Working of LC as a vco..

[dineshbabumm]

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.

 

[akg]

i haven't analysed the ckt , but this is what i think , in the signal analysis the power supply is appearing as a short ckt , and hence the transistor capacitence will appear in parallel to the tank ckt , as the voltage in the base varies , the CB capacitence also varies , which causes the effective C of the tank ckt to change in accordance to the i/p voltage.

 

[Nigel Goodwin]

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.

The feedback path is the capacitor between collector and emitter, it's NOT for 'cancelling harmonics'? - which doesn't make any sense anyway!.

Also it's NOT specifically an FM transmitter, it's more a very poor AM one, the FM is caused by the capacitance changes in the transistor as it's AM modulated.

So you're varying C to get the FM.