Need help on understanding FM transmitters

I've built a couple FM transmitters before and they somewhat worked, but i never really understood how exactly they work. Now i need your help to design and build a little transmitter to send audio about 15 meters away, from my computer to my kitchen radio.

I need it to take a line level input and get power from a 6v or 12v supply. I do have a very well regulated power supply from my old 3-transistor tx so i don't need a regulator in the transmitter circuit. Pre-emphasis will be done in software.

I took this example circuit which is as simple as it gets: http://www.boondog.com/tutorials/rfT...ransmitter.htm

I will be using a line input so R1 will be omitted. Also to my understanding C3 represents the internal capacitance of the transistor. I also assume that i will have to lower the gain of the circuit so i don"t overdrive it with the line input.

Now here's what i don't understand. For the audio being modulated, the transistor acts like a common-emitter amplifier, but the collector resistor is replaced by the LC tank circuit. What effect does it have on the bias resistors R2 and R3, and the emitter resistor R4? Any formulas please? Every transmitter circuit i've found seems to use different values for those resistors but no equations are given. Also, what is the purpose of C2?

To my understanding i will need to decouple the antenna from the oscillator so frequency doesn't drift if the antenna is moved. This can be done with a second transistor in emitter follower configuration, tuned to the same frequency as the oscillator, correct? Any other things i missed?

Thanks in advance.

hi there,

No C3 is an actual capacitor as part of the RF tuned cct in the output of the
system

here's a better 2 transistor cct http://www.sentex.ca/~mec1995/circ/fmt4.html

for line level replace the microphone with a say 10kOhm trimpot resistor
Note you DONT need the (Rx) 10k resistor shown as that is for supplying power
to the electret mic insert

feed the Line input across the trimpot and connect the wiper of the pot to the
4.7uF capacitor. That will give you reasonable control over the input level

The antenna as suggested could be a length of wire, personally I would use a
telescopic antenna from an old transistor radio etc

cheers
Dave
VK2TDN

Yes i understand i do not need Rx, as 9v flowing thru my soundcard wouldn't be such a great idea.

But my question still isn't answered - i want to know what influence do the value of the resistors have, and how to calculate them (say i want to use a different transistor with different beta).

Edit: Also the pot would only attenuate the input signal. My intention is to lower the gain of the circuit for better S/N ratio. My old transmitter had a noise floor of about -20dB, and i had to keep the soundcard volume at 20% tops so that i don't overdrive it because it had been designed with a mike input in mind.