USDX

uSDX is an open-source, experimental software-defined radio (SDR) transceiver designed for low-power (QRP) single sideband (SSB) and continuous wave (CW) HF communications. It operates across multiple amateur radio bands from 160m to 10m and supports all common modes including USB, LSB, CW, AM, and FM. The transceiver leverages a highly power-efficient Class-E amplifier design combined with a software-driven SSB transmit stage implemented on an ATMEGA328P microcontroller. This approach minimizes hardware complexity and cost while maintaining reasonable performance for practical QSOs, including contest conditions.

The uSDX transceiver features a fully digital SDR receiver with software-based 90-degree phase shifting, DSP filtering, automatic gain control (AGC), noise reduction, and multiple analog front-end attenuators to optimize dynamic range. The transmit stage reconstructs the SSB signal by controlling the SI5351 PLL phase and power amplifier envelope via PWM, enabling a compact, low-heat, and efficient design without bulky linear amplifiers. Firmware is open source and built with the Arduino IDE, allowing users to customize and extend functionality. The device supports VOX for fast RX/TX switching and CAT control for remote operation.

What sets uSDX apart is its minimalist hardware design—only four ICs plus a microcontroller and a few passives—combined with advanced software DSP techniques to achieve sideband suppression better than -45dBc and output power around 5W PEP from a 13.8V supply. Its continuous tuning capability across HF bands and support for digital modes like FT8 via VOX make it versatile for amateur radio enthusiasts who want a low-cost, DIY SDR transceiver. The project is actively maintained with a community forum for support and collaboration. Getting started involves assembling the hardware or purchasing kits, then uploading the open-source firmware via ISP or Arduino IDE.