1、 第 1 页阻抗匹配摘 要在长线传输中，总是伴随着反射的发生，反射会造成信号源的发热甚至烧毁，还会使得信宿在接受信号时带着干扰。使用阻抗匹配技术使得信源阻抗匹配，就能有效的降低反射干扰，从而解决这些问题。内阻为实数且等于特性阻抗的信号源称为匹配源。当始端接了匹配源，即使终端负载和特性阻抗不匹配，负载产生的反射波也会被匹配源所吸收，不会再产生二次反射，从而避免了在通信中由于反射引起的重音、重音现象同时也避免了反射电压烧毁反射机的可能。因此，在始端接入匹配源具有重要的现实意义。关键词：输入部分，推动，混频器第 2 页阻抗匹配:素数我们经常会遇到这样一个专业名词阻抗匹配。在各方面的电子、射频和音响等
2、工程领域。然而,即使是在这些领域。 阻抗匹配也经常被误用,许多人就不知所措,。或许可能是他们并不是真正的理解它背后的概念。在本文章中，我们试图弄清楚阻抗匹配是什么,为什么它在一些领域是十分重要的，而在其他领域却不重要。 教科书通常用一个发电机带动一个电阻负载的一个很简单的例子来解释阻抗匹配的概念,如图 1 所示。由于发电机有其自己的内阻（RG） （就像所有真正的发电机一样）每当我们连接负载输出端子时，这往往作为热量从发动机的输出功率中消散。所以来自电力的机械动力不能完整的输入发电机，因为总有些会在 RG 被浪费掉。当早期的电气工程师面对这个问题，他们很自然地尽其所能，以减少发电机的内阻。然而，
3、他们仍不可避免地留下一些内阻,因为减少内阻为零是不可能的,除非你的发电机运行在温度接近绝对零度(0 Kelvins，或 -273C)。一旦他们将内阻减到最小，下一步就是试用不同的负载电阻来最大限度地减少浪费在其中的功率量。他们的发现如图 2 所示，该图描绘了功率转换到负载 RL 上的数量与电阻的关系。正如你看到的，发电量达到峰值时，最大负载电阻是与发电机电阻相同或相匹配的。不管是高于还是低于这个数值都会失去价值，如果我们要最大限度地能够达到负载的功率，显示匹配这两个显然是可取的。第 3 页图片 2因此，这是匹配电阻与发电机负载的想法的出处。不久，它被扩展至一般情况任何连接到电源的负载阻抗能源或
4、电压（电动势） ，有其自身的内部源阻抗。此后，这个匹配负载和发电机阻力的想法成为一种匹配源和阻抗，即阻抗匹配。现在这也许听起来简单而直观,但是重要的是要记住这个想法从何而来，也意识到当我们这样做究竟是什么，匹配负载和发电机/ 源电阻或阻抗。诚然，功率传送到负载将是一个最大值，但在同一时间，实际在发电机内部的功率正在消退，和达到负载相同！换句话说，有一半的总功率正在变成热能发电机内的 RG ，因为它的电阻是总连接整个发电机电路的一半。另一半则是负载电阻 RL。对同一个原因， RG 和 RL 现在作为横跨发电器的一台 2:1 分压器，以便一半仅发电器的产品电压(即) 横跨装载出现。根据电压调动，然
5、后，匹配阻抗不是特别高效率的： 它实际上有 6dB 损失。这是否意味着，阻抗匹配真的只适用于发电厂的发电机？不，其实它并没有真正申请或者至少，不仅仅如此。它的真正含义是，当你以减少负载来增大发电机的功率，达到一个临界点时，会有一半输出功率被浪费掉了。显然具有很高的功率的发电机，它不是一个好主意，加载它们甚至严重更不用说下降的 RL 更进一步，甚至更多的功率内丢失比达到负载的发电机。 （见图中的蓝色曲线，显示电源失去了发电机） 。大多数电站发电机加载与 RL 比的 RG 略高，尽可能少的浪费电源。第 4 页所以，什么时候阻抗匹配是一个好主意？很高兴你能问。基本上，它的情况与图 1（我们坚持与特定
6、负载或电缆的阻抗，我们仍然想要么最大限度地转移到负载的功率，或最大限度地减少发电量从它反射回成缆，或两者兼而有之）相当不同。图片 1射频电缆匹配例如在许多射频的情况下，我们往往有一个相对固定的负载阻抗如谐振四分之一波天线，阻抗 50 欧姆的电阻。为减少干扰我们也有使用同轴电缆将天线连接到发射机或接收器。现在你可能知道，同轴电缆表现为在无线电传输线频率，造成了它有自己的特性阻抗。这只是意味着因为电容电感（或 L / C ）比值，射频电缆能源倾向于沿着它与移动之间的电场和特定的比例磁场（即电压电流） 。在大多数情况下，当能量达到电缆的一端，我们要尽可能可能转移到我们的负载天线，发射器的情况下，或在
7、接收器的情况下输入 RF 阶段。对于发射机，这使电源效率最高，而对于一个接收器，它提供了最好的噪声表现。你猜怎么着？为了确保最佳能源传输的这一点，我们需要合适的特性阻抗相匹配电缆负载电阻/阻抗。因此，对于一个 75W 的天线或接收器的输入，我们需要使用 75W 同轴电缆。我们需要使用一个 50W 天线 50W 电缆等。 （见图 3）第 5 页图片 3那么，这是一个阻抗匹配很重要的方面。因为如果发生了这些，如果电缆天线（或接收）的阻抗不匹配是一些射频能量到最后电缆不会被传输到负载，却沿电缆朝着源头反射回来。这可以建立电缆驻波（另一个造成的电力损失，并可能使电缆损坏） ，并在发射机输出阶段，也可能
8、导致过热。在一个接收器，不匹配的降低有效接收器增益和噪声系数。你如何确保正确的阻抗匹配在此类型的射频情况?一般电缆的阻抗或多或少是固定的，可能和天线的阻抗相同。但不少技术已演变调整两者之间的匹配：调整存根，四分之一波变压器等。类似的事情在接收器的输入任可发生。对于这些细节你有一个良好的参考 RF 匹配技术射频教科书，像 ARRL 手册。请注意，虽然，到目前为止，我们只考虑在负载端的射频电缆的情况。那么源到底是不是阻抗相匹配的重要之处？太少了，特别是对于发射机。更主要的是确保发射机的输出阶段送入电缆的射频能量尽可能输入阻抗。甚至可以是一个优势，故意不匹配阻抗（即，具有发射器阻抗比电缆低很多） ，
9、减少在最后阶段的功率损耗，并确保如果从天线端的 RF 反映，最它右后卫再次反弹。因此，这种情况是有点像在电站的发电机.视频互连现在让我们考虑的另一个领域阻抗匹配再次往往是很重要的：视频互连。在这里，我们正在处理的信号从 DC 跨度大约为 6MHz 或使之顺利进入射频范围。我们也往往会发现自己使用同轴电缆，以减少干扰。所以，我们需要再次匹配电缆的阻第 6 页抗和负载阻抗，以防止信号反映。随着视频，这些反射可以导致在振铃和鬼影最后的画面。 （振铃多条边在画面的轮廓，而鬼影是多个影像的每个转移水平。 ）大多数视频设备的设计 75W 电缆互连，其目的是为了投入出示此相同的输入阻抗。所以匹配往往自动发生
10、，提供您使用正确的电缆。如何对视频输出阻抗匹配过这些吗？是的，他们一般是，不是因为它最大信号传输的结果，但视频信号，因为我们不希望任何信号从负载反射回来被再次反射回所有，这会使振铃更糟。所以经常摄像机，录像机， DVD 播放机的视频输出等方面都配备了一个 75W 的串联电阻内，提供电缆（图 4）终止。这是另一名在源阻抗匹配输出电缆的一端。图片 4需要注意的是，正如我们在图 1 的原始发电机这种附加的阻抗匹配电阻产生不可避免的信号一半的视频输出 6dB 损耗在电阻中。这就是阻抗的缺点在源端匹配，这是为什么输出视频设备的缓冲放大器通常给予，两次获得所需要的，以便让不可避免的 6dB 损耗时，电缆和
11、正确的终止负载连接。音频阻抗匹配如何阻抗匹配音频应用程序？有几个应用程序在其中很重要的，但也许并不像许多像你想象的那样。由于音频信号是相当低频率，它通常只有在他们有要在相当长的电缆发送传输线效应，使得有必要进行阻抗匹配，以防止反射。在大多数情况下，我们可以得到相当有效的信号传输只需安排我们的输出阻抗音频信号源（如放大器）远低于我们的负载（如扬声器） 。第 7 页在大多数 HIFI 功放和音箱的情况下，例如，我们一般安排放大器的输出阻抗是非常比扬声器阻抗。一个典型的扬声器阻抗为 8W ，例如，但最高保真放大器输出阻抗 0.1W 或更少（图 5） 图片 5这不仅确保大多数的音频能量被转移到扬声器，
12、但也为放大器的输出低阻抗提供了良好的电气阻尼扬声器的移动的音圈提供更高的保真度。旧的放大器需要一个不同的形式阻抗匹配，因为输出的阀门一般有相当固定和相对较高的输出阻抗，这样他们就可以不提供音频能源到有效一个典型扬声器的低负载阻抗。因此，一个输出变压器，必须使用，产生更紧密阻抗匹配。变压器加紧扬声器的阻抗，因此它给输出阀门几千欧姆的有效载荷，这是至少与阀门自己的输出阻抗，所以只有少量的热能浪费在阀门。在音频领域唯一的其他地方阻抗匹配（一种不同的） ，往往是重要的是传感器，像话筒，留声机拾音器磁头等。这里的传感器通常需要提供一个特定的负载阻抗，但不是为了最大限度地提高电源或信号传输。一般是换能器的
13、性能，以确保电气减震效果更好的控制负载。例如，当正确加载这些传感器可能有清洁的输出，减少不必要的共振，从而更平和的响应。第 8 页概要但愿这给你一个更好的理解阻抗匹配的想法，它来自何处和它如何真正实现。正如你可以看到，一般真正的阻抗匹配只需要用于 RF 和视频互连主要是在负载端的同轴电缆或其他输电线路。别忘了，当阻抗匹配电缆在源端进行的，总是有坏处：功率（ -3dB）的信号电平（ - 6dB）的损失，因为当发电机阻抗和其负载阻抗是相等的，一半的功率是不可避免地浪费在发电机的内阻上。第 9 页附：英文原文IMPEDANCE MATCHING: A PRIME. From time to time
14、 you.ll come across the term impedance matching. in various areas of electronics, andespecially in fields like RF and audio engineering.However even in these fields it.s often misused, probablybecause many people don.t really understand the concepts behind it.In this primer we.ll try to clarify what
15、 impedancematching is really all about, why it.s important in somesituations . and notimportant in others.Textbooks usually explain the idea of impedancematching with a very simple example of an electricalgenerator feeding a resistive load, as shown in Fig.1.Because the generator has an internal res
16、istance of itsown (RG) . as all real generators do . this tends todissipate some of the generator.s output power as heat,whenever we connect a load to its output terminals. Sothe full mechanical power fed into the generator can.tbe drawn from it as electrical power, because some willalways be wasted
17、 in RG.When early electrical engineers were faced with thisproblem, they naturally enough did everything theycould to reduce the internal resistance of their generators. However they were inevitably still left withSOME internal resistance, because it.s impossible toreduce the resistance to zero unle
18、ss you run the generator at a temperature of close to absolute zero(0Kelvins, or -273C).Once they had minimised the internal resistance, theirnext step was to see if there was some way that theycould minimise the amount of power wasted in it, byvarying the resistance of the load. And what they disco
19、vered is shown in the Fig.2, which plots the amountof power transferred into the load RLas its resistance isvaried. As you can see, the 第 10 页amount of power reaches apeak or MAXIMUM when the load resistance is thesame as . or .matches. the generator resistance. It fallsaway for values both higher a
20、nd lower than this figure,showing that matching the two is clearly desirable if wewant to maximise the power able to reach the load.So this is where the idea of matching the resistance ofthe load to that of the generator came from. Beforelong, it was extended to cover the general situation ofany loa
21、d impedance connected to a source of electricalenergy or voltage (EMF), with its own internal sourceimpedance. And the idea of matching the load and generator resistance became one of matching the sourceand load impedance . impedance matching.Now this may sound simple and straightforward, butit.s im
22、portant to remember where the idea came from,and also realise what exactly is going on when we domatch load and generator/source resistances or impedances. True, the POWER transferred to the loadwill be a maximum; but at the same time, the actualpower being dissipated in the generator.s internal res
23、istance is EXACTLY THE SAME as that reaching theload! In other words, HALF the total power from thegenerator is now being turned into heat inside RG,because its resistance is now half the total connectedacross the generator. The .other half. is the load resistance RL.For exactly the same reason, RGa
24、nd RLwill now beacting as a 2:1 voltage divider across the generator .so that only HALF the generator.s output voltage (EG)will be appearing across the load. In terms of voltagetransfer, then, matching the impedances isn.t particularlyefficient: it actually gives a 6dB loss.Does this mean that imped
25、ance matching really onlyapplies to generators in power stations? No, and in factit doesn.t really apply there either . or at least, notsimply. All it really means is that as you draw more andmore power from a generator by reducing the loadresistance, a point is reached where half the generator.s ou
26、tput power is being wasted inside it.Obviously with very high power generators it.s not agood idea to load them even this heavily . let alonedropping the RLeven further, where even more poweris lost inside the 第 11 页generator than reaches the load. (Seethe blue curve in Fig.2, showing the power lost
27、 in thegenerator.) Most power station generators are loadedwith an RLsomewhat higher than RG, to waste as littlepower as possible.So when IS impedance matching a good idea? Gladyou asked. Basically it.s for situations rather differentfrom that in Fig.1, where we.re stuck with a particularload or cab
28、le impedance, and we still want to eithermaximise the power transferred into the load, or minimise the amount of power reflected back from itinto the cable, or both.RF CABLE MATCHINGFor example in many RF situations, we tend to havea relatively fixed LOAD impedance . quarter-wave antenna, with an im
29、pedance of50 ohms 第 12 页resistive. To minimise interferencewe also have to use coaxial cable to connect the antenna to a transmitter orreceiver.Now as you may be aware, coaxial cablebehaves as a transmission lineat radio frequencies, and as a result it has its owncharacteristic impedance. This simpl
30、y meansthat because of the inductance-to-capacitance (or L/C) ratio of the cable, RFenergy tends to move along it with a particular ratio between the electric andmagnetic fields (i.e., voltage to current).In most cases when the energy reachesthe end of the cable, we want as much aspossible to transf
31、er into our load . theantenna, in the case of a transmitter, or theinput RF stage in the case of a receiver. For a transmitter this gives the highest power efficiency, whilefor a receiver it gives the best noise performance.And guess what? To ensure this optimum energytransfer, we need to match the
32、characteristic impedanceof the cable to the impedance/resistance of the load. Sofor a 75W antenna or receiver input, we need to use75W coaxial cable. For a 50W antenna we need to use50W cable, and so on. (see Fig.3)This, then, is an area where impedance matching ISquite important. Because what happe
33、ns if the cable andantenna (or receiver) impedances are NOT matched isthat some of the RF energy reaching the end of thecable won.t be transferred into the load, but isREFLECTED back along the cable, towards the source.第 13 页This can set up standing waves in the cable (another cause of power loss, a
34、nd possibly cable damage), and canalso cause overheating in the transmitter output stage.In a receiver, the mismatch degrades the effective receiver gain and noise figure.How do you ensure correct impedance matching in this type of RF situation? Generally the cable impedanceis more or less fixed, an
35、d the antenna impedance may be the same. But quite a few techniques have been evolvedto .tweak. the matching between the two: tuned stubs,quarter-wave transformers and so on. Similar things canbe done at the input of a receiver. For details of theseRF matching techniques you.ll have to refer to a go
36、odRF textbook, like The ARRL Handbook.Notice though that so far we.ve only considered the situation at the LOAD end of the RF cable. How aboutthe source end . isn.t impedance matching importantthere too? Less so, especially for transmitters. The mainthing is to ensure that the transmitter output sta
37、ge willfeed as much RF energy as possible into the cable.sinput impedance. There can even be an advantage indeliberately mismatching the impedances (i.e., having thetransmitter impedance much lower than the cable), tominimise power loss in the final stage and also ensurethat if RF is reflected back
38、from the antenna end, mostof it is bounced right back up again. So this situation is abit like the generator in a power station.VIDEO INTERCONNECTIONSNow let.s consider another area where impedancematching again tends to be quite important: video interconnections. Here we.re dealing with signals whi
39、chspan from DC up to about 6MHz or so . well into the.RF. range. And we also tend to find ourselves usingcoaxial cables, to reduce interference. So again we needto match the cable impedance and theload impedance, to prevent signalreflection. With video, these reflectionscan cause ringingand ghosting
40、in thefinal picture. (RingIng is multiple edgeson outlines in the picture, while ghosting is multiple images . each shifted horizontally.) Most video equipment is designed tobe interconnected with 75W cables, andhas inputs which are designed to present this same input impedance. Somatching tends to
41、第 14 页occur automatically,providing you use the correct cables.How about video outputs . arethese impedance matched too? Yes, generally they are, not because itresults in maximum signal transfer butbecause with video signals we DON.T want any signalreflected back from the load to be reflected back a
42、llover again . this would make ringing even worse. Sooften the video outputs of cameras, VCRs, DVD playersand so on are fitted with a 75W series resistor inside, toprovide .back termination. for the cable (Fig.4). This isjust another name for impedance matching at the sourceend of the output cable.N
43、ote that just as with our original generator in Fig.1,this added impedance matching resistor produces aninevitable 6dB loss of signal . half the video output islost in the resistor. That.s the penalty of impedancematching at the source end, and it.s why the outputbuffer amplifier in video equipment
44、is usually given again of twice what is needed, to allow for the unavoidable 6dB loss when a cable and correctly terminated load are connected.AUDIO IMPEDANCE MATCHINGHow about impedance matching in audioapplications? There are a few applicationswhere it.s important, but perhaps not asmany as you mi
45、ght think.Because audio signals are quite low in frequency, it.s generally only where they haveto be sent over quite long cables that transmission-line effects make it necessary toperform impedance matching to preventreflections. And in most cases, we can getquite 第 15 页efficient signal transfer sim
46、ply by arranging for the output impedance of our audio source (such as an amplifier) to be much lower than that of our load (such as aloud speaker).In the case of most hifi amplifiers and speakers, for example, we generally arrange for the amplifier output impedance to be very much LOWER than the sp
47、eaker impedance. A typical speaker impedance is 8W, for example, but most hifi amplifiers have an output impedance of 0.1W or less (Fig.5). This not only ensures that most of the audio energy is transferred to the speaker, but also that the amplifiers low output impedance provides good electrical DA
48、MPING for the speakers moving voice coil . giving higher fidelity.Older valve amplifiers needed a different form of impedance matching, because output valves generally had a fairly fixed and relatively high output impedance, so they couldnt deliver audio energy efficiently into the low load impedanc
49、e of a typical speaker. So an output transformer had to be used, to produce a closer impedance match. The transformer .stepped up. The impedance of the speaker, so that it gave the output valve an effective load of a few thousand ohms; this was at least comparable with the valves own output impedance, so only a small amount of energy was wasted as heat in the valve.The only other area in audio where impedance matching (of a different kind) tends to be important is with transducers like microphones, gramophone pickups, tape heads and so on. Here the transducer often needs to be provi
