Polar Modulation for the masses
We have been working on generating SSB signals using Polar Modulation.
Hardware stack: RP2350-Zero MCU board + Fast Si5351 module + AP63301 buck regulator (for 5W mode).
Please see https://github.com/kholia/PolarModulationSSB for more details.
The audio fidelity and general quality is ok'ish at the moment but it can only improve from here. We are yet to hookup the amplitude ("envelope") restoration part.
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 have an initial rough-and-dirty "port" running on the cheap and widely available ColorLight 5A-75B FPGA board now!
The hope is to be able to do faster phase updates which may result in much better SSB quality output.
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)