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 generated by a filter connected to GPIO-pin 'GP0' which provides the binary PWM signal. You can probe it by 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).

Image: Line-out signal (see below) probed by a DSO. We clearly see the 1200 Hz and 2200 Hz tones of the 1200 Bd 2-AFSK.

Basically, the data/signal flow is as follows:

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.

Prerequisites

Your host platform for cross-compilation is assumed to be LINUX.

1. Install the Pico-SDK following the instructions given in the Raspberry Pi 'Getting Started' Guide (pdf)
2. Set the PICO_SDK_PATH environment variable to point to your Pico-SDK installation directory
3. Install the pico-extras libraries:

  cd $PICO_SDK_PATH/..
  git clone -b master https://github.com/raspberrypi/pico-extras.git

Hardware

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

The line-out voltage can be as high as 2.7 V_pp (~1 V_rms) (at full-scale volume setting in the software and high-impedance load).

Build the library and the example application

git clone https://github.com/eleccoder/raspi-pico-aprs-tnc.git
cd raspi-pico-aprs-tnc
cmake -S . -B build
cmake --build build

build/lib/libaprs_pico.a and build/aprs_pico_example.uf2|.elf|.xxx will be generated.

Run the example application

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

The analog AFSK audio signal will be available at the filter's line-out. You can probe it by 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 (Sept. 2021)

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

Ingredients / Acknowledgements

• For APRS => AX.25 => PCM conversion I'm using my modified version of fsphil's ax25beacon
• For PCM => PWM conversion I'm using the pico_audio_pwm library from pico-extras (NOTE: ATTOW, maturity seems to be rather alpha/beta)