The circuit shown here is Stereo FM transmitter which will transmit high quality signals up to a range of 70 feet. The circuit is based on BH1417 PLL stereo transmitter IC from Rhom semiconductors. The IC has separate audio processing sections for the left and right channels, pre emphasis circuit for improving signal to noise ratio, crystal control circuitry for correct frequency locking, multiplex circuit for creating sum ( left and right) and difference ( left minus right).Another vital feature of this IC is that the transmission frequency can be set employing a 4 channel DIP switch. The IC can be powered from something between 4 to 6V DC and has an output power around 20mW. At full output power the circuit consumes only 20mA and has a channel separation of 40dB. There are 14 potential preset transmission frequencies, starting from 88.7MHz and incrementing in steps of 0.2MHz that can be selected using the DIP switch. The PLL circuitry of the IC is so precise that there's practically no frequency drift.
