PLL Stereo FM Transmitter

  
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The circuit shown here is of a good Stereo FM transmitter that can 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, crysta
PLL Stereo FM Transmitter - schematic

l control circuitry for accurate frequency locking, multiplex circuit for making sum ( left plus right) and difference ( left minus right) {see this article for better understanding Stereo decoder circuit} etc. Another important feature of this IC is that the transmission frequency can be set using a 4 channel DIP switch. The IC can be powered from anything 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 possible 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 is practically no frequency drift. Circuit Description Capacitors C18 and C19 are DC decoupling capacitors for the left and right channel inputs. Capacitors C1 and C17 are used to set the amount of pre-emphasis required and here with the used values it is 50uS. Capacitors C2 and C16 sets the roll-off point of the low pass filter. The crystal X1 is a 7. 6MHz crystal which sets the oscillator frequency while capacitors C13 and C14 associated with are used for providing the appropriate loading. Resistors R8 to R11 are the pull-up resistors for the D0 to D3 (pins 15 to 18 of the IC) respectively. These pins can be held low by closing the corresponding switches. The RF oscillator of the IC is tuned using the components L1,...



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