Anders Nielsen's PhaseLatch Mini Is an STM32 Blue Pill-Inspired Low-Cost Software-Defined Radio

Anders Nielsen is back with a second software-defined radio design, the PhaseLatch Mini — part of an ongoing effort to build a highly-capable modular SDR platform powered by, of all things, the 50-year-old MOS 6502 eight-bit microprocessor.

"The PhaseLatch Mini [is] an [STMicroelectronics] STM32-based direct-conversion SDR front-end paired with simple Python host scripts," Nielsen explains of his creation. "GQRX reads samples through a USB FIFO, and tuning is handled on the hardware side through the [Silicon Labs] SI5351 synthesizer. With this setup, I was able to cleanly receive HF, FM broadcast, and even experiment (with mixed success) around 144MHz."

This isn't the first software-defined radio Nielsen has built: the PhaseLoom, a chunkier design built as an add-on shield for the MOS 6502-powered 65uino single-board computer, served as a starting point for the project. This time the classic chip has a little help: an STMicro STM32F103 microcontroller, which provides dual simultaneous-sampling analog to digital converters (ADCs) — "essential for properly digitizing I/Q channels," Nielsen explains, and something missing from the last project that required the signal be piped into the soundcard of a separate computer.

The board's design is inspired by the classic STM32 Blue Pill development board, though with two SMA antenna inputs that connect through a 100kHz low-pass filter offering 210kHz of complex baseband bandwidth. There's a USB Type-C connector for power and data, and this time around it doesn't need a separate analog output to a soundcard to carry the received signals.

"Since ADC performance depends heavily on layout," Nielsen explains, "I upgraded the board to include a proper ground plane and clean separation between the sensitive analog input and noisy USB lines. The design is simple: capture I and Q through a passive LC [inductor-capacitor] low-pass filter for anti-aliasing, then feed them straight into the MCU's ADCs."

More details are available in the video embedded above and on Nielsen's website; source code and design files, including Gerbers for manufacturing, have been released on GitHub under an unspecified open-source license, with boards available to order on Nielsen's store at €26.71 (around $31) plus shipping.