A TX-only TNC (Terminal Node Controller) to generate the AFSK (Audio Frequency-Shift Keying) audio tones for APRS (Automatic Packet Reporting System) messages using a RP2040 microcontroller on a Raspberry Pi Pico board.

Block diagram showing the signal flow

An analog line-out audio signal will be generated by a band-pass 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.

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

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
  cd pico-extras
  git submodule update --init

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).

NOTE: In case you want to allow the Pico to control the PTT (Push-To-Talk) input of your transmitter, set the appropriate #define parameters in src/aprs_pico_beacon_demo.c.

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_beacon_demo[.uf2|.elf|.bin|.hex] will be generated, as well as the testing application build/aprs_pico_tone_test[.uf2|.elf|.bin|.hex].

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).

But for testing the signal integrity, you can feed the signal into the soundcard of your computer and let decode its data content by Dire Wolf - see down below.

cd build

Flash aprs_pico_beacon_demo[.uf2|.elf|.bin|.hex] to the Pico board as you're usually doing.

We can use the famous Dire Wolf CLI software to decode the APRS data after sampling our APRS audio signal by means of a soundcard.

1. Connect the line-out of our circuit above to the microphone input of your soundcard of your (ALSA-supported) LINUX system.
2. Check if you can hear the typical APRS 'modem-sound' on your audio output device by monitoring the input signal (44.1 kHz, mono, WAVE):

arecord -f cd -c 1 - | aplay

3. Install Dire Wolf on your system. Probably, you just have to run:

sudo apt install direwolf

4. Let's sample the APRS audio signal fed to the soundcard and forward the audio stream to Dire Wolf (44.1 kHz, mono, WAVE):

arecord -f cd -c 1 - | direwolf -

5. Enjoy the decoded APRS message:

Here is a recorded audio wav-file (44.1 kHz, mono, WAVE) captured by the soundcard using:

arecord -f cd -c 1 > aprs_pico_beacon_demo.wav

You can replay and decode this file by:

cat aprs_pico_beacon_demo.wav | direwolf -

If you actually want to RF-transmit the APRS signal, you can simply do it by one of the many cheap HTs, like the Baofeng UV-5. The complete schematic including PTT control looks like this:

Don't forget to set the appropriate #define parameters in src/aprs_pico_beacon_demo.c to configure PTT control.

• Send the APRS message on the console (USB or UART) rather than hard-coding

• For APRS Payload => AX.25 => AFSK (PCM) conversion I'm using my modified version of fsphil's ax25beacon project.
• For PCM => PWM conversion I'm using the pico_audio_pwm library from pico-extras (NOTE: ATTOW, maturity seems to be rather alpha/beta).
• Basic PWM audio rendering with the Pi Pico has been spotted in the pico-playground.