Working of LC as a vco..

I am attaching a Fm transmitter circuit here. The common method of frequency modulating and transmitting is by connecting a L-C(tank) circuit to the collector of a transistor. The message signal is provided to the base. There is a capacitor between collector and emitter for cancelling harmonics.

Now for frequency modulation,the frequency changes only by varying either L or C. In this circuit which one varies and how are we getting frequency variation according to the message signal?

Both inductance and capacitor will not vary as per the circuit,but still we are getting the frequency modulation. Can anyone explain why it is so?? This circuit really works,but dont know how.. Also where is the much needed feed back path??

Please answer this question as we are discussing about this for a long time,but yet to get one solid explanation.. Please anyone help me.

Frequency of an LC oscillator is inherently unstable. If you vary the
9V supply, the frequency will change. If you change the load (antenna
length, in this example), frequency will change.
As the audio is applied to the base of the oscillator transistor, its bias
will change and its current will change, which leads to change of
interelectrode capacitance values: Cbc, Cbe, etc.
You may consider the capacitor between Collector and Emitter is the
feedback element.
Regards,
S. H.

Hi
Modulating signal will vary Cbe of the transistor and so the frequency of oscillation will vary.
We made this circuit four years ago

Thanks
BR
Haytham

shwoo said:

Frequency of an LC oscillator is inherently unstable. If you vary the
9V supply, the frequency will change. If you change the load (antenna
length, in this example), frequency will change.
As the audio is applied to the base of the oscillator transistor, its bias
will change and its current will change, which leads to change of
interelectrode capacitance values: Cbc, Cbe, etc.
You may consider the capacitor between Collector and Emitter is the
feedback element.
Regards,
S. H.

Click to expand...

Thanks for replying.. But isn't the capacitor between emitter and collector is for cancelling the higher harmonics and the parasitic capacitance?? Will it also act as feed back network??

If we go by this,Output is fed back to emitter instead of to base where the message signal is applied.. Once again thanks for replying.. Please try to answer this question too..

Added after 2 minutes:

haytham said:

Hi
Modulating signal will vary Cbe of the transistor and so the frequency of oscillation will vary.
We made this circuit four years ago

Thanks
BR
Haytham

Click to expand...

Thanks for replying.. Can you please explain the feed back path in this circuit please?? Is the capacitor between collector and emitter act as both feed back and for cancelling parasitic effect??