FM transmitter on colpitts oscillator

[zeller]

Hi,

I have designed a colpitts oscillator which works fine at 27HZ I have to turn it into an FM transmitter, what and where on the circuit should I add i.e diode microphone etc.I.e how do i turn this into a radio transmitter?

Many thanks in advance!

 

[vfone]

Because this is a common base type oscillator you can input audio from the microphone directly to the base of the transistor (using a DC coupling capacitor), turning the circuit into an FM modulator. And if using an antenna mutual coupled to L1 this can be a low range FM transmitter.
Other option is to place a varicap in parallel to C3, and modulate the bias of the varicap with the signal from the microphone

 

[zeller]

vfone,
Okay if i use the microphone, will the battery stay or do i need to remove it?also, how do I determine capacitor size?

 

[vfone]

Better is to remove the battery when doing the experiments.

Use a 100nF in series with the microphone.