Polar Modulation for Everyone
We have been working on SSB generation using polar modulation.
Hardware stack: RP2350-Zero MCU board + Fast Si5351 module + AP63301 buck regulator (for 5W mode).
See https://github.com/kholia/PolarModulationSSB for details.
Audio fidelity is acceptable at the moment and should improve further. We still need to hook up the amplitude ("envelope") restoration stage.
Motivation
Why should Guido, Hans, and FlexRadio folks have all the fun? ;)
Samples
Original audio 1:
Transmitted SSB quality (received by SDR) 1 @ 1 MHz I2C (FM+ mode):
PRs, ideas, and feedback are welcome!
Notes
The most critical factor for the SSB quality is the I2C speed we can practically achieve. 1KΩ pull-ups really help (our setup is currently using 2KΩ pull-ups).
Useful commands:
pactl load-module module-null-sink sink_name=Virtual0 arecord -f cd -t wav Fresh-Recording-Feb-2026-21.wav
Updates
We now have an initial rough "port" running on the inexpensive and widely
available ColorLight 5A-75B FPGA board.
The goal is to support faster phase updates, which should improve SSB output quality.
References and Credits
https://www.pe1nnz.nl.eu.org/2013/05/direct-ssb-generation-on-pll.html - Classic modern reference
https://dl0tz.de/polar/index_en.html - Modern FOSS work - The reference!
https://daveshacks.blogspot.com/2025/03/polar-modulation-deep-dive-into-phase.html
https://daveshacks.blogspot.com/2025/02/guidos-ssb-modulation-analysis-and.html
https://daveshacks.blogspot.com/2025/12/testing-revised-phase-calculation.html
https://github.com/F5OEO/rpitx - The SSB TX quality is pretty good!
https://github.com/tmcqueen-materials/pio-i2c-hs (3.4 Mbps I2C!)
https://github.com/kholia/Colorlight-5A-75B/tree/master/dds_ssb_uart - (FPGA SSB PM PoC)