Mighty Little FM Transmitter (part 1)

fmtrx_icon_bigWell there is one thing that’s most annoying about laptops: their speakers. People often choose to listen to their music with headphones, but still, sitting with headphones on for a number of hours is not too comfortable nor healthy. Many of us use speaker systems but that often leads to quite a mess with all those cables and power cords.

Since I’m in possession of Phillips Music Center with integrated FM receiver I wanted to make some use out of it. In order to avoid a long piece of audio cable going on my floor from my laptop to music center itself, I thought of a device that would allow me to send my audio signal in a wireless manner. Here is the whole story how this simple idea materialized.

Step One: Concept.

To make this device functional it had to satisfy following requirements:

  • Ability to transmit high quality audio signal in FM Broadcast Band (stereo, high bandwidth, etc.)
  • Implement USB Audio Sound Card (driver-less solution) to get all the audio from my laptop, so that additional stereo jack connection wouldn’t be necessary.
  • Power the whole system from USB port (again – no additional cables)
  • Optional: Implement FM receiver, so that I could send audio from one PC to another
  • Support as many different architectures/OSes/etc. as possible
  • Implement an easy way to modify parameters (such as transmit frequency) over USB (single interface to rule it all)

Step Two: Choosing the right components for the job.

… a.k.a choosing the obvious. Implementing whole FM circuitry all by myself would be pointless and painful (to say the least). The overall outcome in terms of performance would be doubtful, so I decided to choose a single IC solution that integrates all that I needed: Si4721, from Silicon Labs. Just a single glance at it’s datasheet and you know you’ve scored a home-run.

What about MCU? Well, considering my stock, again – I chose the obvious candidate for the job: STM32L151, from ST Microelectronics. I have couple of those as leftovers from my previous projects. It embeds all that is needed to make a perfect ‘heart’ of entire system and I really like working with ST MCUs :)

I also decided to design and implement a simple RF Power Amplifier just to pump up the output antenna swing a little bit, mainly because I’m about to use short monopole antenna, known as ‘the wire’ Using full size monopole antenna for FM Band would be a project-killer IMHO (75 cm of wire sticking out of my USB dongle? no thanks, maybe another time..)

Additional parts included: some LDO that will be capable of powering all ICs and PA as well, LEDs just to notify the user of current mode of operation, USB state, etc.., passive components, blah blah…

Stay tuned for more !

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