simulate this circuit – Schematic created using CircuitLab
This is a minimalist FM transmitter I've found from http://www.talkingelectronics.com/projects/Spy%20Circuits/SpyCircuits-1.html
I'm a bit confuse about three things
1)How does this modulates the base input signal to frequency modulation ?
Because I don't see any change happening neither in Inductance nor in Capacitance of that LC tank oscillator. Is that because of C1 somehow ?
2) For what purpose capacitor C5 is used ?
Best Answer
The frequency of this generator is determined by \$C_2||C_{bc}\$ of the transistor. \$C_5\$ connects the \$C_{bc}\$ in parallel with L and C2.
As long as the junction capacitance is voltage dependent, changing the base voltage changes \$C_{bc}\$ and thus the resonance frequency of the oscillator.
Of course, the quality of this modulation is not very high, but for such simple device it is acceptable.