raspi-pico-aprs-tnc

A TX-only TNC (Terminal Network Controller) to generate the AFSK audio tones for APRS (Automatic Packet Reporting System) messages using a Raspberry Pi Pico microcontroller board.

An analog line-out audio signal will be produced at GPIO-pin 'GP0'. You can observe it by using a scope, listen to it by using an audio amp, or connect it to any RF transceiver to send it on the air (ham radio license required).

Basically, this is the data/signal flow:

APRS (text msg + geo-coordinates + meta-data) -> AX.25 -> PCM -> PWM -> Band-Pass filtering -> AFSK audio signal

Both a static library libaprs_pico.a and an example application will be generated by the build.

Preliminaries

Your host platform is assumed to be LINUX. If you have already installed the Pico-SDK, set the PICO_SDK_PATH environment variable accordingly to avoid installing the SDK twice.

Hardware

We just need a simple band-pass filter to extract the AFSK-signal from the PWM signal:

Build the library and the example application

(cd into the cloned dir)
cmake -S . -B build
cmake --build build

build/lib/libaprs_pico.a and build/aprs_pico_example.xxx will be created.

Run the example application

cd build
(flash 'aprs_pico_example.uf2' or 'aprs_pico_example.elf' to the Pico board as usual)

The GPIO-pin 'GP0' is the line-out for the analog AFSK-signal. You can observe it by using a scope, listen to it by using an audio amp, or connect it to any RF transceiver to send it on the air (ham radio license required).

TODO (Aug 2021)

• Thorough evaluation, in general
• Send the APRS message on the console (USB or UART) rather than hard-coding
• Show how to connect to a Baofeng HT
• PTT control for RF tranceivers

Acknowledgements

• For APRS => AX.25 conversion I'm using a modified version of fsphil's ax25beacon
• For PCM => PWM conversion I'm using the audio lib from pico-extras (WARNING: maturity seems to be alpha/beta)