FM transmitter circuit which is build using single chip of MAX2606. A very simple FM transmitter connects your home-entertainment system to a portable radio that will be carried surrounding the house and into the back yard. As an example, it is possible to play music on the CD player in your private room, and listen to it on a portable radio by the back-yard barbeque.
IC1 is a voltage-controlled oscillator with integrated varactor. Its nominal frequency of oscillation is set by inductor L1, and a 390nH value places that frequency at 100MHz. Potentiometer R1 then allows you to choose a channel by tuning above the FM band of 88MHz to 108MHz. Output power is about -21dBm into 50 (most nations accept emissions below 10dBm in the FM band).
The home system's left and right audio signals are summed by R3 and R4, and attenuated by the (optional) potentiometer R2. R2's wiper signal serves as a volume adjustment by modulating the RF frequency. Signals above 60mV introduce distortion, so the pot attenuates down from that level.
In the absence of a common FM radio antenna, 75cm (30 inches) of wire will suffice as a transmitting antenna. For most effective reception, it need to be mounted parallel with the receiving antenna. The IC operates on a single power supply voltage inside the range 3V to 5V, but it is best to regulate the applied voltage to reduce frequency drift and noise.
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