I found the FM transmitter below in a thread on this forum and I was wondering how can I calculate the frequency deviation? Or at least someone knows of any book where this specific calculation is explained?

Thanks

 

This is not a really good design since the frequency is derived from an LRC time constant, which varies with temperature and age.
The output of Q1 (audio) varies the base of Q2, which modifies the frequency of the radio by switching the current flow through L1 and R5 (probably affecting the charge on C4 through C7).

 

This is not a really good design since the frequency is derived from an LRC time constant, which varies with temperature and age.
The output of Q1 (audio) varies the base of Q2, which modifies the frequency of the radio by switching the current flow through L1 and R5 (probably affecting the charge on C4 through C7).

Thanks for your answer.

I see the problem you mention. However, just for the sake of knowing the math, do you have any idea of how to calculate the frequency deviation?

 

That would be difficult since in involves the change in capacitance of Q2 caused by the modulation signal and that's not easily calculated. A Spice simulation of the circuit would likely be the best way to determine the deviation.