滤波器的选择与应用.htm

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    <p class=MsoNormal style='line-height:14.0pt'><span lang=EN-US
    style='font-size:9.0pt;font-family:Arial'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    Arial;mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>　　电路设计人员如何确定在哪种场合该选用哪种滤波器呢？本文旨在帮助他们作出这种决定。</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'><br>
    <br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    Arial;mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>　　滤波器的选择看似神秘，但实质上并非如此。不过在很多场合，即使竭尽全力采取以下所述方法来选择，也还是需要实验多个滤波器后才能挑出最合适的一只。</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    Arial;mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>　　那么，为什么要煞费苦心去正确的选择滤波器呢？按这里提供的准则来进行滤波器的筛选，至少可满足滤波器的正确尺寸和类型的要求，因此，试用滤波器仅仅是用一只滤波器替换另一只滤波器，同时检查传导及辐射发射，看哪只滤波器具有最佳的费效比。</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    Arial;mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>　　如果在设计过程中没有足够的耐心去选择滤波器，墨菲法则（好象所有的物理、医疗和财政方面的公式都是从这里派生出来的）表明：最终证明是最合适的滤波器会与产品的其它要求完全不兼容。要么滤波器太大或太重而不能安装在铸塑模机壳内，需要一笔昂贵的重新制造模具的费用，要么需要一种不易实现的安装方法，要么由于滤波器的泄漏电流，将使推向市场的产品存在安全隐患问题。确实，如果没有仔细选择正确型号及类型的滤波器，那么按照墨菲法则，挑选合适的滤波器将增加研发和生产费用，同时也会推迟产品的上市时间。</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'><br>
    <br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    Arial;mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>　滤波器有关指标的计算</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    Arial;mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>　　通过将产品的发射频谱与相关的电磁兼容标准比较，可以估算用滤波器控制发射所需要的衰减量。对于抗扰性控制，可以通过比较外部电噪声（通常取自有关的电磁兼容抗扰度标准）与产品电子线路的敏感性以及干扰期间希望达到的性能等级来估算一个粗略值。</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    Arial;mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>　　当明确知道一个产品实际的发射或敏感性能时，就可采取精确的计算而不去进行估测。不过，如果不是在一个可控的</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'>50</span><span
    style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:Arial;
    mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>Ω阻抗环境中工作，在购买滤波器时，厂家提供的产品指标是靠不住的。</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'><br>
    <br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    Arial;mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>　阻抗问题</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    Arial;mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>　　滤波器的工作原理是在射频电磁波的传输路径上形成很大的特性阻抗不连续，将射频电磁波中的大部分能量反射回源处。大多数滤波器的性能是在源和负载阻抗均为</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'>50</span><span
    style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:Arial;
    mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>的条件下测得的，这使我们直接联想到极为重要的一点，这就是滤波器的性能在实际情况下不可能达到最佳。</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    Arial;mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>　　考察一个典型的电源线滤波器，它安装在交流电源线与作为电子产品直流电源的交</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'>-</span><span
    style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:Arial;
    mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>直流变换器之间。白天，交流电源的阻抗在</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'>2</span><span
    style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:Arial;
    mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>～</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'>2k</span><span
    style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:Arial;
    mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>Ω间变化，取决于与它连接的负载以及所关心的频率。连接到电子设备的电源线的特征阻抗大约在</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'>150</span><span
    style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:Arial;
    mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>Ω，当整流器在电源波形的尖峰附近导通时，相当于短路，而在其它时间，相当于开路。</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    Arial;mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>　　滤波器参数是在</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'>50</span><span
    style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:Arial;
    mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>Ω的源和负载阻抗的测试环境下获得的，因为大多数射频测试设备采用</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'>50</span><span
    style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:Arial;
    mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>Ω的源、负载及电缆。这种方法获得的滤波器性能参数是最优化的，同时也是最具有误导性的。</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    Arial;mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>　　因为滤波器由电感和电容组成的，因此这是一个谐振电路。其性能和谐振主要取决于源端及负载端的阻抗。事实上，一只价格昂贵且</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'>50/50</span><span
    style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:Arial;
    mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>性能优秀的滤波器可能在实际中的性能还不如一只价格较低且</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'>50/50</span><span
    style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:Arial;
    mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>性能较差的滤波器好。</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'><br>
    <br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    Arial;mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>　电源线滤波器</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'><br>
    </span><span style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:
    Arial;mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>　　图</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'>1</span><span
    style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:Arial;
    mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>给出的单级电源线滤波器对源和负载的阻抗都很敏感，当工作在实际的源和负载阻抗条件下时，很容易产生增益，而不是衰减。这种增益通常出现在</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'>150kHz</span><span
    style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:Arial;
    mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>～</span><span
    lang=EN-US style='font-size:9.0pt;font-family:Arial'>10MHz</span><span
    style='font-size:9.0pt;font-family:宋体;mso-ascii-font-family:Arial;
    mso-hansi-font-family:Arial;mso-bidi-font-family:Arial'>的频率范围内，幅度可以达到</span><span