For example:
c2=22pf and the capacitor parallel to varicap value Cpar=100pF.
But you have to calculate these capacitors so that the frequency deviation does not exceed 75 kHz. Another way of linearizing is to use two counter-connected varicaps.

 

I'll give you a tip. The C2 capacitor is too large. This increases the depth of modulation, but introduces great distortion. On the varicap, the voltage changes a lot and therefore the capacitance changes a lot. It is possible and necessary to reduce the carrier frequency level on the varicap. This is done by reducing the C2 value and connecting a capacitor parallel to the varicap. This will reduce the distortion.

The capacitance change of a varicap depends on what model is used, and that in turn is a designers selection. There are a whole lot of considerations needed in designing an FM oscillator circuit, this post started with a request for help by one who was not really familiar with those considerations and was not getting it to work at all. To get a circuit to provide the desired functionality it is important to understand what is going on in that circuit. Getting a random circuit to function well on the first try without understanding it requires a great deal of luck.

 

thank for all.
my post began for a circuit( in multisim) that i find in a book for communication system (modern communicatin pearson edition)

i sent an email to author but i cant find a solution .

thank you again for all suggestions !!
George