标签: signals

In my seminars, I often tackle why we should understand at least a little electromagnetics theory, even for purely firmware people. But the subject is hard to understand and sometimes harder to believe, which is why the best book on the subject, " High Speed Digital Design, " is subtitled "A Handbook of Black Magic." Why is it important? You'll surely be probing your design with various tools like scopes and logic analysers, and every such probe has some impedance. As speeds get higher, that impedance is ever more likely to corrupt the operation of the device. But "speed" is poorly understood today. We equate clock rate with speed, which is only part of the story. Almost two hundred years ago polymath Jean-Baptiste Joseph Fourier showed that any periodic function can be expressed as the sum of sine waves of different amplitudes and frequencies. A square wave, like a microprocessor clock, is periodic and its Fourier series is:   In other words, a square wave is composed of the sum of the sine of the wave's frequency and each of its odd harmonics. (A harmonic is an integer multiple of the base frequency.) As you can see, no matter what the square wave's frequency is, it has harmonics that go to infinity, though at progressively lower amplitudes. A useful rule of thumb is that for most digital design we can ignore harmonics that exceed: f=0.5/T r Where T r is the square wave's rise time in nanoseconds. Every real-world signal takes time to transition from a zero to a one. Above f the Fourier components are down about 40 dB. Since a picture is worth a million bits, look at the following scope trace:   Figure 1—A square wave with 20 ns rise time. The top trace is a 1MHz square wave, just like a CPU clock. The bottom is an expanded view of the highlighted portion of the same trace. Note that what looks like a nice, quick zero to one transition actually takes quite a bit of time. In this case the rise time is 20 ns. Running 20 ns through the previous formula and it's clear that anything above 25MHz will be so far down we don't have to worry about them. But it does mean that 1MHz signal has important frequency components that far exceed the fundamental. That square wave comes from my scope's waveform generator. To show the Fourier effect more dramatically I built a circuit to improve the signal's rise time by feeding the signal through a fast gate.   Figure 2—Square wave fixer-upper circuit. It is possible to see the individual frequency components predicted by the Fourier series. Most modern scopes can compute the Fourier transform of a signal, as in the following screen capture.   Figure 3—10MHz sine wave The bottom trace is a simple 10MHz sine wave. Above it is the Fourier transform. Unlike a normal scope display where the horizontal axis is time and the vertical volts, the upper one is displayed in units of dB and frequency. In this case the screen's scale is set to 0MHz at the left and 20MHz all the way to the right. Note there's a very strong peak exactly at 10MHz, because 10MHz is the only frequency component in a 10MHz sine wave. Sure, there are some other things running around on that trace due to an imperfect waveform generator and some artifacts of the Fourier transform process. But the ugly stuff is 57 dB lower than the peak. That's 1/500,000 less than the 10MHz peak. Switching from dB to voltage makes this more obvious:   Figure 4—10MHz sine wave in volts. Here's the Fourier transform of the 20 ns rise time square wave:   Figure 5—Spectrum of a square wave with 20 ns rise time. The bottom trace is the 1MHz square wave from the scope's waveform generator, before going to the fixer-upper circuit. Above it is the Fourier transform. In this case the screen's scale is set to 0MHz at the left and 100MHz at the right. Each peak is one of the square wave's odd harmonics from the Fourier series. Unsurprisingly, the highest peak is at the wave's fundamental frequency of 1MHz. At 33MHz the signal is down 38 dB, and quickly rolls off from there. That's close enough to the rule of thumb for practical work. With the fixer-upper circuit the rise time is now 1 ns:   Figure 6—Spectrum of a square wave with 1 ns rise time. This looks different from the previous picture because now the Fourier transform window goes from 0MHz to 500MHz. There are a lot more harmonics displayed. But notice that the 40 dB point is now at 375MHz (the formula predicts 500MHz, again, close enough for a rule of thumb). It's important to remember that this is the spectrum of the same 1MHz signal; all that has changed is the rise time. If your little 8 bit system is clocking at just a few MHz, or even hundreds ofkHz, with sharp edges it may have all of the same high speed issues we usually attribute to much faster circuits. Sounds hard to believe. Some time ago a company asked for help with their 30 year old, 4MHz, Z80 design. A new batch of boards didn't work, though the design was unchanged. It seems the semiconductor vendor had increased the speed of some of the logic. Those gates that used to switch in 15 ns now did it in 5. The company had to redesign this board as a high-speed digital system simply because of the faster rise time. The takeaway is that we can't think of digital signals as we do simple sine waves. They are composed of a lot of harmonics, and, depending on rise time, those harmonics can have a huge effect on the circuit's operation. A 1MHz clock does not necessarily imply a slow circuit.

In my seminars, I usually discuss the importance of understanding at least a little electromagnetics theory, even for purely firmware people. But the subject is hard to understand and sometimes harder to believe, which is why the best book on the subject, " High Speed Digital Design, " is subtitled "A Handbook of Black Magic." Why is it important? You'll surely be probing your design with various tools like scopes and logic analysers, and every such probe has some impedance. As speeds get higher, that impedance is ever more likely to corrupt the operation of the device. But "speed" is poorly understood today. We equate clock rate with speed, which is only part of the story. Almost two hundred years ago polymath Jean-Baptiste Joseph Fourier showed that any periodic function can be expressed as the sum of sine waves of different amplitudes and frequencies. A square wave, like a microprocessor clock, is periodic and its Fourier series is:   In other words, a square wave is composed of the sum of the sine of the wave's frequency and each of its odd harmonics. (A harmonic is an integer multiple of the base frequency.) As you can see, no matter what the square wave's frequency is, it has harmonics that go to infinity, though at progressively lower amplitudes. A useful rule of thumb is that for most digital design we can ignore harmonics that exceed: f=0.5/T r Where T r is the square wave's rise time in nanoseconds. Every real-world signal takes time to transition from a zero to a one. Above f the Fourier components are down about 40 dB. Since a picture is worth a million bits, look at the following scope trace:   Figure 1—A square wave with 20 ns rise time. The top trace is a 1MHz square wave, just like a CPU clock. The bottom is an expanded view of the highlighted portion of the same trace. Note that what looks like a nice, quick zero to one transition actually takes quite a bit of time. In this case the rise time is 20 ns. Running 20 ns through the previous formula and it's clear that anything above 25MHz will be so far down we don't have to worry about them. But it does mean that 1MHz signal has important frequency components that far exceed the fundamental. That square wave comes from my scope's waveform generator. To show the Fourier effect more dramatically I built a circuit to improve the signal's rise time by feeding the signal through a fast gate.   Figure 2—Square wave fixer-upper circuit. It is possible to see the individual frequency components predicted by the Fourier series. Most modern scopes can compute the Fourier transform of a signal, as in the following screen capture.   Figure 3—10MHz sine wave The bottom trace is a simple 10MHz sine wave. Above it is the Fourier transform. Unlike a normal scope display where the horizontal axis is time and the vertical volts, the upper one is displayed in units of dB and frequency. In this case the screen's scale is set to 0MHz at the left and 20MHz all the way to the right. Note there's a very strong peak exactly at 10MHz, because 10MHz is the only frequency component in a 10MHz sine wave. Sure, there are some other things running around on that trace due to an imperfect waveform generator and some artifacts of the Fourier transform process. But the ugly stuff is 57 dB lower than the peak. That's 1/500,000 less than the 10MHz peak. Switching from dB to voltage makes this more obvious:   Figure 4—10MHz sine wave in volts. Here's the Fourier transform of the 20 ns rise time square wave:   Figure 5—Spectrum of a square wave with 20 ns rise time. The bottom trace is the 1MHz square wave from the scope's waveform generator, before going to the fixer-upper circuit. Above it is the Fourier transform. In this case the screen's scale is set to 0MHz at the left and 100MHz at the right. Each peak is one of the square wave's odd harmonics from the Fourier series. Unsurprisingly, the highest peak is at the wave's fundamental frequency of 1MHz. At 33MHz the signal is down 38 dB, and quickly rolls off from there. That's close enough to the rule of thumb for practical work. With the fixer-upper circuit the rise time is now 1 ns:   Figure 6—Spectrum of a square wave with 1 ns rise time. This looks different from the previous picture because now the Fourier transform window goes from 0MHz to 500MHz. There are a lot more harmonics displayed. But notice that the 40 dB point is now at 375MHz (the formula predicts 500MHz, again, close enough for a rule of thumb). It's important to remember that this is the spectrum of the same 1MHz signal; all that has changed is the rise time. If your little 8 bit system is clocking at just a few MHz, or even hundreds ofkHz, with sharp edges it may have all of the same high speed issues we usually attribute to much faster circuits. Sounds hard to believe. Some time ago a company asked for help with their 30 year old, 4MHz, Z80 design. A new batch of boards didn't work, though the design was unchanged. It seems the semiconductor vendor had increased the speed of some of the logic. Those gates that used to switch in 15 ns now did it in 5. The company had to redesign this board as a high-speed digital system simply because of the faster rise time. The takeaway is that we can't think of digital signals as we do simple sine waves. They are composed of a lot of harmonics, and, depending on rise time, those harmonics can have a huge effect on the circuit's operation. A 1MHz clock does not necessarily imply a slow circuit.  

2013年6月6日     由Dave Bursky的特约编辑 越来越高的无线网络带宽的需求已经从早期的无线网络的一个微不足道的10兆位/秒的数据传输速率推雇硬件基于IEEE 802.11b标准的1.3 Gbit / s的峰值数据传输速率，通过利用最新的IEEE 802.11交流标准。 这家新近批准的标准，利用先进硅集成收拾丰富金额多个无线电信号处理，设立多（MIMO）子系统，采用多达4个发射和4个接收通道，还有更多的功能。 虽然蜂窝无线电标准有没有关系，很多人指5G无线系统11AC，11AC标准以来基本上是第五大无线网络标准（以前的“代”的标准与802.11b开始，然后802.11克，其次是802.11 a，然后802.11n标准，每一代人提供更高的数据传输速率，并与移动的经营载体11A和11n，从2.4 GHz频段的5 GHz频段，与802.11设备通常提供双频段能力（2.4 / 5 GHz）的 802.11ac标准向下兼容802.11a和11n的5 GHz的频率，但没有一个“传统”模式与802.11b连接，和11g的无线接口。 通过消除较低频率的无线电，设计师开辟一些地区在芯片上添加了许多新的功能，如波束形成和增强功能，以提供更好的服务质量（QoS）的。 此外，其他的无线接口已添加一些厂商 - 由少数的802.11ac芯片供应商已经集成了蓝牙和NFC（近场通信）接口。 在无线网络上的QoS已经成为一个关键的问题，因为现在很多的网络流广泛大量的视频和音频内容，并没有任何人喜欢的视频或音频内容，休息时或启动和停止。 各种芯片供应商为他们的系统级芯片（SoC）解决方案，在MIMO信道，包括蓝牙，NFC，甚至FM无线电接收器的数量的差异，各自采取了不同的集成方法目前只有极少数的芯片供应商 - 博通，Marvell公司，Redpine公司信号，高通Atheros公司，Quantenna公司提供的802.11ac解决方案。 博通，例如提供BMC4335，它调用一个完整的单数据流5G WiFi系统。 由于这种芯片只包含一个发送和接收通道，其最大数据传输速率是有限的433.3兆位/秒。 在芯片设计采用40纳米CMOS工艺，已经集成了媒体访问控制器（MAC），物理接口（PHY），2.4和5 GHz操作的射频电路（传统设备的兼容性与支持802.11a/b/g/n ），调频收音机，以及蓝牙无线电设备处理4.0低能量的协议，以及高速标准。 该芯片是平台无关的，可以添加到任何手机，平板电脑或其他平台。 为了确保可靠的连接和良好的区域覆盖，该芯片还采用了先进的波束形成优化的天线辐射模式，既低密度奇偶校验（LDPC）和空时分组编码（STBC），以减少传输和接收错误。 的基础上，基本芯片，博通有电路的多个变体，包括2×2和3×3 MIMO无线射频，以达到更高的数据吞吐量。 BCM4360和BCM43460有三个空间复用信道，可以实现高达1.3 Gb / s的数据传输速率，，而BCM4352和BCM43526有两个通道，最大出866.6兆位/秒的数据传输速率。 4 MIMO信道走向全口径Marvell的Avastar 88W8864提供了一个1.3 Gbit / s的峰值数据速率，并利用波束成形和LDPC确保信号质量。 Avastar 88W8897提供最高的数据传输速率为866.6兆位/秒，4×4通道芯片提供了一个成本更低的替代品。 与Broadcom公司的芯片，88W8864和88W8897不包括FM收音机，但他们添加NFC功能的支持点至点使用Miracast规格的高清视频流。 高通Atheros公司有1 - ，2 - 和3流解决方案在其VIVE家庭提供433.3兆位/秒至1.3 Gb / s的数据速率范围。 该芯片还包括蓝牙4.0低能量的无线电，也可以工作在高速模式。 片剂，WCN3680移动802.11ac解决方案集成了蓝牙4.0和FM功能，而笔记本电脑的QCA9862与QCA9860为2 - 3流，双频802.11ac的解决方案集成了蓝牙4.0的连接。 该公司还开发了与Wilocity结合，三频芯片QCA9005，共同集成的60-GHz 802.11ad的标准称为WiGig联盟。 WiGig联盟的界面提供多千兆位网络，数据同步，视频和音频流，同时保持其无线总线扩展对接的能力。 QAC2300从Quantenna的两芯片解决方案，提供一个完整的4×4 MIMO收发器相结合的802.11ac和802.11n信道。 平行802.11ac和802.11n信道通过使用该芯片组可以实现高达2千兆位/秒的传输速度。 这两个芯片组成与4×4的MIMO信道的数字基带和RF芯片，该芯片支持在5 GHz的802.11ac标准。 Redpine公司信号也分割成两个芯片，其解决方案已制定了一个单一的通道和一个三通道的基带芯片，RS9117和RS9333，分别。 二者都采用了蓝牙4.0的收音机和一个ZigBee接口。 补充几个基带芯片射频收发器选项 - RS8221，8331和8112。 RS8221是CMOS双频段（2.4GHz和5GHz）射频功率放大器支持1×1或2×2的信道配置，而RS8331可以处理1×1），（2×2或3×3 MIMO配置（图3）和RS8112有单信道的输出，可以在2.4和5 GHz频段上同时操作。 开始使用的运动鞋网的网络接口迁移到每一代的人，是令人印象深刻的，有能力提供以千兆位速度的数据。 而且，它不会停在那里。 未来流程和集成的进步将允许更高的数据速率和更好的流媒体 - 尤其是重要的超高清视频系统（4K分辨率），现在已经开始出现改善的QoS。

  2013年6月7日   RTLS跟踪货物在运输位置识别系统是完美的，但有轻微的扭曲。 实时定位系统（RTLS）位置识别系统已被证明有用内预先定义的area.That的的定位，识别和跟踪物体或人实时跟踪货物在运输为什么他们是完美的，但有轻微的扭曲。 通常与RTLS，资产，设备，人与动物的物体，如可以通过自动的，事件驱动的机制跟踪一个常数communication.Through使用无线标签附着的物体，固定参考点（设备）接收无线信号/数据从标签来确定位置。 使用此信息在许多应用程序在不同的行业，包括政府（国防，军工，航空航天），医疗保健，工业和制造业，物流和retail.The最常见的应用已在这些市场中的资产跟踪/管理，人力资源管理，供应链管理，检测和监控。 标签已经适应不同形式的例子包括徽章标签，手腕标签和资产标签（见图1）。 智能标签启用RTLS enginesto轨道位置。 图1：左到右：徽章标签，手腕标签和资产标签。 可实现实时定位系统使用现有的Wi-Fi基础设施。 接收信号强度指示（RSSI）andtime飞行（TOF）两种常用的RTLS技术。 此外，通过使用多个天线的方向的到达角（AOA）可以被额外地利用。 对于交通行业的资产跟踪，GPS和/或基于蜂窝的定位和RTLS的组合是必需的。 GPS或基于蜂窝的定位有助于跟踪在更大的地域，远远超出了RTLS的Wi-Fi覆盖。 实时定位系统可以帮助跟踪资产在较小的地理区域的无线局域网络（WLAN）扩展。 在需要更大的地理区域的情况下，单独实时定位系统就足够了。 消耗功率和电池寿命的标签设计和整个系统的部署体系结构中是重要的方面。 很多时候，资产可能是在休息的大部分时间。 除非标签的移动，其位置不会改变，因此没有必要沟通与接入点（AP）。 标签在这种情况下，可以采用运动检测。 当资产的移动，运动检测器被唤醒的标记，然后允许它被跟踪的AP进行通信。 在一些应用中，目标可能是只知道资产时，离开源的处所，当它到达其目的地的处所，无需跟踪沿线。 在这些情况下，扼流点都可以使用。 源处所的出口处的扼流点醒来的标记，然后与所述接入点进行通信。 定位引擎和数据库寄存器该资产已经离开处所。 其后，标签可以进入省电模式，并保持其沿路线到目的地的处所。 目标物业的切入点的扼流点再次唤醒它，并定位或与供应链管理软件相关联的数据库注册该资产已经到达了目的地。 资产然后，可以进一步跟踪宅内。 在大型货物运输和航运，集装箱可垂直堆叠。 同样是真实的箱子或物品的大型仓库，可以竖直堆放在货架。 目前，大多数的RTLS解决方案在两个方面找到对象。 在建筑物中，每个楼层有一组单独的接入点或用于定位的装置，并因此在平面图上的两维定位的对象可能就足够了。 这种方法是不可行的航运码或仓库。 因此，为了获得一个仓库中的容器或箱的垂直位置，立体定位是必需的。 图2：系统。WiSeMote代表了一种低成本，高精度的实时定位系统的硬件架构的标记，的SuperMote代表的廉价版的AP，的NetworkMote代表与网络连接的设备。 的SuperMote获得的RSSI或从WiSeMote和继电器的NetworkMote的进货信息的时间差。 定位和RTLS发动机被安置在CLAAS服务器。 RSSI指纹图谱中常用的两个尺寸可以延长，但现在具有指纹，可在第三个维度有一个显着的负担。 另外，根据AP的部署上，有可能不会是一个显着的变化RSSI。 使用RSSI计算模型，纳入明确的信息，周围的环境可能会受到错误或在周围有变化时，需要更新模型。 因此，飞行时间可以用来帮助三维分配。 此外，可以使用多个天线，以提高通过添加进货信息的角度定位。 一些的TOF技术涉及标签和AP之间的帧交换，因此需要花费额外的电源相比使用常用的RSSI技术，比如与思科兼容扩展（CCX）标签。 存在替代技术，找到标记使用相同的传播时间，但不需要这个额外的电源。 有与两个相关联的权衡，但为受影响的数量增加，是优选的替代技术。 这样的一个替代方法是只发送标签，多个接入点的时间同步的接收到此传输和进一步喂饱当前信息定位引擎。 提高准确性，可靠性和覆盖受影响的数量增加，但是，这也显着增加了成本。 一种替代的体系结构采用成本较低的设备，主要只从标签信息中继到另一个网络连接的设备，这是连接到互联网或企业内部网，通过它连通的定位引擎。 这些都可以更换AP或与他们一起使用，以增加定位。  

  美国Redpine Signals公司总裁兼首席执行官Venkat Mattela （点击放大） 　　在美国开发无线LAN用IC的企业中，有一家企业因积极扩展产品而备受关注。这就是位于加利福尼亚州硅谷地区的风险企业Redpine Signals公司。该公司曾面向传感器网络等提供无线芯片和收发模块，今后则将通过率先提供支持无线LAN新一代标准“IEEE802.11ac”的收发IC，来挑战由博通及高通创锐讯等知名企业垄断的市场。本站就Redpine Signals将在无线LAN市场采取的举措采访了该公司总裁兼首席执行官Venkat Mattela。 ——贵公司真的已经开发出符合IEEE802.11ac标准的无线LAN用IC了吗？ Mattela：11ac的芯片已开发完成。正与移动领域的某知名企业就授权提供电路技术进行磋商。我们曾向德国英飞凌授权提供过支持IEEE802.11n的电路技术，这次也一样。 ——现在很多半导体厂商都在开发支持11ac标准的无线LAN用IC。您认为贵公司的优势在哪里？ Mattela：我们正在自主开发大部分要素技术。比如，我们正在设计无线LAN基带电路、RF收发器电路及功率放大器。并且，CPU内核不使用ARM及MIPS等，而采用自主技术的内核。拥有这些自主技术是我们的优势所在。 　　我们不只开发IC。还提供包括外围电路在内的无线LAN用收发模块。另外，还能够提供包括评测软件、中间件及主机MCU在内的评测工具包和参考设计。因此，我们与瑞萨电子及美国赛普拉斯半导体（Cypress Semiconductor）等MCU厂家签订了合作协议。 　　我们115名员工中，有95％是工程师。其中大部分是印度人。这些优秀的工程师正努力扩展软件和硬件均领先于其他公司的产品。另外，我们的目标是2013年实现IPO（公开发行股票）。 ——无线LAN用收发IC和模块将瞄准哪些市场？ Mattela：智能电网及产业领域的传感器网络等是发展非常缓慢的市场，还不需要尖端技术。不过，供货量非常大，因此我们非常关注这个市场。到2020年元件供货量有可能达到几十亿个以上，非常令人期待。这些市场现在对每个IC和模块厂家的订货量要比智能手机等小，但价格高。我们打算通过这个市场赚取资金，用作开发资金，同时针对大市场——智能手机及便携产品等消费类电子领域进行研发。总之，我们的主战场还是移动和电脑市场。 ——贵公司不针对美国GainSpan从事的传感器网络市场吗？ Mattela： 我们并不专门针对传感器网络领域。从这个意义上说，我们跟GainSpan这样的公司不同。我们最终要与博通及高通创锐讯进行同台竞争。 　　回过头来想一想，2000年开发无线LAN的风险企业超过90家。其中也有创锐讯（Atheros）等企业。而现在已经锐减到7家左右。我们是存活下来的企业之一。我们已经拥有400多家客户。与博通等知名企业相比，客户多是我们的优势。我想到2017年，在无线LAN市场上，我们能够跻身世界5强。 ——博通和高通创锐讯都是非常强大的对手。贵公司将如何挑战？ Mattela： 我非常清楚博通和高通创锐讯是不容易战胜的对手。我们并不是单*匹马。而是与伙伴企业一起作战。比如，上面提到的瑞萨电子和美国赛普拉斯半导体等就有望成为我们的合作伙伴。这些企业为扩大MCU市场而吸引苹果公司等采用时，就需要独立的无线LAN芯片厂家。这时，我们就能成为良好的合作伙伴。 ——博通和高通创锐讯都在开发采用毫米波的IEEE802.11ad标准IC。贵公司准备开发11ad芯片吗？ Mattela：就在接受采访前，我们公司刚召开了有关11ad的会议。我们正考虑开发整合11n和11ad的芯片。 　　关键是单独开发11ad完全没有意义。只有将11n与11a/b/g等功能结合起来才有价值。11ad利用毫米波，因此只能近距离使用。11ac也能单独使用，但11ad不行。 　　凭借毫米波通信的单功能芯片取胜的厂家都很艰难。比如美国SiBEAM公司。如果着手毫米波通信，那就需要整合型解决方案。 ——除11ac和11ad以外，贵公司还在开发具有哪些功能的芯片？ Mattela：现在我们正致力于拟于2012年供货的无线LAN整合型芯片。这种芯片不仅集成了11a/b/g/n，还集成了蓝牙、GPS、FM收发及NFC功能。2012年，我们还将供货追加了ZigBee功能的芯片。 （记者：蓬田 宏树，《日经电子》）