pll fm circuit

Posted on Feb 4, 2014

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.

pll fm circuit
Click here to download the full size of the above 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, I found the LMX1601 frequency synthesizer chip and thought: "Just maybe, the PLL approach is finally within my grasp. " The LMX1601, which apparently was designed for use in cell phones, includes everything need to make two phase locked loops except for the VCOs. More importantly, one of the PLLs, specifically the "AUX" PLL, is specified to work in the FM broadcast band. The LMX1600 and the LMX1602 were also considered, but the LMX1601 was selected because it has a "500 MHz option", meaning that it can work down to about 50 MHz. In order to function, the LM1601`s registers need to be loaded. In particular, they need to be loaded with the divide ratios for the reference and signal (VCO in this case) counters, and to set some control parameters. To load the registers, I used a Atmel AT90S2313. It could have been any microcontroller with 3 I/O pins, but I had chosen the AVR processor, and I still have a...

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