Project : PLL FM Transmitter

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Joined Nov 13, 2015
Here is a PLL FM transmitter I recently completed without any error and this was my one of the biggest project till now. The designer of the transmitter is Dick Cappels. His first prototype was very experimental circuit board which was hard to duplicate and I decided to give it professional touch like this (click on the pictures to enlarge): :)
LMX1601 PLL Layout.jpg
Because of my hard labor while designing PCB layout, I got the transmitter working exactly at first test, an exciting moment! I applied some modification (added a pre-amp/mixer/pre-emphasis) around audio section for microphone, for mixing microphone voice with music and for OpAmp based pre-emphasis as below (click to enlarge the pictures):
Input modified  PLL transmitter schematic.png
Audio section:
component side audio pre-amp.jpg

For extended transmitting rage I added a simple 2N3904 based RF booster stage and result was very nice. I got more than 100 meters on my cell phone receiver (cell phone are poor in receiving FM) without peak tuning its collector coil/capacitor (LC). Once I got almost 500meter with peak tuned collector LC using same type of RF booster with another FM transmitter. I want to say thanks to Audioguru (Canada of course) for entire modification idea for pre-amp and for RF booster. I want to say thanks to Rod Elliott ( for core idea about OpAmp based pre-emphasis and mixer. I want to say BIG thanks to Dick Cappels ( for helping me in everything!

Transmitter components view:
component side transmitter.jpg

Transmitter layout view:
circuit side transmitter.jpg

All features are exactly as programmed on AVR. It starts to transmit exactly at 100.0 MHz and tunes 100 KHz up and down. It beeps on every start, beeps on every click, LED shows OSC ON, Audio detecting and OSC OFF. After 5 minutes without proper audio, the oscillator goes OFF by indicating on LED. It is amazing that I can power it from AC to DC adapter and it is not catching horrible hum noises. I had to use just battery as a power supply for my other simple FM transmitters because of horrible hum noise from AC to DC adapter. ESD precaution has been taken in care while handling and soldering parts. As I said the project is an improvement, so we can get more vast details about the projects in the original page here: PLL FM Transmitter.html

  • The printable high resolution PCB artwork file has been uploaded below, download 'PLL Tx layout_high rezolution.pdf file'. Print it with 'Actual size' setup in your printer.
  • To modify PCB artwork with ExpressPCB, 'LMX1601 PLL Layout_ExpressPCB' has been uploaded.
  • To burn the HEX in the AVR controller, the hex file (written by the designer: Dick Cappels) has been uploaded with his kind permission below, named 'Dick Cappels's PLL HEX program.hex'.
- Raj Kumar Khadka (my formal name)
Project completed: 24 Oct 2015


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