined by the varactor specs) to determine the resonance

frequency.

If, on the other hand, the input alters the capacitance significantly,

then the behavior is highly nonlinear and probably a numerical

simulation is required to find the resonance. But doing that would

be well beyond the scope of what's describable in a newsgroup, and

possibly more time-consuming than measuring the resonance for a few

different values of L1 and C5.

Mark