Digital controlled radio frequency oscillator for Raspberry Pi Pico

The library for Raspberry Pi Pico includes the headers and source code and all necessary build files to build a custom application which turns pico into precise PLL digital frequency oscillator of the whole of HF radio spectrum (1 Hz to 32.333 MHz) with millihertz resolution.

Precise frequency resolution

The library provides about 23 milli-Hz frequency resolution. This resolution is limited by 24-bit register which is used in algorithm. The working WSPR beacon which has been built on the base of this project proves that the quality of generated signal is sufficient to such precise (~1.46 Hz step) frequency manipulation digital modes. The upper freq. limit is ~32.333 MHz and it is achieved only using Pico overclocking to 270MHz.

Here is an example of narrowband FSK (9.4 MHz carrier, 5 Hz step, 20 Hz range in total).

Phased locked loop in C

The DCO uses phase locked loop principle programmed in C.

NO additional hardware

The DCO provides the output signal on the GPIO pin. However if you want to transmit the signal, you should calculate and provide a lowpass filter of appropriate frequency. Please also figure out whether you possess rights to emit radio frequency energy on desired frequency.

GPS reference frequency correction (optional) since v.0.9

GPS reference frequency correction option provides an absolute frequency error within about ~1Hz in long term.

Dual-core

The DCO uses extensively the secodary core of the pico. The first one is for your ideas how to modulate the DCO to obtain a desired signal. The DCO does NOT use any floating point operations - all time-critical instructions run in 1 CPU cycle.

Radio transmitters

Owing to the meager frequency step, it is possible to use 3, 5, or 7th harmonics of generated frequency. The practical resolution will be quite the same - far below 1 Hz. Such solution completely cover all HF and VHF band up to ~233 MHz.

Console

It is possible to work without compling the sources: load .uf2 to the Pico and start minicom serial console via USB: minicom -D /dev/ttyACM0 Enter the command 'HELP' in the console. Follow instructions. Here is an example: https://www.youtube.com/watch?v=np87nLzC88o

Tests of RF spectrum quality

Sweep test: https://youtu.be/nYC1VDBiz4o Pseudorandom MFSK test: https://www.youtube.com/shorts/CEPW8hwlG7k There is WSPR beacon project based on this oscillator: https://github.com/RPiks/pico-WSPR-tx

For what?

This is an experimental project of amateur radio hobby and it is devised by me in order to experiment with QRP narrowband digital modes. I am licensed radio amateur who is keen on experiments in the area of the digital modes on HF. My QRZ page is https://www.qrz.com/db/R2BDY

Feedback

I appreciate any thoughts or comments on that matter. I strongly appreciate if you use this project as a part of your one in accordance with the Licence. I have plans of building a transceiver on the base of this library. The WSPR beacon is working and available here: https://github.com/RPiks/pico-WSPR-tx

Quick-start

1. Install Raspberry Pi Pico SDK. Configure environment variables. Test whether it is built successfully.

2. git clone this repository. cd pico-hf-oscillator ; ./build.sh Check whether output file ./build/pico-hf-oscillator.uf2 appears.

3. Prepare the surrogate antenna (if you possess an SSB receiver) or pin-out for an oscilloscope or a spectrum analyser. The default output pin is GPIO6.

4. Load the .uf2 file (2) into the Pico.

5. Initialy the running frequency is 29.977777 MHz.

6. Set any other frequency up to 32.333333 MHz by #define GEN_FRQ_HZ and build the project.

7. Provide the feedback by clicking like (a star) on the github page of the project :)

Cheers, Roman Piksaykin, amateur callsign R2BDY https://www.qrz.com/db/R2BDY