I wanted to know how is frequency modulated in this circuit. In some transmitters there is LC tank circuit on collector of transistor, so when audio is changed, then base-collector capacitance is changed too and LC resonant frequency is changed. But in this circuit LC tank circuit is between base and collector so could someone explain it to me? Thanks!

 

You're still modulating the operating conditions of the oscillator (by varying base bias) and any time you vary operating conditions you end up varying inter-element capacitances. It's not particularly linear, but it works.

 

The FM occurs because of Cd, diffusion capacitance of fwd biased BE junction.

It is proportional to current, inversely to voltage.

Regards, Dana.

 

You're still modulating the operating conditions of the oscillator (by varying base bias) and any time you vary operating conditions you end up varying inter-element capacitances. It's not particularly linear, but it works.

There's a name for that transistor stage, but I can't remember it - worth trawling a few ham radio sites.