Low Power PLL FM Transmitter

There are some very interesting projects over at Dick Cappels’ Project Pages. They range from simple LED voltage boost circuits to this complex PLL FM Transmitter. The video of it in action is quite interesting since he installed an LED to monitor what the transmitter is doing.

• Dark: No power is applied to the circuit.
• Steady red: Power is applied and oscillator is off.
• Steady blue: Oscillator is on and a audio signal is not detected.
• Blue, occasionally blinking red: Oscillator is on and an audio signal is being detected.

“The common characteristic of all of the previous low power FM transmitters I’ve built over the decades, is that their operating frequency is determined by an LC resonant circuit. Some of them had excellent stability, some of them didn’t, but I had always wanted to make one that is crystal controlled. Various schemes had been considered from time-to-time, including the direct approach of modulating the load capacitance of a a crystal oscillator, a whimsical phase modulation scheme involving a phase shifter, some balanced modulators, and limiting amplifiers, and at times, the down-to-earth and sober approach of modulating a VCO within a phase locked loop (PLL). While browsing Digikey’s online catalog”