What do we have so far
Many of you may ask - how is the project going? Let me tell you, what has been done so far. This page may miss some details, as I'm still editing it. TL;DR: scroll to the bottom
First of all, I have a concept of a digital system that is open, eg. that everyone can order a PCB along with a bunch of components and create a radio. But that's not all. Imagine, that you can also contribute to that system, by writing your own or modyfying existing software and hardware. That's what M17 project is all about, a community-driven DMR alternative.
I have assembled four versions of M17 compatible radio so far. First one was the most messy one - everything was hooked up on a breadboard and wired to an STM32-Nucleo board. But it worked. I was able to send and receive data frames using an Si4463 module. The next step was to send voice packets instead. That's where the second version was born. It had an electret microphone, a TFT display, a small speaker and a Press-to-talk tactile switch. It also worked nice, but the Si4463 chips I used were apparently counterfeit (damn RFM26W modules like to freeze). Also the RF power output was low - only 100mW. The next step was to use a genuine Si chip and add some good quality passives. Of course it worked OK, there were no problems with Si chip freezing doing nothing or generating a CW sine wave at the output. The last step was to re-design the mainboard and add a 3 to 5 watts power aplifier.
The heart of the radio is an STM32F777 micro. It basically just samples the voice signal from the microphone using ADC, encodes it with Codec2 and sends as a packet via radio. Seems like a simple task, but it was actually a pain in the ass to implement all of it.
In the meantime, I have prepared a device, which I have called the M17 analyser. It is basically a USB radio modem: a board with the FT232 chip, a low-end STM32 micro and the Si4463 RF front-end. It can be used for transmitting and receiving packets. With additional PC (even RPi) software, it can be used as a local point-of-access for the M17 system. AT commands are used for communication. More detailed info can be found in my GitHub repo M17_ANL.
Power output of the repeater is limited to 15W. That wouldn't be possible without an external power amplifier. I have assembled one using S-AU82L module by Toshiba. It is supplied by 12V line.
To sum it all up
- There are 3 working radio sets
- I'm building a repeater using RPi 3 and an STM32-based motherboard (pictured in the banner above)
- I have received a radio permit for a digital repeater (SR5MS, QTH locator KO02IK)
- M17 analyser works great and enables creation of a point-of-access
- M17 repeater re-sends received voice frames as UDP packets - a one way QSO has been made between Warsaw and Poznań (SP5WWP - SO3ALG)
- We have a 50W duplexer and a 60W RF module (of course it may work only with power output limited to 15W - as the radio permit states)