FM modulation in miniature transmitter

[helix1]

I've been trying to understand the circuit shown below. I have a few questions that it would be great if someone could help me with.

I don't really understand how frequency modulation happens in this circuit. From reading around, the general explanation seems to be that the amplified signal from the electret microphone somehow alters the internal capacitance of the transistor Q2 to create the modulation. Which junction capacitance is this that is getting altered? Is it the capacitance between base and emitter? How exactly is the capacitance being altered by the input signal?

Also, C5 is providing the positive feedback to the emitter of Q2 in common base configuration. This looks like a Colpitts type setup, but why is the signal fed back through just one capacitor instead of the usual capacitive divider?

[flynwill]

The varying capacitance is the base-collector junction -- which, thanks to to C4, is effectively in parallel with C6.

[helix1]

I'm still not clear how the capacitance of the base-collector junction changes with base voltage. Is this a documented effect?

And the capacitor C4 is not always present in these kinds of circuits to put the base-collector junction in parallel with the tank circuit. For example, the circuit below. Is this still working the same way?

[PA0PBZ]

I'm still not clear how the capacitance of the base-collector junction changes with base voltage. Is this a documented effect?

And the capacitor C4 is not always present in these kinds of circuits to put the base-collector junction in parallel with the tank circuit. For example, the circuit below. Is this still working the same way?

Yes, look at the 1N and 22N

[helix1]

@PA0PBZ that graph looks good. Where did you get it? I'd like to know how those curves are derived 

[PA0PBZ]

@PA0PBZ that graph looks good. Where did you get it? I'd like to know how those curves are derived 

It was the 7^th hit when I typed "collector base capacitance" in google image search, I just wanted to find something to show you that it was not imaginary 

[alterbaron]

There's some formulas here:

http://ecee.colorado.edu/~bart/book/book/chapter5/ch5_6.htm

The capacitance in the base emitter junction is a consequence of voltage-dependent charge separation in the depletion layer. (When a change in voltage changes the amount of charge separation, you have a capacitance!)

More details here:

http://ecee.colorado.edu/~bart/book/book/chapter4/ch4_3.htm

In short, it turns out that you can treat the capacitance between the base and collector terminals as being due to a parallel plate capacitor, where the plates are separated further apart the higher the reverse bias the junction sees. I.e. the capacitance goes down with increasing reverse bias.