Koppar LTE Modem: Integrating Cellular Connectivity via USB-C
A custom four-layer hardware board enabling LTE data, SMS, and voice calls over USB.
1Hardware Architecture Overview
The Koppar LTE modem is a four-layer board designed to provide computers with a dedicated phone line. It supports LTE data, SMS, and voice calls, routing audio directly through USB-C. The device lacks internal batteries or speakers, relying on a 5V USB-C power source and a 3.8V buck converter.
2Core Components and Power
The system utilizes an RP2040 microcontroller alongside a USB hub to interface with the host computer. Power management includes a 3.8V, 3A buck converter to handle 2A transmit bursts, plus a 3.3V rail for the hub and flash memory. Three onboard LEDs indicate network status, incoming calls, and active call states.
3Design and Fabrication Workflow
The board was developed using KiCad 10, starting from a single-page specification file. Fabrication files, including Gerbers and drill data, are optimized for JLCPCB assembly. The design requires a specific 1.6mm stackup, JLC04161H-3313, ensuring antenna feeds maintain a 49-ohm impedance. All vias are 0.45mm with 0.2mm holes to avoid pad interference.
4Firmware and Control Logic
The RP2040 manages the modem's power state by driving the PWRKEY and RESET_N pins. Before firmware initialization, the host computer only detects the USB hub and the microcontroller. The project includes custom sound-card firmware to process audio, enabling integration with scripts or AI agents for automated call handling and text messaging.
5Iterative Design Improvements
The current Rev B iteration addresses critical flaws identified in the initial Rev A prototype. Key fixes include correcting the antenna feed impedance from 35 ohms, relocating the buck converter inductor, and reorienting the SIM socket. The design review document, REVIEW.md, provides a detailed breakdown of these changes and remaining open items.
6VoLTE and Network Compatibility
The modem utilizes the EC25-EUX module, which supports optional VoLTE functionality. In regions where 2G and 3G networks are decommissioned, users must ensure their module firmware and carrier support VoLTE for voice calls. Compatibility should be verified with the module seller to ensure proper functionality on modern cellular infrastructure.
7Open Source Documentation
The project is released under the CERN Open Hardware Licence Version 2, Permissive. All design files, including KiCad project data, custom symbols, and 3D models, are available in the repository. Users can convert missing 3D models to .wrl format using the easyeda2kicad tool to complete their visual assembly renders.
8System Integration Potential
By bridging cellular hardware with USB-C, this project enables advanced automation possibilities for developers. The ability to pipe call audio directly to a computer allows for sophisticated AI-driven telephony applications. This hardware serves as a bridge between traditional telecommunications and modern software-defined workflows, simplifying complex cellular integration for custom computing projects.