标签: rfi

一、应用环境与客户需求 USB-C的问世不仅显著提升了USB装置的普及率和功能性，也连带对装置设计提高了要求。除了传统的即插即用特性和透过Windows OS提供的通用驱动程序外，USB-C还支持15W的供电与USB Power Delivery(USB PD)技术，进一步扩展了USB装置的应用范围，但这些增强的功能也引入了一些设计挑战，尤其在射频干扰(RFI)的管理方面。 USB-C的应用环境和需求 1.扩展功能和普及 USB-C的普及，主因能够提供更高的供电能力和数据传输速度。15W的供电能力使得更多设备能够藉由USB-C进行充电或供电，而USB PD技术则允许设备之间进行双向的供电和电量协商，进一步增强它的灵活性和通用性。此外，USB-C的多功能接口支援影片输出、音频传输等多种功能，从而应用在更多设备中。 2.设计上的挑战 具备更多功能和更高的性能，也使USB-C增加了设计的复杂性，特别是在与其他无线射频装置共同工作时，USB-C装置需要有效管理射频干扰，包括对USB装置本身的干扰以及与其他射频装置(如 Wi-Fi 和蓝牙设备)之间的干扰进行管理。 二、问题与难处 在USB-C问世之前，针对USB速度对无线射频装置的干扰问题并未受到足够重视，然而随着USB速度提升，尤其在5Gbps和10Gbps的运行模式下，这一问题变得更为明显。这种干扰主要表现在2.4 GHz和5 GHz的无线射频装置上，可能会导致USB装置性能下降，甚至瞬间断线。同时，无线射频装置也会受到干扰，造成性能下降、讯号范围缩小以及连接不稳定。 【问题描述】 百佳泰经手过一典型的实际案例，客户产品为一款配备USB-C接口、Wi-Fi和蓝牙功能的笔记本电脑，该型号支援的USB速度包括5Gbps和10Gbps，但当Wi-Fi和蓝牙功能启用时，笔记本电脑的USB-C接口性能显著下降，仅达到略高于USB 2.0(480Mbps)的传输速度，同时Wi-Fi和蓝牙的连接也变得不稳定，常出现延迟或瞬间断线，主要面对的问题跟挑战如下： 性能下降： 在Wi-Fi和蓝牙启用的情况下，USB-C接口的实际传输速度大幅下降，从5Gbps或10Gbps降至接近USB 2.0的速度。不仅影响数据传输效率，也限制了高带宽应用的性能。 无线连接不稳定： Wi-Fi和蓝牙连接的不稳定性是另一个严重问题。当USB-C接口进行高速数据传输时，无线连接经常出现延迟或瞬间断线，对用户体验造成极大的困扰。 设计和兼容性问题： 对于设计师和工程师来说，在设计和实施USB-C接口时，如何有效地管理和减少射频干扰是个巨大的挑战。尤其是在集成多种功能(如USB、Wi-Fi和蓝牙)的复杂系统中，要确保兼容性和稳定性变得更加困难。 应对策略和成本： 针对上述问题，可能需要采取额外的设计和工程措施，如改进屏蔽和过滤设计、进行更严格的RFI测试等。这些措施会增加设计和生产成本，并且需要在产品开发早期就进行有效的测试和调整。 三、解决方案 针对该客户USB-C接口干扰无线射频讯号的问题，经由百佳泰分析发现了几个关键因素，并提出了对应的解决方案。 【问题分析】 1.阻抗不匹配和信号skew： RFI测试失败的主要可能原因包括trace上组件的阻抗不匹配、PCB贯孔过多以及trace不等长。这些问题会导致信号skew，产生共模讯号，进而影响RFI测试结果。 2.USB-C设备的整体屏蔽不足： USB-C设备的屏蔽设计未能有效阻挡讯号干扰，导致USB运行时对周围的无线射频装置造成干扰。 3.USB-C连接器的讯号外溢： 尽管USB-C连接器的屏蔽有所改进，但USB讯号仍可能从连接器的针脚之间外溢，干扰无线射频讯号，尤其在高速度模式（如 5Gbps 或以上）更为明显。 4.Wi-Fi和蓝牙天线走线过于接近PCB上的USB走线： Wi-Fi和蓝牙的天线走线与PCB上的USB走线距离过近，造成了相互干扰，影响无线讯号的稳定性。 【解决方法】 我们依照问题的分析，提供以下建议给客户进行调整的参考。 ✔ 改进屏蔽设计 目标： 提升USB-C接口的屏蔽效果，减少对无线射频装置的干扰。 建议措施： 增强屏蔽材料：使用高效屏蔽材料包覆USB-C接口，减少外部干扰。 改进屏蔽结构：设计完善的屏蔽结构，与PCB良好接触并确保USB-C 接口的讯号不会轻易外泄。 增强接地：提高屏蔽结构的接地效果，强化USB-C的GND、减少PCB GND与USB-C GND之间的阻抗不匹配，降低干扰。 ✔ 优化过滤措施 目标： 在设计中添加过滤器，以抑制高频噪声和干扰。 建议措施： 添加EMI滤波器：在USB-C连接器及其相关讯号在线安装EMI滤波器，同时为trace Tx±配备CMCC（共模电感），以抑制Tx±产生的共模信号并减少高频噪声。 设计滤波电路：集成滤波电路，过滤掉不必要的高频讯号，降低对无线射频的干扰。 ✔ 调整设计 目标： 在产品设计中考虑USB-C和无线功能的协同工作，进行设计调整和优化。 建议措施： 合理布局天线：确保Wi-Fi和蓝牙天线与USB走线保持适当距离，避免干扰。 优化PCB布局与设计：合理规划USB和无线模块的位置，避免高频信号线与敏感讯号线过近。减少PCB贯孔数量，防止讯号skew导致共模讯号，并确保trace等长设计，以降低讯号skew。 设计隔离区域：为USB-C和无线模块设计隔离区域，减少相互干扰。 ✔ 严格的RFI测试 目标： 全面测试电磁干扰，确保 USB-C 接口高速传输不会干扰无线功能。 建议措施： 遵循标准测试程序：根据USB-IF的USB 3.2 RFI System-Level Test Procedure进行测试。 测试Compliance Mode：确保USB-C进入Compliance Mode，发出CP0 pattern，符合干扰标准。 进行迭代测试：多次RFI测试和调整，确保产品在实际使用中的可靠性。 客户根据上述建议进行修正后，产品成功通过了测试，并在实际使用中表现正常，解决了之前讯号干扰的问题，改进措施不仅提升了产品的性能和稳定性，也改善了用户的使用体验，确保USB-C设备在高速传输和无线功能协同工作时的良好表现。

随着无线通信越来越普及，电磁辐射干扰的问题也越加被重视。依照USB-IF的规范，以往是用兼容性测试的方法来验证RFI的影响，但兼容性测试的判定较主观，所以至今年四月起（2021.04.01），USB-IF规定了新的测试项目：USB 3.2 RFI System Level Test。 本文将由多个主题：什么产品需要测试、怎么测试、以及如何判定测试结果，带您逐步了解RFI。 ◆ 什么类型的产品需通过USB3.2 RFI System Level Test（以下简称RFI测试）？ 符合以下全部条件的产品，皆须经过RFI测试： 使用 USB Type-C® 接头 支持 USB 3.2 Gen 1x1 以上的传输速率 产品类型为 Downstream Facing Port (DFP) ◆ 测试USB 3.2 RFI System Level Test的仪器列表 以下为您整理出必要使用的仪器、治具、厂商与型号（表一）： 表一 ◆ USB 3.2 RFI System Level Test 的测试流程 重点测试流程简述如下（如表二） ： 表二 ◆ 如何判定RFI测试结果呢？ 经过上述的测试流程后即可得到测试结果，以下撷取一个测试规范如图一： 图一 而测试结果判定标准如表三： 表三 细心的您一定可以发现4GHz~5GHz没有规范定义，这是因为无线通信目前尚未使用到该频段。 ◆ 测试环境 图二：标准连接方式 图三：GRL测试环境 参考文献 ◆ USB RFI System Level Test Procedure, Rev1.0, August 21, 2020 ◆ USB 3.2 RFI System-Level Test Procedure, Rev1.1, November 24, 2020 ◆ https://graniteriverlabs.com.cn/usb-compliance/ ◆Granite River Labs, USB-IF Compliance Tests 作者 GRL台湾测试工程师 曾威华 Wing Tseng 擅长USB、PCIe、SATA及DDR接口测试。GRL技术文章作者及演讲讲师。希望藉由帮助大家顺利测试拿到接口Logo，彼此互相交流共同成长飞翔。

The phenomenal proliferation of wireless devices (Wi-Fi, IoT, handheld products) has forced engineers who don't have much RF background to become somewhat RF-knowledgeable and meet performance standards. Fortunately, in many cases, the required tests are well-defined, the needed equipment and test suites are available, and there's often a specialist with experience on the team or one within reach.   Optical test, however, is a foreign area to most electronic engineers, and with good reason. Unless they are designing the display itself or the light-source drivers, most EE's usually don't need to know much about optical issues. Many of the issues are fairly basic, such as LED brightness and color. In contrast, advanced optical specialists are viewed somewhat the way RF engineers were perceived twenty or thirty years ago: a small, esoteric subculture involved with hard-to-grasp rituals and mystical idiosyncrasies. In-depth optical test is very different world, with parameters such as optical power, photon counting, strange spectrum issues, dispersion, radiance, efficiency, efficacy, and many others which are either alien to the RF world or have different meanings and measurements (even though both are based on Maxwell's equations, of course).   Soon, though, there may be a need for more optical expertise, just as RF went from being niche to mainstream. There is a proposal for a Wi-Fi-like equivalent—often called Li-Fi—for short-range mobile use in an office, meeting room, large hall, or retail store, but based on free-space (non-fiber) optical links; see Li-Fi Gets Ready to Compete With Wi-Fi , Retailers Test Visible Light Communications and A Bright Idea: Using Light to Transmit Data for some perspective .   The idea is to take advantage of the installation of LED-based lighting, by modulating the LEDs at roughly 100-Mb/sec data rates (obviously well beyond what the eye can perceive) so the LED serves two roles: area illumination and data transmitter. The marketing pitch from pureLiFi, a major innovator and proponent of the technology, is simple: "wireless data from every light bulb." A Li-Fi system uses optical links rather than RF for wireless connectivity, but is not limited to fixed, point-to-point use (image from pureLiFi).   Will VLC (visible light communications) be a success? I'll give my usual answers: a) I certainly don’t know, nor does anyone else, and b) it depends on the situation. It may not become widespread, but it may well be a good solution in well-defined cases, such as offices, meeting halls, or classrooms where RF links are being bogged down by too much traffic. Also, the mostly line-of-sight performance of optical inks (reflections from walls do work, to a lesser extent than a direct link) is both a limitation and a virtue, if you are concerned about sniffing and security, as is their inherent RFI immunity.   Implementing an optical link that supports mobile PCs and smartphones, and is also as easy to use as Wi-Fi has become, will take lots of work beyond the just deploying the optical emitters and receivers. Even if the LED-based lamps that provide the major nodes are simply screw-in replacements for illumination-only bulbs (as some proponents presume they can be), there's the issue of creating the backhaul network to link all these dual-purpose bulbs into a high-speed system. Providing that part of the total system may be as big a challenge as establishing the optical link between the user and the in-ceiling lamp itself. If Li-Fi does get some traction, even in niche situations, both designers of these systems and especially those who install them will have to learn about the realities of sophisticated high-speed, free-space optical test and measurement. There's a whole new world of instrumentation, test set-ups, and criteria, all very different from RF or even constrained optical-fiber situations. Do you think that Li-Fi optical links have a future? Are engineers ready for the optical test and measurement challenges?

Most designers are aware of the technique of using a spread-spectrum clock to diminish apparent EMI/RFI emissions. By deliberately dithering the system clock, the radiated energy is spread across the spectrum and thus its peaks are mnimized, which allows the product to meet regulatory or industry specifications. The technique is now well-established, and vendors offer clock ICs with adjustable spread widths and rates, as well as advanced pseudorandom spread algorithms. But when discussing the use of spread -spectrum clocking to meet EMI/RFI requirements, I find that engineers are divided on the approach. Some feel it is a legitimate, very-low-cost tool in the designer's kit that helps the design meets market requirements. Others feel it is short-cut cheat, too often used instead of proper EMI design methods. Still others feel it should be used only after all other conventional steps have been taken, such as shielding, grounding, layout changes, ferrite beads, to cite a few. One argument against using spread-spectrum is that you are very likely actually making your design "problem" into someone else's. As you spread the energy, yes, you may meet a specification, but you also introduce the likelihood of unexpected problems when your spread energy mixes with as-yet unknown or undefined energy in other nearby or connected systems, each with their own frequencies and amplitudes. In short: "hey, I met the spec—after that, it's your problem, not mine!" As with most engineering designs, there is no right or best answer. What makes sense depends for your project's priorities, budget, constraints, market forces, and the balance among all the tradeoffs which every design encompasses. Perhaps in the ideal world, the design would first be made as EMI-robust as possible, and then spread spectrum would be added for a little extra insurance, but only if needed. Have you ever used spread spectrum to reduce EMI and meet a spec? What's your view on spread spectrum as an EMI reduction technique? - It's a great idea, go ahead and use it right away; - Use it only after everything else that should be done has been done, and you are still stuck; - Use it only after you have already met the spec, just to buy a little extra margin; - Or don't use it at all, since the unforeseen consequences to the overall system and broader application are too risky? What do you think?

Most designers know the technique of using a spread-spectrum clock to minimize apparent EMI/RFI emissions. By deliberately dithering the system clock, the radiated energy is spread across the spectrum and thus its peaks are diminished, which allows the product to meet regulatory or industry specifications. The technique is now well-established, and vendors offer clock ICs with adjustable spread widths and rates, as well as advanced pseudorandom spread algorithms. But when discussing the use of spread -spectrum clocking to meet EMI/RFI requirements, I find that engineers are divided on the approach. Some feel it is a legitimate, very-low-cost tool in the designer's kit that helps the design meets market requirements. Others feel it is short-cut cheat, too often used instead of proper EMI design methods. Still others feel it should be used only after all other conventional steps have been taken, such as shielding, grounding, layout changes, ferrite beads, to cite a few. One argument against using spread-spectrum is that you are very likely actually making your design "problem" into someone else's. As you spread the energy, yes, you may meet a specification, but you also introduce the likelihood of unexpected problems when your spread energy mixes with as-yet unknown or undefined energy in other nearby or connected systems, each with their own frequencies and amplitudes. In short: "hey, I met the spec—after that, it's your problem, not mine!" As with most engineering designs, there is no right or best answer. What makes sense depends for your project's priorities, budget, constraints, market forces, and the balance among all the tradeoffs which every design encompasses. Perhaps in the ideal world, the design would first be made as EMI-robust as possible, and then spread spectrum would be added for a little extra insurance, but only if needed. Have you ever used spread spectrum to reduce EMI and meet a spec? What's your view on spread spectrum as an EMI reduction technique? - It's a great idea, go ahead and use it right away; - Use it only after everything else that should be done has been done, and you are still stuck; - Use it only after you have already met the spec, just to buy a little extra margin; - Or don't use it at all, since the unforeseen consequences to the overall system and broader application are too risky? What do you think?