Show HN: A USB-C LTE 调制解调器为电脑提供手机通信功能

开发者在 Hacker News 展示了一款名为 Koppar 的开源硬件项目，该设备是一个 USB-C 接口的 LTE 调制解调器，旨在通过单根线缆为计算机提供 LTE 数据、SMS 短信以及语音通话功能 1。这款基于 spec.txt 规范设计的设备集成了 EC25-EUX LTE 模组和 RP2040 微控制器，并通过单个 USB-C 接口向主机呈现为三个普通设备 1。

该项目的技术细节涵盖了从电源管理到硬件制造参数的完整规格。在电源方面，设备接受 5V 输入，其中 3.8V/3A 用于模组发射，3.3V 则供给 Hub 和 MCU 1。PCB 采用 4 层板设计，厚度为 1.6mm，堆叠型号为 JLC04161H-3313，天线馈线阻抗设定为 49Ω，过孔设置为 0.45/0.2mm 且无 Pad 内过孔 1。调试接口包括用于 UF2 刷写的 BOOT+GND 短接点、用于 CMSIS-DAP 调试的 SWD 接口，以及用于 EC25 紧急下载模式的 USB_BOOT+1V8 组合 1。

项目采用 CERN 开放硬件许可证版本 2（Permissive）发布 1。开发者特别指出，VoLTE 功能为可选项，使用前需确认模组固件及运营商的支持情况；此外，早期 Rev A 版本存在的天线阻抗和布局问题已在当前版本中修复 1。

A developer has unveiled an open-source hardware project that transforms a standard computer into a cellular device using a single USB-C connection. The design, created by Koppar based on a spec.txt specification file, enables the host computer to access LTE data, SMS messaging, and voice call functionality 1. The device presents itself to the operating system as three distinct generic devices through its single interface, eliminating the need for multiple cables or complex drivers 1.

The hardware architecture centers on an EC25-EUX LTE module paired with an RP2040 microcontroller 1. Power management is handled internally, accepting 5V input which is stepped down to 3.8V at 3A to support the module’s transmission needs, while a separate 3.3V rail powers the USB hub and microcontroller 1. For manufacturing, the project utilizes a four-layer PCB with a thickness of 1.6mm, following the JLC04161H-3313 stackup 1. Critical signal integrity details include a 49Ω antenna feed impedance and specific via configurations of 0.45/0.2mm without pad-in vias 1.

Debugging and flashing procedures are integrated directly into the board layout. Users can enter UF2 bootloader mode by shorting BOOT to GND, use an SWD interface for CMSIS-DAP debugging, and trigger emergency download mode for the EC25 module by connecting USB_BOOT to 1V8 1. The entire design is released under the CERN Open Hardware Licence Version 2 (Permissive) 1. While VoLTE functionality is optional and dependent on both module firmware and carrier support, previous issues regarding antenna impedance and layout found in Rev A have been resolved in the current version 1.