关于电缆屏蔽的问题.htm

该资料时关于EMC的相关问题

    宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>千兆）比特的数据流，我们用一对作为发送，而另一对作为接收，对</span><span
    lang=EN-US style='font-size:9.0pt'>150</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>平衡电缆进行了全面的屏蔽。下面我们将讨论屏蔽电缆是在一端接地，还是两端都接地的问题。</span><span
    lang=EN-US style='font-size:9.0pt'><br>
    <br>
    Joe</span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    "Times New Roman";mso-hansi-font-family:"Times New Roman"'>：</span><span
    lang=EN-US style='font-size:9.0pt'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    "Times New Roman";mso-hansi-font-family:"Times New Roman"'>　　针对如何将屏蔽体接地（用金属片或使用电容）、过大的地电流将烧坏屏蔽体等问题，根据我在对低电平信号进行保护使其免受地环路干扰的经验，我建议：采用电阻而不是电容。</span><span
    lang=EN-US style='font-size:9.0pt'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    "Times New Roman";mso-hansi-font-family:"Times New Roman"'>　　特别地，机壳与地之间的电压波动，很少超过</span><span
    lang=EN-US style='font-size:9.0pt'>10V</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>，要是把</span><span lang=EN-US style='font-size:9.0pt'>100</span><span
    style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:"Times New Roman";
    mso-hansi-font-family:"Times New Roman"'>、</span><span lang=EN-US
    style='font-size:9.0pt'>1W</span><span style='font-size:9.0pt;font-family:
    宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>的碳膜电阻串接在屏蔽体的任一端，另一端直接接到机壳上，这样地电流被箝制在</span><span
    lang=EN-US style='font-size:9.0pt'>0.1A</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>，正好在屏蔽体可承受的范围内。双绞线可以使屏蔽体免遭</span><span lang=EN-US
    style='font-size:9.0pt'>EMC</span><span style='font-size:9.0pt;font-family:
    宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>等其它因素的影响，适当的差动接收器可以很好地抑制</span><span
    lang=EN-US style='font-size:9.0pt'>10V</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>左右的电源频率和谐波共模电压。</span><span lang=EN-US style='font-size:
    9.0pt'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    "Times New Roman";mso-hansi-font-family:"Times New Roman"'>　　事实上，我发现实验室中的电压波动仅仅几个伏特，均采用</span><span
    lang=EN-US style='font-size:9.0pt'>10</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>、</span><span lang=EN-US style='font-size:9.0pt'>1W</span><span
    style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:"Times New Roman";
    mso-hansi-font-family:"Times New Roman"'>串联碳膜电阻在大型建筑物和船上观测到的结果也仅有几伏。因此我针对有效值</span><span
    lang=EN-US style='font-size:9.0pt'>10V</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>的电压设计。</span><span lang=EN-US style='font-size:9.0pt'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    "Times New Roman";mso-hansi-font-family:"Times New Roman"'>　　对于哪端硬接地，哪端用串联电阻，这个未在通信信号上试过，但我认为接收端应硬接地，因为接收机是处理极低电平信号的，所以零阻抗接地比</span><span
    lang=EN-US style='font-size:9.0pt'>100</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>接地效果好得多，因为屏蔽体对付的是高阻抗噪声源，与这些噪声源阻抗相比，</span><span
    lang=EN-US style='font-size:9.0pt'>100</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>不算大。但在甚高频也许例外。因此，</span><span lang=EN-US
    style='font-size:9.0pt'>100</span><span style='font-size:9.0pt;font-family:
    宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>电阻可用一电容旁路，这可防止单用</span><span
    lang=EN-US style='font-size:9.0pt'>100</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>电阻引起的</span><span lang=EN-US style='font-size:9.0pt'>ESD</span><span
    style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:"Times New Roman";
    mso-hansi-font-family:"Times New Roman"'>击穿。</span><span lang=EN-US
    style='font-size:9.0pt'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    "Times New Roman";mso-hansi-font-family:"Times New Roman"'>　　顺便说一下，地噪声可以是电源频率的三倍，要是用户系统有大量输入电容，</span><span
    lang=EN-US style='font-size:9.0pt'>5V</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>电源将从三相基本电源系统的三根引线馈送出。在飞行器自动控制系统中，我在</span><span
    lang=EN-US style='font-size:9.0pt'>240Hz</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>的频率上测量到</span><span lang=EN-US style='font-size:9.0pt'>2.4V</span><span
    style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:"Times New Roman";
    mso-hansi-font-family:"Times New Roman"'>有效值的电压，直到绿色和白色地线被分开才消失，有效的源阻抗大约</span><span
    lang=EN-US style='font-size:9.0pt'>1</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>左右，我没记错的话，其波形很类似正弦波。当其能够驱动电流通过</span><span lang=EN-US
    style='font-size:9.0pt'>VME</span><span style='font-size:9.0pt;font-family:
    宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>总线的逻辑地时，系统状态立即翻转。当我把一根地线与另一条地线碰到一起时，我看到有火花冒出，意识到自己遇到了麻烦。</span><span
    lang=EN-US style='font-size:9.0pt'><br>
    <br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    "Times New Roman";mso-hansi-font-family:"Times New Roman"'>答</span><span
    lang=EN-US style='font-size:9.0pt'>Joe </span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>（</span><span lang=EN-US style='font-size:9.0pt'>JOHNSON</span><span
    style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:"Times New Roman";
    mso-hansi-font-family:"Times New Roman"'>博士）</span><span lang=EN-US
    style='font-size:9.0pt'><br>
    <br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    "Times New Roman";mso-hansi-font-family:"Times New Roman"'>　　我不同意你所建议的在屏蔽体一端使用电阻来获得有效的电磁屏蔽，这在高速数字电路场合并不适用。在高速数字中，欲使屏蔽体发挥其作用，必须在机壳两端与屏蔽体之间进行低阻抗连接，且连接阻抗要在屏蔽体作用的频率范围内都很低。相对高速连接器来说，屏蔽连接功效的度量称之为地线传输阻抗或屏蔽传输阻抗，这是个至关重要的参数，在你提到的例子中，在一端上的地线传输阻抗将达到</span><span
    lang=EN-US style='font-size:9.0pt'>100</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>，使屏蔽体无效。</span><span lang=EN-US style='font-size:9.0pt'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    "Times New Roman";mso-hansi-font-family:"Times New Roman"'>　　在低速场合，包括高阻抗电路系统，围绕导体的大部分近场能量是电场模式（与磁场模式相对），屏蔽体仅需在一端接地，这时，屏蔽体作为一个包围导体的法拉第盒，阻止电场的进出。</span><span
    lang=EN-US style='font-size:9.0pt'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    "Times New Roman";mso-hansi-font-family:"Times New Roman"'>　　在高速场合，包括高阻抗电路系统，围绕导体的大部分近场能量处于磁场模式，因此，仅磁场屏蔽会起作用，这即是两点接地屏蔽的作用。两端接地的屏蔽体允许高频电流在其内环流，这将抵消信号线中的电流，这些起反作用的电流产生磁场，进而抵消来自信号导体的磁场，从而取得磁场屏蔽效果。为了使磁场屏蔽正确地发挥作用，必须让电流可从电缆两端进出。因此就要求屏蔽电缆两端具有到机壳的低阻抗连接。</span><span
    lang=EN-US style='font-size:9.0pt'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    "Times New Roman";mso-hansi-font-family:"Times New Roman"'>　　有办法使地线传输阻抗在高频时仍处于比较低的水平，但同时在</span><span
    lang=EN-US style='font-size:9.0pt'>60Hz</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>时出现非常高的阻抗。这些方法是：屏蔽体以电容方式耦合到机壳，这用于需要高频屏蔽但要抑制</span><span
    lang=EN-US style='font-size:9.0pt'>60Hz</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>电流环流的场合。</span><span lang=EN-US style='font-size:9.0pt'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    "Times New Roman";mso-hansi-font-family:"Times New Roman"'>　　为了使容性耦合的屏蔽体正常工作，电容的阻抗在工作频率段要很低。比如，要是信号线通过</span><span
    lang=EN-US style='font-size:9.0pt'>70</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>阻抗耦合到屏蔽体（这是电缆共模阻抗为</span><span lang=EN-US
    style='font-size:9.0pt'>75</span><span style='font-size:9.0pt;font-family:
    宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>的另一种情形），屏蔽体通过</span><span
    lang=EN-US style='font-size:9.0pt'>0.1</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>的阻抗线接到地上，我们会在屏蔽体上测量到一个电压，该电压等于</span><span lang=EN-US
    style='font-size:9.0pt'>0.0013</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>倍共模信号电压，此例中的屏蔽体可以有</span><span lang=EN-US
    style='font-size:9.0pt'>57dB</span><span style='font-size:9.0pt;font-family:
    宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>的屏蔽效能。</span><span
    lang=EN-US style='font-size:9.0pt'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    "Times New Roman";mso-hansi-font-family:"Times New Roman"'>　　为了保证屏蔽体能应用在千兆位以太网络中，因此，要求地线传输阻抗（这是电缆屏蔽层与机箱之间的阻抗）在</span><span
    lang=EN-US style='font-size:9.0pt'>625MHz</span><span style='font-size:
    9.0pt;font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>时小于</span><span lang=EN-US style='font-size:9.0pt'>0.1</span><span
    style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:"Times New Roman";
    mso-hansi-font-family:"Times New Roman"'>，如果对比</span><span lang=EN-US
    style='font-size:9.0pt'>BERG Metacug</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>屏蔽连接器规范，你会发现这种做法是不符合要求的。这种屏蔽连接器提供了一个直接连接屏蔽层与机箱的金属件，并且在整个连接器中，它包围着信号线导体。</span><span
    lang=EN-US style='font-size:9.0pt'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    "Times New Roman";mso-hansi-font-family:"Times New Roman"'>　　为了用容性耦合屏蔽获得同样的效果，电容的有效串联自感应小于</span><span
    lang=EN-US style='font-size:9.0pt'>16pH</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>（皮亨），引线元件是不可能有这样小的电感的，必须用具有极低自感系数的分布电容。例如可能通过在连接器外壳与机壳之间加一片薄衬垫来获得，我们经常看到这种连接器的有关介绍，但却没有遇到一例应用在实际工作中。</span><span
    lang=EN-US style='font-size:9.0pt'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    "Times New Roman";mso-hansi-font-family:"Times New Roman"'>　　我不赞成在工作中应用容性耦合屏蔽，原因如下：</span><span
    lang=EN-US style='font-size:9.0pt'>1</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>、这样做将增加复杂度</span><span lang=EN-US style='font-size:9.0pt'>
    2</span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    "Times New Roman";mso-hansi-font-family:"Times New Roman"'>、没有实例证明它有作用</span><span
    lang=EN-US style='font-size:9.0pt'> 3</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>、这不会超出实际应用的范围。记住：我们这里所讨论的铜线连接在近距离设备之间，它仅长</span><span
    lang=EN-US style='font-size:9.0pt'>25m</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>，它可专用于要连接到同一接地点的设备之间，且有意地将它接地在同一点，在设备之间将没有大的循环地电流。对于较长的连接，我们提供其它连接方式，这些方式根本不需要在任何一端接地（多模光纤、单模光纤、</span><span
    lang=EN-US style='font-size:9.0pt'>5</span><span style='font-size:9.0pt;
    font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
    "Times New Roman"'>类非屏蔽双绞线）。屏蔽体两端直接接地是正确的。</span><span style='font-size:
    9.0pt'> </span><span lang=EN-US style='font-size:9.0pt;font-family:宋体'><o:p></o:p></span></p>
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