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