标签: TDR

什么是TDR？ TDR是英文Time Domain Reflectometry 的缩写，中文名叫时域反射计，是测量传输线特性阻抗的主要工具。TDR主要由三部分构成：快沿信号发生器，采样示波器和探头系统。   TDR测试原理 TDR通过向传输路径中发送一个脉冲或者阶跃信号，当传输路径中发生阻抗变化时, 部分能量会被反射, 剩余的能量会继续传输。只要知道发射波的幅度及测量反射波的幅度，就可以计算阻抗的变化。同时只要测量由发射到反射波再到达发射点的时间差就可以计算阻抗变化的相位。   图(1) TDR示意图 根据反射原理，反射系数 公式(1)中，ZDUT是待测器件的阻抗，Z0是TDR的输出阻抗，通常为50ohm标准电阻，Vrefelected和Vincident分别是反射波幅度和入射波幅度，可以通过示波器测得，算出反射系数ρ，从而算出待测器件的阻抗ZDUT。 算出待测器件的阻抗，接下来再来看看待测器件的电气长度如何计算。 TDR产生一个阶跃信号到待测器件中，会产生入射波，入射波经过时延TD之后在待测器件中遇到阻抗不连续的地方，又会产生发射波，反射波将会叠加在入射波上，再经过时延TD到达TDR的输出端。 通过仿真工具模拟TDR，如图(2)  图(2) 模拟TDR 模拟采样示波器上看到的电压和阻抗曲线，如图(3)，图(4)   图(3) 电压曲线   图(4) 阻抗曲线 在图(4)中可以看到，当负载呈容性不连续时，阻抗会偏低；当负载呈感性不连续时，阻抗会偏高。PCB中常见的阻抗不连续的地方, 过孔、焊盘、拐角通常呈容性，跨分割处、breakout等通常呈感性。 图(5) 感性阻抗不连续   图(6) 容性阻抗不连续   作者：一博科技 深圳市一博科技有限公司专注于高速PCB设计、PCB制板、PCB贴片、物料供应等服务: 作为全球最大的高速PCB设计公司，我司在中国、美国、日本设立研发机构，全球研发工程师500余人。超大规模的高速PCB设计团队，引领技术前沿，贴近客户需求。 一博旗下线路板厂可提供2-64层，从样品到批量的PCB多层板生产服务。最快仅需2天。 一博旗下SMT贴片加工厂有12条SMT加工产线，最快8小时交付。  

全新PicoSource® PG900系列脉冲信号发生器是一款高速，低成本的仪器，可用于单端和差分脉冲测量应用。快速过渡脉冲可利用光谱信号立即激发传输线路，设备和网络。此种脉冲对我们要做的很多高速宽频测量至关重要，比如时域反射计（TDR），半导体测试，千兆互联和端口测试以及雷达系统测试。 通常使用该发生器将产生光谱含量连接到50 ohm的线缆，连接器，RF半导体或其他被测设备中。然后可通过带宽或采样示波器监测和显示反射或透射脉冲。被称为“时域发射或透射”的分析（TDR/TDT）可替代矢量和标量网络分析。它们已广泛用于高速数据路径的开发、评估和测试中，如以太网，USB，HDMI和SATA，和RF，雷达和微波装置，线缆，网络和设备。   与传统笨重昂贵的台式仪器不同，PicoSource PG900信号发生器是紧凑的USB设备，连接到windows操作系统的PC上。这使得紧凑型便携仪器具备卓越微波性能有点，并配置有高分辨率图形显示屏，可通过键盘、鼠标或触摸屏轻松进行设置。这种低成本便携仪器能够让RF和微波测试走出实验室，执行现场测量。凭借这些优点，此款全新脉冲发生器在所有宽带设别设计人员、安全工程师以及维修技术人员的工具箱中占有一席之地。   “PicoSource PG900脉冲发生器对于现代差分测量挑战不同寻常但是至关重要，其具备可调节差分输出，可调节度达到1ps分辨率，”业务发展经理Mark Ashcroft解释说。“如此一来可忽略测试连接和固定装置中定时不均衡问题；或将时序偏差慎重引入系统压力测试中。”同时，还可在单端模式中运行所有输出。   PicoSource PG900系列提供两种不同的触发阶跃信号发生技术，以满足不同应用需求。PG911配有集成阶跃恢复二极管输出，可提供60 ps的过渡时间，其中每个输出具有2.5V到6V的较大输出调节范围。这些脉冲能够支持高动态范围和远距离测量，可操作多数传输系统和设备中所有信号振幅。PG912使用外部隧道二极管脉冲源提供更加快速的过渡时间（40 ps），其中界面固定振幅正好为200mV。第三个型号PG914在一节省空间的经济型设备中集成了这两种技术。 图：脉冲源 图：脉冲源内部结构   所有模块均配有低抖动外触发器输入和输出以及多种宽度、周期和时延可调的外部触发时钟。与触发输入和输出相关的脉冲边缘抖动小于3 ps RMS。   价格最低的为60 ps PG911，4905英镑(US$8095 or €6495)，其中包含软件和2年保修。Pico Technology还提供种类齐全的TDR和TDT附件，以便差分脉冲发生器与PicoScope 9000系列12GHz和20GHz采样示波器配合使用。索要详细资料：sales@hkaco.com

Pico9300系列的20GHz采样示波器带有TDR测量功能，但是一直以来对这个功能都是一知半解的，在网上搜集了大量资料，这几天有时间抽空学习，记录学习的过程跟大家分享，有不对的地方可以一起交流。欢迎指正。   一、TDR的定义 TDR是（Time Domain Reflectometry）时域反射技术。该技术是产生沿传输线传播的时间阶跃电压，用示波器检测来自阻抗的反射，测量输入电压与反射电压比，从而计算出不连续的阻抗。 简单说：就是相对一个已知的标准阻抗上的反射，测量一个未知的器件的反射。 从定义上看，脉冲源是进行TDR测量的必要条件。Pico9000系列示波器自带的TDR脉冲源分别是100ps，60ps，40ps上升时间。（下图是60ps上升时间）   所以，您只需要购买一台示波器（12GHz或者20GHz），就标配有进行TDR测量的脉冲源，而不需要再另外购买昂贵的脉冲信号发生器，就能进行单端或者差分 TDR测量。   而且，集合了示波器+脉冲信号发生器（差分）+TDR测试，只是一个小小的模块，尺寸是：170 x 255 x 40 mm，重量1.1kg，能完全取代传统的笨重的台式示波器。   二、TDR的工作原理 1、TDR的工作原理示意图：   如上提到的，Pico示波器自带有阶跃信号发生器，跟示波器集成在一个小盒子里面，所以大大节省了空间和价格。     阶跃信号发生器向被测系统产生一个正向（负向）的阶跃信号，该信号沿着传输线向前传输。如果负载阻抗等于传输线的特性阻抗，将没有信号反射，示波器上能看到的只有发送的阶跃信号；假如负载存在失配，将有部分的输入信号被反射，示波器上将出现反射信号和输入信号的叠加。如下图所示：  

近期拜访客户（北京高校和研究所），演示PicoScope9311的TDR功能，测量线缆的阻抗等，客户都会问到我们标称的60ps的上升时间，怎么测量。 所以抽空整理了一下步骤，跟大家分享一下。 按照下面配置确保正确的TDR上升时间。 使用PicoScope9311提供的30cm线缆，直接连接30cm从TDR输出到通道1输入。 步骤： 1、连接示波器，接通电源60分钟之后，进行水平和垂直方向的校准。 2、按照图片连线 3、设置PicoScope9300软件：       右击 这个按钮 ，然后选择 Factory Setup。       点击TDR/TDT ，然后选择Simulus 菜单。      点击Output1的On ，然后Output2.      设置限制电压为7V。      设置正输出幅值到5。      取消通道2.      设置通道1垂直标尺到200mV/div      设置通道1偏置offset为0mV。 4、设置测量：       点击Measure，然后点击Display菜单中的Statistics       点击X Parameters菜单中的Rise time       点击Definition，然后Method菜单中的User Defined。 5、经过100次采集之后（看Total Wfms 列）在测量区域Measurement Area记录Rise time平均值结果。它将小于60ps。 差分TDR PicoScope9311-统计 英国PicoScope9300系列示波器，20GHz带宽，4通道输入，1通道9.5GHz光输入，TDR/TDT测量，60ps和40ps上升时间，眼图测量，Fiber Channel， SONET/SDH，以太网，PCI Express， 无限带宽（ InfiniBand ）， RapidIO等内置遮罩测试。内置时钟恢复，外部触发等，包含频谱分析，任意波形发生器等功能。更多详细请与Pico Technology中国总代理广州虹科电子科技有限公司联系，020-38743030.

When talking about high-speed circuits, it is imperative to take proper care of signal terminations. Yes, I'm talking about transmission lines. We all know that, even at the printed-circuit level, improper termination of a high-speed signal can lead to all kinds of nasty realities in RF, analogue signalling, or digital circuitry. In RF circuitry, it's often called the voltage standing wave ratio, or (V)SWR, which represents the amount of reflected energy seen by the output amplifier. In an ideal system, regardless of type, the SWR is 1:1. That is, the power sourced by the driver is the same as the power sunk by the receiver. Again, regardless of system type, if the (V)SWR is too bad, the reflected power not only can cause distortions, but also can burn out the driver or even the receiver. What am I talking about? Well, this video explains what I did in a recent project. I have a digital signal with a programmable frequency being output on a pin. At low frequencies, you can see that the output looks pretty good, but as the frequency increases, you can see quite a lot of oddball behaviour. What causes reflections? I won't go into great detail, because many books have been written on this subject. For now, I'll say it's a result of transmission line theory. It's caused by an impedance mismatch between different parts of the circuit. An antenna is actually matching the impedance of free space to the impedance of your transmitter's output driver or receiver's input stage. I think (and correct me if you know better, but please do it in nice, easy terms that I can understand) that this has to do with the propagation velocity of a signal through the medium. Energy travels through the medium at a certain speed but is not absorbed at the same rate as it is being received. This causes a reflection (echo) to go back through the medium. Reflections will occur at any point where there is an impedance mismatch. When a wire, cable, or PCB trace becomes a transmission line is a function of frequency and distance. A rule of thumb is that, when the length of the transmission line is more than about 2x the wavelength of the highest frequency being carried, you have a transmission line. That is, if my frequency is 1MHz (wavelength = 300m), then I have a transmission line when my cable becomes somewhere in the neighbourhood of 600m long. This is because there are now multiple values on my line at any given time, and the reflection of one value can interfere with the value at any given point on the line, distorting the signal. Each time the signal echoes, some energy is lost (heat), but as the signal bounces back and forth between the various discontinuities, the signals can stack up because of superposition. Now, let's talk specifically about digital circuits. Given the explanation on the previous page, you'd think you never need to worry about a 1MHz digital signal, right? Well, there's another rule of thumb specifically for digital circuits: You should treat a wire or interconnect as a transmission line if the propagation delay is more than 1/6 of the rise time of the digital signal. A square wave is made of many frequencies. Recall your college days (way back when) when your instructor talked about the Fourier Series? This is a perfect example. A square wave is the prime frequency plus the odd harmonics. The propagation delay of a signal through a medium is also dependent on the signal's frequency. Different components of our square wave reach the end of the line at different times. If I have a square wave with a slow edge (say, 5µs), then a three-inch trace is no big deal. The distance traveled is so small that all the frequencies get there at almost the same time. But if the edge is, say, 1ns, maybe we'll have ringing on the signal. "Yes, we already know all this, and your explanation isn't all that good," you say. "Why are you bugging us with this ancient news?" When I was working on those F-4s, we occasionally had a broken wire. Look closely at the picture below. This bird has cables that go from the near the tip of the nose all the way to the tip of the tail. That would be more than 40 feet (convert to m) if it were a straight line, but there's nothing straight about it. Believe me.   A real F-4 aircraft. There are two ways to find the location of a fault in one of those wires. The first is long, hard, and laborious. It involves finding an access panel where the cable runs. You open the panel, break the wire (this is usually a splice point), and use an ohm meter ('re' for unit of measurement) to shoot the cable both ways. Now you know if the problem is in the front half or the back half. You keep going like this until you isolate the bad section and replace it. Then you go about repairing all the breaks you made and sealing the jet back up. Now, let's talk about the second way to find the fault—one of the greatest troubleshooting tools of all time. It's a time domain reflectometer (TDR). It makes use of the principle of reflections to help find faults in long cables. It is really an oscilloscope combined with a pulse generator.   A TDR is really an oscilloscope with a signal generator. (Source: ePanorama.net) The pulse is sent down the cable, and you use the oscilloscope to measure the signals on the wire. A break in the wire looks like a pulse up. A short looks like a pulse down, and a connector usually looks like a glitch. Places where the cable is going bad (e.g., due to corrosion from water infiltration) look, well, crummy. TDRs are essentially oscilloscopes, except the time-per-division knob is continuously variable and is calibrated in tenths of feet. We could just start dialling the knob until we saw the fault, look at the knob, and know it's about 37.5 feet down the cable (likely in the engine compartment). That's a lot easier than the other way of doing it. Some people think TDRs can be used only with coax cables, but I'll tell you that just isn't true. Of course, that's how they work best, but they work well on twisted pairs, as well. In fact, they work rather well if you have any kind of multi-conductor cable where you can probe both lines together. In fact, the TDR is so sensitive that you could just lay a long wire on the ground outside the plane and still get a pretty decent reading on the location of the fault—within a few feet. (I've done this once, and the manual for the TDR referenced it.) TDRs come in two varieties—electronic (like I've been discussing) and optical (for optical fibres). If you ever need to deplay or troublwshoot some long cables, look into one of these devices. Unfortunately, they're kind of expensive (imagine that) to have one of your own, especially given how rarely you need them. The cheapest electrical TDR I've seen recently costs more than $400. However, if you'd like to do a fun project at home, take a look at this . This guy is basically building his own and telling us all about it. As for my opinion about the best electrical troubleshooting tool of all time, take a look at this video . What's your favourite troubleshooting tool? Tom Burke Senior Systems Engineer