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<div class=Section1 style='layout-grid:15.6pt'>

<p align=center style='text-align:center'><b><span style='font-size:12.0pt;
letter-spacing:4.0pt'>实例分析</span><span lang=EN-US><o:p></o:p></span></b></p>

<p><b><span style='letter-spacing:4.0pt'>故障现象：</span></b><span lang=EN-US><o:p></o:p></span></p>

<p><span style='letter-spacing:4.0pt'>　　某直流放大器在进行<span lang=EN-US>GJB151中的CS06试验时，放大器出现饱和现象而失效。</span></span><span
lang=EN-US><o:p></o:p></span></p>

<p><b><span style='letter-spacing:4.0pt'>故障原因分析：</span><span lang=EN-US><o:p></o:p></span></b></p>

<p><span style='letter-spacing:4.0pt'>　　该直流放大器安装在一块线路板上，为了安装方便，整块线路板通过一条电缆与其它电路模块连接起来，如图<span
lang=EN-US>1所示。</span></span><span lang=EN-US><o:p></o:p></span></p>

<p align=center style='text-align:center'><span lang=EN-US><!--[if gte vml 1]><v:shapetype
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<p align=center style='text-align:center'><span style='letter-spacing:4.0pt'>图<span
lang=EN-US>1 某直流放大器安装结构</span></span><span lang=EN-US><o:p></o:p></span></p>

<p><span style='letter-spacing:4.0pt'>　　由于电缆中导线平行，因此导线之间的寄生电容较大。<span
lang=EN-US>GJB151中的CS06试验是在电源线上施加400V的尖峰脉冲干扰电压。由于尖峰脉冲信号的高频成分较多，干扰能量会通过导线之间的寄生电容耦合到放大器的输入端。尽管这个放大器是直流放大器，但设计者并没有限制放大器的带宽，结果放大器对耦合到输入端的高频信号进行了放大。由于放大器的输入线与输出线之间也有较大的寄生电容，因此放大后的输出信号又被耦合到输入信号线上，结果形成正反馈，导致放大器饱和。</span></span><span
lang=EN-US><o:p></o:p></span></p>

<p><b><span style='letter-spacing:4.0pt'>解决措施：</span><span lang=EN-US><o:p></o:p></span></b></p>

<p><span style='letter-spacing:4.0pt'>　　要解决这个问题可以有两个方案，首先可以将导线分开，减小导线之间的寄生电容，特别是将电源线与放大器的输入线分开，这样可以避免试验脉冲的能量耦合进放大器的输入端。</span><span
lang=EN-US><o:p></o:p></span></p>

<p><span style='letter-spacing:4.0pt'>　　另一个办法是压缩放大器的频带，使放大器对耦合进输入端的高频信号没有响应。因为，既然是直流放大器，就应该使放大器仅对直流附近的信号有放大作用。而根据电容耦合的理论，频率较低的信号是不容易耦合的。</span><span
lang=EN-US><o:p></o:p></span></p>

<p><span style='letter-spacing:4.0pt'>　　与设计者商量以上两个方案，设计者认为，将导线分开会影响使用的方便性，不能通过一条电缆将放大器接入系统，总体设计师不同意。如果更换一个带宽较窄的放大器，虽然可以解决这个问题，但是在航天产品推荐目录上没有合适的产品。</span><span
lang=EN-US><o:p></o:p></span></p>

<p><span style='letter-spacing:4.0pt'>　　因此为了解决这个问题，只有在放大器的外围安装滤波器件，压缩放大器的带宽，使放大器对耦合进输入端的高频信号没有响应。频率较低的信号耦合效率较低，不会造成放大器饱和的问题。采取措施后的电路如图<span
lang=EN-US>2所示。在放大器的输入端安装一个低通放大器。这样相当于压缩了放大器的带宽。采取这个措施后，放大器顺利通过了CS06试验。</span></span><span
lang=EN-US><o:p></o:p></span></p>

<p align=center style='text-align:center'><span lang=EN-US><!--[if gte vml 1]><v:shape
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src="./实例分析.files/image002.gif" alt="图2改造后的放大器电路.gif (2264 字节)"
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<p align=center style='text-align:center'><span style='letter-spacing:4.0pt'>图<span
lang=EN-US> 2 改造后的放大器电路</span></span><span lang=EN-US><o:p></o:p></span></p>

<p><b><span style='letter-spacing:4.0pt'>启示：</span><span lang=EN-US><o:p></o:p></span></b></p>

<ul type=disc>
 <li class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;
     mso-list:l0 level1 lfo1;tab-stops:list 36.0pt'><span style='font-size:
     15.0pt;font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
     "Times New Roman";letter-spacing:4.0pt'>在保证功能的前提下，尽量压缩电路的带宽，不要使用超过需要的带宽。</span><span
     style='font-size:15.0pt'> <span lang=EN-US><o:p></o:p></span></span></li>
 <li class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;
     mso-list:l0 level1 lfo1;tab-stops:list 36.0pt'><span style='font-size:
     15.0pt;font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
     "Times New Roman";letter-spacing:4.0pt'>线路板布线时，要考虑信号线之间的耦合问题。</span><span
     style='font-size:15.0pt'> <span lang=EN-US><o:p></o:p></span></span></li>
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