88 108 MHz PLL FM Synthesizer

This fm synthesizer circuit is played with and optimised design for an 88-108 MHz synthesiser, programmable in 25 kHz steps. It produces about 50 mW output (and thus feeds nicely into the amplifier shown below) with no tuning required other than to set the inputs of the divide-by-N counter to the wanted frequency.

FM modulation is achieved just by injecting audio on the audio input (audio response is pretty flat from 10 Hz to over 200 kHz so can be used for stereo and RDS). This versatile design has also been used for link transmitters at around 48, 52 and 200 MHz with a few changes in the VCO and output filter, and by changing the reference crystal you can alter the channel spacing too. The power output is switched off until the synthesiser achieves lock to prevent transmissions on the wrong frequency (which can be disasterous if youve amplified it to high power and got it connected to a highly resonant antenna).