Some question on simple FM transmitter

[fm101]

hello,
Can anybody tell me the function of the capacitor C3 between collector and emitter in FM transmitter? How is value is selected? Also what is the function of the capacitor C1 and how its value is selected? The coil L1 is of 6 turns but what is approximate inductance?

 

[FvM]

The circuit is common base Colpitts oscillator, C3 providing feedback, C1 is RF short. Inductance can be estimated by empirical formulas or tables, of course all dimensions must be known. E.g. https://www.qsl.net/in3otd/indcalc.html

 

[biff44]

yes. C3 feedbacks rf voltage, but most importantly gives it the correct phase shift so that the common base amplifier part of the circuit plus the C3 phase shift adds up in-phase..

i believe its called the Barkhausen critria