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, 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.
Circuit PLL FM Transmitter
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 lot of AT90S2313 chips left over from another project. To the best of my knowledge, the ATtiny2313 can be substituted without changing the source code. The 20 pin DIP appears monstrously large compared to the diminutive LMX1601. The size difference is an indication of the passing of the three of four decades that elapsed between the introduction of these two packages.The LMX1601 has a reference divider, which I used to divide the 4 MHz clock from the micro controller's crystal oscillator, down to 12.5 kHz reference frequency. The feedback divider, referred to as "16 Bit AUX N Counter" in the data sheet, is proceeded by a divide by 8 prescaler. These factors determine the channel spacing for the phase locked loop. 8 X 12.5 kHz = 100 kHz. The AUX N register contains the divide ratio for the feedback divider, and the phase locked loop tries to make the VCO operate at a frequency equal to the value written to the AXU N register, times 100 kHz. I say "tries" because the actual frequency of oscillation depends on the range of the VCO itself. This means that the center requency of the resulting frequency modulated oscillator can be set with 100 kHz resolution, enabling it to be set either on or between allocated FM broadcast channels throughout most of the world.
Layout PLL FM Transmitter
If a value of 1000 is written into the AUX N register, the PLL will try to run at 100 KHz X 1000 = 100 MHz. The math isn't so hard, and I skipped the hard math -the loop gain and bandwidth calculations- so this should be an easy project, mathwise.
In the firmware, an 8 bit count is used as the "channel
" number. The highest frequency, which occurs when the channel number is 255, is set to 108 MHz by adding an offset of 825 to the channel number. I did this because the part of the spectrum just above the FM broadcast band is used for aviation communications in most, if not all, parts of the world. When the frequency increment and decrement buttons are pressed, the channel number increments or decrements, respectively. The button increment and decrement routines that limit the channel number to those that correspond to frequencies between 88.0 Mhz and 108.0 MHz.