计算机产生的射频电磁干扰的解决.htm

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    <p class=MsoNormal style='line-height:14.0pt'><span lang=EN-US
    style='font-size:9.0pt'><br>
    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"'>、</span><span
    style='font-size:9.0pt'> </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"'>简述</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"'>　　当计算机与无线电广播设备共同工作时，无论它是用于控制还是解码，一个最烦人的问题是这些计算机产生的射频干扰。这些射频干扰不仅会占据一些没有使用的频段，还会在有用的频段中淹没所有的弱信号，干扰你所要接收的信号或使信号失真。在处理数字信号甚至是连续波时，这通常是不可接受的。因此，最近经常听到的问题是怎样才能减小或消除射频干扰。</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"'>　　　让人感到失望的回答是：在大多数场合，没有办法完全消除计算机产生的射频干扰。</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"'>　　　但也不要灰心：有一些具体的措施可以帮助你减小射频干扰，使其降低到可以接受的水平，在有些场合，甚至可以全部消除。</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"'>　　本文是许多技术人员和我自己在工作中处理这类问题时遇到的一些现象及其建议的汇编。大家经常读到的有关射频干扰的文章可能都会涉及到解决发射机产生射频干扰的问题，但本文从接收机的角度进行讨论。</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"'>　　我想很多人已经尝试过通过增加接收机和天线与射频干扰源（在本案例中，指计算机和监视器）之间的距离来改善信号的质量，但这通常受到各种限制而无法获得所需要的距离。还有一个办法是试着改变计算机、监视器、接收机和天线的方向，看看情况是否变好一些。若所有这些都试过，还有别的办法吗？这就是本文主要讨论的问题。</span><span
    lang=EN-US style='font-size:9.0pt'><br>
    <br>
    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"'>、</span><span
    style='font-size:9.0pt'> </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"'>射频干扰源</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"'>　　在我们真正开始处理这个问题前，了解计算机产生射频干扰的原因及如何对接收机造成干扰的原理很有必要。计算机的两个主要部件是</span><span
    lang=EN-US style='font-size:9.0pt'>CPU</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"'>和监视器（方便起见）。计算机运行在由内部晶振决定的特定时钟频率上。通常，时钟频率值大约是</span><span
    lang=EN-US style='font-size:9.0pt'>4.77 MHz</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'>8 MHz</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'>12 MHz</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'>16 MHz</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'>20 MHz</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'>25 MHz</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'>33 MHz</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'>40 MHz</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'>50 MHz</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'>66 MHz</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"'>和</span><span lang=EN-US
    style='font-size:9.0pt'>80MHz</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"'>（高速时钟永远是计算机制造业追求的目标，现在已有</span><span lang=EN-US
    style='font-size:9.0pt'>GHz</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"'>频率的计算机出现）。但计算机中并不是只有一个时钟，显示卡和其它电路卡也有自己的时钟，因此计算机内会包含有几个晶振。正如你所看到的，这些时钟的震荡频率均分布在</span><span
    lang=EN-US style='font-size:9.0pt'>HF</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"'>和</span><span lang=EN-US style='font-size:9.0pt'>L-VHF</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"'>频段内，这将干扰我们想要接收的信号。更糟的是，这些时钟频率经常在计算机内部分成许多其他频率时钟。因为计算机是一个数字系统，这些信号的特征波形是方波，而方波含有大量的谐波。因为像素数据的速率相当高，处于高频区域，所以显显示卡也易产生射频干扰。所有这些原因解释了为什么计算机及监视器是产生射频的高手。</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"'>　　当计算机采用没有屏蔽作用的廉价塑料机箱时，这个问题更加突出。许多人认为，监视器是主要的射频干扰源之一，这是没错的。但我也发现，别看微机的键盘很不起眼，却也产生相当强的射频干扰。这是因为其电路板上有一个微处理器，该处理器的时钟频率在</span><span
    lang=EN-US style='font-size:9.0pt'>3MHz</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"'>范围以内。由于键盘电路板上走线的布置就象一个良好的环路辐射天线，因此这个问题变得更为复杂，在数次谐波范围内均可发生干扰。监视器线路板上的时钟通常为</span><span
    lang=EN-US style='font-size:9.0pt'>14.316MHz</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"'>，因此你在这个频率会发现较强的辐射。</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"'>　　我们要做的第一件事就是决定</span><span
    lang=EN-US style='font-size:9.0pt'>CPU</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"'>和监视器所产生的射频干扰有多大。可能</span><span lang=EN-US
    style='font-size:9.0pt'>CPU</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"'>根本不辐射而监视器才是辐射源，或相反。这可以通过关掉监视器，而仅让</span><span
    lang=EN-US style='font-size:9.0pt'>CPU</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"'>处于工作状态，观察射频干扰对无线电广播设备造成的影响来确认。</span><span
    style='font-size:9.0pt'> </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"'>以下是一些建议：</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"'>　</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"'>快速扫描整个频段，弄清射频干扰在什么频率处最强。这是很必要的，因为要是我们能在这些频率减小射频干扰，通常其它频率的射频干扰也会降低。</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"'>　</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"'>从主机上依次断开鼠标、串口电缆、打印机电缆、键盘、视频电缆、显示器电源线等，将接收机用电池供电（如果可能），观察射频干扰是否减小。</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"'>　</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"'>然后再依次连接上键盘、鼠标、串口电缆、并口电缆、显示电缆、显示器电源线（但先不要打开显示器），在连接的同时，注意射频干扰的变化。作完这些后，你对哪部分是主要的射频干扰源就有了一些认识。如果在以上的过程中，射频干扰一点也没增加，你真是太幸运了。</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"'>　</span><span
    lang=EN-US style='font-size:9.0pt'>(4) </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"'>打开显示器，同时注意射频干扰的增加，运行文字和图形两种模式的应用程序，看射频干扰是否有变化。</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"'>　</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"'>重新把无线电广播设备连接到电源上，注意观察射频干扰的增加。</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"'>　</span><span
    lang=EN-US style='font-size:9.0pt'>(6) </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"'>断开与接收机连接的所有天线（包括内部和外部天线），观察射频干扰是否消失。注意有一些接收机，当你拔掉外部天线时，内部天线自动起作用。为了防止这个现象，可在天线插座上插入一个虚假的插头。</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"'>　　至此，你应该相当清楚哪些部件在产生射频干扰。要对射频干扰源精确定位，可按下面的建议来做：</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"'>　　“当通过以上的方法你不能确定干扰源时，需要检查来自监视器和计算机机壳的辐射。把同轴电缆的屏蔽层向下剥出几厘米，露出芯线，将芯线绕几圈，作成一个直径大约</span><span
    lang=EN-US style='font-size:9.0pt'>2—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"'>厘米的小环，然后将芯线与你剥离的屏蔽层焊起来。把这个小环与你的接收机天线输入端连起来，用它来探测计算机系统。屏蔽不好的计算机可用指形簧片、金属编织丝网、或其他导电材料非常容易地进行修补。对显示器进行屏蔽则困难得多，并且可能引起安全问题（高压）或过热。</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"'>　　若觉得以上方法太麻烦，你可拿一个由电池供电的中短波收音机（其内部有磁棒天线）在计算机周围移动，并转动收音机。通常当收音机中的噪声最强时，它所处的位置就是主要的射频干扰源的位置。当然，收音机应调谐到已经可以听到射频干扰的频率上。在可疑设备附近按上述方法微调收音机，你可以知道射频干扰是何种性质的，它主要集中在什么频率范围内。</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"'>下面我们将讨论射频干扰路径的问题。</span><span
    lang=EN-US style='font-size:9.0pt'><br>
    <br>
    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"'>、</span><span