FM transmitter

NEED HELP!!

This FM transmitter works. I get it from a post while surfing the net. The original circuit is actually the thing inside the yellow box.

Minor changes were made. I made its input to be either from an electret microphone or from RCA jack (input from computer, MP3, etc..). I also regulated the input voltage (red box).

But can you explain and elaborate any information about it (the things inside the yellow box)?? There are just things that I don’t know.

The oscillator used?

The purpose of some capacitors and resistors?

The LC combination?

And any thing you can add..

Thanks a Lot..

It is an extremely simple VHF Colpitts oscillator used as a very poorly performing FM tramsmitter.
Look in google for FM Transmitter or ask your teacher.

Hi audioguru,

so it's a colpitt's oscillator.. but how can you say that it is a very poor fm transmitter..?? honestly, it's working perfectly for me, transmitting at 105.5 MHz. The sound was very clear and it has a very long range..

also, i wanna ask something. when i finished it, it's working at 107MHz, using just a 3inch wire.. but when i added an antenna (i get it from an old nonworking radio) it shifted to 105.5MHz. there is no change in the quality of sound. why is that??

i also noticed that when my input is from an electret mic, it seems that the mic is very sensitive that i can even hear from a receiver voices and sounds of almost everyone around the transmitter (all are clear). how can i lower its sensitivity??

btw, thanks for your reply..

Horcrux said:

Hi audioguru,


also, i wanna ask something. when i finished it, it's working at 107MHz, using just a 3inch wire.. but when i added an antenna (i get it from an old nonworking radio) it shifted to 105.5MHz. there is no change in the quality of sound. why is that??

Click to expand...

The antenna is part of the tuned circuit for your oscillator. Change the antenna, you change the tuned circuit. Also, objects moving close to the antenna will affect the frequency, as they will change the antenna impeadance.

Of course it is a very poor FM transmitter.

The quality of the sound is muffled because the extremely simple circuit does not have pre-emphasis (treble frequencies boost like all FM radio stations have). The de-emphasis in all FM radios will reduce treble frequencies making the sound like your stereo with its treble tone control turned all the way down.

The circuit needs a simple audio equalizer to provide pre-emphasis.
The circuit also needs an RF amplifier driving the antenna to isolate the antenna and anything capacitive-coupled to it from the oscillator. Then the frequency will not change and the range will be increased too.