ADX-MI3

Acknowledgement: ADX stands for Arduino Digital Xceiver and it was originally conceived and designed by Barb WB2CBA. The key features of the ADX architecture include a CD2003GP receiver chip and a class-E RF amplifier. The ADX firmware runs on an AtMega 328 micro-controller and uses the Arduino IDE for firmware development.   The ADX-MINI, ADX-MI2 and ADX-MI3 were designed by Scott KJ7NLA and are modifications of the original ADX design.   A heartfelt appreciation goes to Barb for the original ADX design, and to Scott for his ongoing enhancements to the ADX family of transceivers.

NOTICE: THIS PRODUCT IS EXCLUSIVELY AVAILABLE TO LICENSED RADIO HAM OPERATORS

Note on the Filter Module : ADX-Mini and ADX-MI2 Band filters are not compatible with this Version ADX-MI3

The ADX-MI3 LPF Modules have a band ID for verification of band selection

When placing your order, ensure to include your Callsign.

ADX MI3 – OLED Digital QRP HF SDR Transceiver Platform

The ADX MI3 is a modern, compact, and highly refined digital HF QRP transceiver system designed for advanced amateur radio operation using PC-controlled digital modes. It represents the next evolution in the ADX family, improving usability, stability, and system integration while maintaining a minimalist SDR-style architecture.

Built for FT8, FT4, JS8Call, and WSPR digital operation, the MI3 removes unnecessary complexity from earlier versions and focuses on a clean, efficient, and highly reliable operating experience. Most radio functions are handled through the computer interface, allowing seamless integration with software such as WSJT-X.

The MI3 introduces a simplified control system with CAT-based operation, reducing the need for multiple manual controls. Band selection, mode control, and operating parameters are managed directly from the digital software environment.

A key upgrade in this version is the inclusion of a 128×32 OLED display, providing real-time system status, voltage monitoring, band information, and operational feedback in a clear and modern format. This eliminates the need for legacy LED indicator decoding and improves usability significantly.

The system also uses encoded band-pass filter modules, ensuring correct band detection and preventing mismatches between software-selected bands and physical hardware modules. A built-in safety check prevents transmission if incorrect band hardware is detected.

Additional improvements include a resettable protection fuse on the 12V input rail, improved power handling, and onboard voltage monitoring displayed on the OLED. These enhancements increase reliability, especially in portable or field operations.

The MI3 maintains a modular RF architecture with plug-in band filters, efficient transmit and receive stages, and USB-based audio and control integration, making it a highly capable SDR-style QRP platform.

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Key Features

• Digital QRP HF transceiver (SDR-style architecture)
• Optimized for FT8, FT4, JS8Call, WSPR operation
• 128×32 OLED display for status and voltage monitoring
• CAT-controlled operation (software-defined control via PC)
• USB audio interface for digital mode integration
• Encoded band-pass filter modules with validation system
• Automatic band detection and safety interlock
• Resettable fuse protection on 12V input rail
• Modular HF band operation via plug-in filters
• Low power, compact QRP design
• Based on ADX MI2 / MINI / original ADX architecture lineage

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System Overview

The ADX MI3 operates as a software-defined transceiver front end, where signal processing, modulation, and demodulation are handled by the connected PC. The radio hardware provides RF conversion, filtering, and amplification, while the computer manages digital signal processing via WSJT-X or similar software.

This separation of RF and DSP allows for excellent flexibility, low hardware complexity, and strong performance in digital communication modes.

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Applications

• Amateur radio HF digital modes (FT8, FT4, JS8, WSPR)
• QRP portable and field operation
• SDR-style PC-controlled transceiver systems
• Multi-band HF experimentation
• Educational RF and communications projects

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Improvements Over MI2

• OLED display replaces LED status system
• Simplified single-button user interface
• CAT-controlled operation (no complex button sequences)
• Encoded band module validation system
• Added resettable fuse protection
• Improved voltage monitoring and system feedback

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What’s Included

• ADX MI3 main PCB (assembled or bare depending on kit)
• OLED display (installed or included)
• Band-pass filter interface support
• Build and configuration documentation