高速PCB设计入门技术问答集

<p><font face="Verdana">高速PCB设计入门技术问答集</font><br/></p>
<p>
<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><b><span style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'times New="New"  Roman'; mso-hansi-font-family: 'Times New="New"  Roman'"></span></b><b><span lang="EN-US" style="FONT-SIZE: 12pt"><?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p></span></b></p><span style="COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-size: 9.0pt">要做高速的<span lang="EN-US">PCB</span>设计，首先必须明白下面的一些基本概念，这是基础。<span lang="EN-US"><br/>1</span>、什么是电磁干扰（<span lang="EN-US">EMI</span>）和电磁兼容性<span lang="EN-US">(EMC)? <br/>(Electromagnetic InteRFerence)</span>，有传导干扰和辐射干扰两种。 传导干扰是指通过导电介质把一个电网络上的信号耦合（干扰）到另一个电网络。辐射干扰是指干扰源通过空间把其信号耦合（干扰）到另一个电网络。在高速<span lang="EN-US">PCB</span>及系统设计中，高频信号线、集成电路的引脚、各类接插件等都可能成为具有天线特性的辐射干扰源，能发射电磁波并影响其他系统或本系统内其他子系统的正常工作。<span lang="EN-US"><br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span></span>自从电子系统降噪技术在<span lang="EN-US">70</span>年代中期出现以来，主要由于美国联邦通讯委员会在<span lang="EN-US">1990</span>年和欧盟在<span lang="EN-US">1992</span>提出了对商业数码产品的有关规章，这些规章要求各个公司确保它们的产品符合严格的磁化系数和发射准则。符合这些规章的产品称为具有电磁兼容性<span lang="EN-US">EMC</span>（<span lang="EN-US">Electromagnetic Compatibility</span>）。<span lang="EN-US"><br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span>2</span>、什么是信号完整性<span lang="EN-US">(signal integrity)? <br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span></span>信号完整性是指信号在信号线上的质量。信号具有良好的信号完整性是指当在需要的时候，具有所必需达到的电压电平数值。差的信号完整性不是由某一单一因素导致的，而是板级设计中多种因素共同引起的。主要的信号完整性问题包括反射、振荡、地弹、串扰等。常见信号完整性问题及解决方法<span lang="EN-US"><o:p></o:p></span></span></p>
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<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">问<span lang="EN-US"><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span></span>题<span lang="EN-US"><o:p></o:p></span></span></p></td>
<td style="BORDER-RIGHT: windowtext="windowtext"  1pt="1pt"  solid; PADDING-RIGHT: 0.75pt; BORDER-TOP: windowtext="windowtext"  1pt="1pt"  solid; PADDING-LEFT: 0.75pt; BACKGROUND: lime; PADDING-BOTTOM: 0cm; BORDER-LEFT: medium="medium"  none; WIDTH: 110pt; PADDING-TOP: 0.75pt; BORDER-BOTTOM: windowtext="windowtext"  2.25pt double; HEIGHT: 30.75pt; mso-border-top-alt: solid="solid"  windowtext="windowtext"  1.0pt; mso-border-bottom-alt: double="double"  windowtext="windowtext"  2.25pt; mso-border-right-alt: solid="solid"  windowtext="windowtext"  .5pt" width="147">
<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">可 能 原 因<span lang="EN-US"><o:p></o:p></span></span></p></td>
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<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">解 决 方 法<span lang="EN-US"><o:p></o:p></span></span></p></td>
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<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">其 他 解 决 方 法<span lang="EN-US"><o:p></o:p></span></span></p></td></tr>
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<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">过大的上冲<span lang="EN-US"><o:p></o:p></span></span></p></td>
<td style="BORDER-RIGHT: windowtext="windowtext"  1pt="1pt"  solid; PADDING-RIGHT: 0.75pt; BORDER-TOP: medium="medium"  none; PADDING-LEFT: 0.75pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: medium="medium"  none; WIDTH: 110pt; PADDING-TOP: 0.75pt; BORDER-BOTTOM: windowtext="windowtext"  1pt="1pt"  solid; HEIGHT: 30.75pt; mso-border-bottom-alt: solid="solid"  windowtext="windowtext"  .5pt; mso-border-right-alt: solid="solid"  windowtext="windowtext"  .5pt" width="147">
<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">终端阻抗不匹配<span lang="EN-US"><o:p></o:p></span></span></p></td>
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<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">终端端接<span lang="EN-US"><o:p></o:p></span></span></p></td>
<td style="BORDER-RIGHT: windowtext="windowtext"  1pt="1pt"  solid; PADDING-RIGHT: 0.75pt; BORDER-TOP: medium="medium"  none; PADDING-LEFT: 0.75pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: medium="medium"  none; WIDTH: 110pt; PADDING-TOP: 0.75pt; BORDER-BOTTOM: windowtext="windowtext"  1pt="1pt"  solid; HEIGHT: 30.75pt; mso-border-bottom-alt: solid="solid"  windowtext="windowtext"  .5pt; mso-border-right-alt: solid="solid"  windowtext="windowtext"  1.0pt" width="147">
<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">使用上升时间缓慢的驱动源<span lang="EN-US"><o:p></o:p></span></span></p></td></tr>
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<td style="BORDER-RIGHT: windowtext="windowtext"  1pt="1pt"  solid; PADDING-RIGHT: 0.75pt; BORDER-TOP: medium="medium"  none; PADDING-LEFT: 0.75pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: windowtext="windowtext"  1pt="1pt"  solid; WIDTH: 110pt; PADDING-TOP: 0.75pt; BORDER-BOTTOM: windowtext="windowtext"  1pt="1pt"  solid; HEIGHT: 30.75pt; mso-border-left-alt: solid="solid"  windowtext="windowtext"  1.0pt; mso-border-bottom-alt: solid="solid"  windowtext="windowtext"  .5pt; mso-border-right-alt: solid="solid"  windowtext="windowtext"  .5pt" width="147">
<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">直流电压电平不好<span lang="EN-US"><o:p></o:p></span></span></p></td>
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<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">线上负载过大<span lang="EN-US"><o:p></o:p></span></span></p></td>
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<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">以交流负载替换直流负载<span lang="EN-US"><o:p></o:p></span></span></p></td>
<td style="BORDER-RIGHT: windowtext="windowtext"  1pt="1pt"  solid; PADDING-RIGHT: 0.75pt; BORDER-TOP: medium="medium"  none; PADDING-LEFT: 0.75pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: medium="medium"  none; WIDTH: 110pt; PADDING-TOP: 0.75pt; BORDER-BOTTOM: windowtext="windowtext"  1pt="1pt"  solid; HEIGHT: 30.75pt; mso-border-bottom-alt: solid="solid"  windowtext="windowtext"  .5pt; mso-border-right-alt: solid="solid"  windowtext="windowtext"  1.0pt" width="147">
<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">在接收端端接，重新布线或检查地平面<span lang="EN-US"><o:p></o:p></span></span></p></td></tr>
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<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">过大的串扰<span lang="EN-US"><o:p></o:p></span></span></p></td>
<td style="BORDER-RIGHT: windowtext="windowtext"  1pt="1pt"  solid; PADDING-RIGHT: 0.75pt; BORDER-TOP: medium="medium"  none; PADDING-LEFT: 0.75pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: medium="medium"  none; WIDTH: 110pt; PADDING-TOP: 0.75pt; BORDER-BOTTOM: windowtext="windowtext"  1pt="1pt"  solid; HEIGHT: 30.75pt; mso-border-bottom-alt: solid="solid"  windowtext="windowtext"  .5pt; mso-border-right-alt: solid="solid"  windowtext="windowtext"  .5pt" width="147">
<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">线间耦合过大<span lang="EN-US"><o:p></o:p></span></span></p></td>
<td style="BORDER-RIGHT: windowtext="windowtext"  1pt="1pt"  solid; PADDING-RIGHT: 0.75pt; BORDER-TOP: medium="medium"  none; PADDING-LEFT: 0.75pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: medium="medium"  none; WIDTH: 110pt; PADDING-TOP: 0.75pt; BORDER-BOTTOM: windowtext="windowtext"  1pt="1pt"  solid; HEIGHT: 30.75pt; mso-border-bottom-alt: solid="solid"  windowtext="windowtext"  .5pt; mso-border-right-alt: solid="solid"  windowtext="windowtext"  .5pt" width="147">
<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">使用上升时间缓慢的发送驱动器<span lang="EN-US"><o:p></o:p></span></span></p></td>
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<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">使用能提供更大驱动电流的驱动源<span lang="EN-US"><o:p></o:p></span></span></p></td></tr>
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<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">时延太大<span lang="EN-US"><o:p></o:p></span></span></p></td>
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<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">传输线距离太长<span lang="EN-US"><o:p></o:p></span></span></p></td>
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<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">替换或从新部线，检查串行端接<span lang="EN-US"><o:p></o:p></span></span></p></td>
<td style="BORDER-RIGHT: windowtext="windowtext"  1pt="1pt"  solid; PADDING-RIGHT: 0.75pt; BORDER-TOP: medium="medium"  none; PADDING-LEFT: 0.75pt; PADDING-BOTTOM: 0cm; BORDER-LEFT: medium="medium"  none; WIDTH: 110pt; PADDING-TOP: 0.75pt; BORDER-BOTTOM: windowtext="windowtext"  1pt="1pt"  solid; HEIGHT: 30.75pt; mso-border-bottom-alt: solid="solid"  windowtext="windowtext"  .5pt; mso-border-right-alt: solid="solid"  windowtext="windowtext"  1.0pt" width="147">
<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">使用阻抗匹配的驱动源，变更布线策略<span lang="EN-US"><o:p></o:p></span></span></p></td></tr>
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<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">振荡<span lang="EN-US"><o:p></o:p></span></span></p></td>
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<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">阻抗不匹配<span lang="EN-US"><o:p></o:p></span></span></p></td>
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<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">在发送断串接阻尼电阻<span lang="EN-US"><o:p></o:p></span></span></p></td>
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<p class="MsoNormal" style="TEXT-ALIGN: center" align="center"><span style="FONT-FAMILY: 宋体; mso-bidi-font-size: 10.0pt">　<span lang="EN-US"><o:p></o:p></span></span></p></td></tr></tbody></table></p>
<p class="MsoNormal" style="TEXT-INDENT: 21pt; mso-char-indent-count: 2.0"><span lang="EN-US" style="COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-size: 9.0pt">3</span><span style="COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-size: 9.0pt">、什么是反射<span lang="EN-US">(reflection)? <br/></span>反射就是在传输线上的回波。信号功率<span lang="EN-US">(</span>电压和电流<span lang="EN-US">)</span>的一部分传输到线上并达到负载处，但是有一部分被反射了。如果源端与负载端具有相同的阻抗，反射就不会发生了。<span lang="EN-US"><br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span></span>源端与负载端阻抗不匹配会引起线上反射，负载将一部分电压反射回源端。如果负载阻抗小于源阻抗，反射电压为负，反之，如果负载阻抗大于源阻抗，反射电压为正。布线的几何形状、不正确的线端接、经过连接器的传输及电源平面的不连续等因素的变化均会导致此类反射。<span lang="EN-US"><br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span>4</span>、什么是串扰<span lang="EN-US">(crosstalk)? <br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span></span>串扰是两条信号线之间的耦合，信号线之间的互感和互容引起线上的噪声。容性耦合引发耦合电流，而感性耦合引发耦合电压。<span lang="EN-US">PCB</span>板层的参数、信号线间距、驱动端和接收端的电气特性及线端接方式对串扰都有一定的影响。<span lang="EN-US"> <br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span>5</span>、什么是过冲<span lang="EN-US">(overshoot)</span>和下冲<span lang="EN-US">(undershoot)? <br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span></span>过冲就是第一个峰值或谷值超过设定电压<span lang="EN-US">——</span>对于上升沿是指最高电压而对于下降沿是指最低电压。下冲是指下一个谷值或峰值。过分的过冲能够引起保护二极管工作，导致过早地失效。过分的下冲能够引起假的时钟或数据错误<span lang="EN-US">(</span>误操作<span lang="EN-US">)</span>。<span lang="EN-US"><br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span>6</span>、什么是振荡<span lang="EN-US">(ringing)</span>和环绕振荡（<span lang="EN-US">rounding</span>）<span lang="EN-US">? <br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span></span>振荡的现象是反复出现过冲和下冲。信号的振荡和环绕振荡由线上过度的电感和电容引起，振荡属于欠阻尼状态而环绕振荡属于过阻尼状态。信号完整性问题通常发生在周期信号中，如时钟等，振荡和环绕振荡同反射一样也是由多种因素引起的，振荡可以通过适当的端接予以减小，但是不可能完全消除。<span lang="EN-US"><br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span>7</span>、什么是地电平面反弹噪声和回流噪声<span lang="EN-US">? <br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span></span>在电路中有大的电流涌动时会引起地平面反弹噪声（简称为地弹），如大量芯片的输出同时开启时，将有一个较大的瞬态电流在芯片与板的电源平面流过，芯片封装与电源平面的电感和电阻会引发电源噪声，这样会在真正的地平面（<span lang="EN-US">0V</span>）上产生电压的波动和变化，这个噪声会影响其它元器件的动作。负载电容的增大、负载电阻的减小、地电感的增大、同时开关器件数目的增加均会导致地弹的增大。<span lang="EN-US"><br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span></span>由于地电平面（包括电源和地）分割，例如地层被分割为数字地、模拟地、屏蔽地等，当数字信号走到模拟地线区域时，就会产生地平面回流噪声。同样电源层也可能会被分割为<span lang="EN-US">2.5V</span>，<span lang="EN-US">3.3V</span>，<span lang="EN-US">5V</span>等。所以在多电压<span lang="EN-US">PCB</span>设计中，地电平面的反弹噪声和回流噪声需要特别关心。<span lang="EN-US"><br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span>8</span>、在时域<span lang="EN-US">(time domain)</span>和频域<span lang="EN-US">(frequency domain)</span>之间有什么不同<span lang="EN-US">? <br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span></span>时域<span lang="EN-US">(time domain)</span>是以时间为基准的电压或电流的变化的过程，可以用示波器观察到。它通常用于找出管脚到管脚的延时<span lang="EN-US">(delays)</span>、偏移<span lang="EN-US">(skew)</span>、过冲<span lang="EN-US">(overshoot)</span>、、下冲<span lang="EN-US">(undershoot)</span>以及建立时间<span lang="EN-US">(settling times)</span>。<span lang="EN-US"><br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span></span>频域<span lang="EN-US">(frequency domain)</span>是以频率为基准的电压或电流的变化的过程，可以用频谱分析仪观察到。它通常用于波形与<span lang="EN-US">FCC</span>和其它<span lang="EN-US">EMI</span>控制限制之间的比较。<span lang="EN-US"><br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span>9</span>、什么是阻抗<span lang="EN-US">(impedance)? <br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span></span>阻抗是传输线上输入电压对输入电流的比率值<span lang="EN-US">(Z0=V/I)</span>。当一个源送出一个信号到线上，它将阻碍它驱动，直到<span lang="EN-US">2*TD</span>时，源并没有看到它的改变，在这里<span lang="EN-US">TD</span>是线的延时<span lang="EN-US">(delay)</span>。<span lang="EN-US"><br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span>10</span>、什么是建立时间<span lang="EN-US">(settling time)? <br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span></span>建立时间就是对于一个振荡的信号稳定到指定的最终值所需要的时间。<span lang="EN-US"><br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span>11</span>、什么是管脚到管脚<span lang="EN-US">(pin-to-pin)</span>的延时<span lang="EN-US">(delay)? <br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span></span>管脚到管脚延时是指在驱动器端状态的改变到接收器端状态的改变之间的时间。这些改变通常发生在给定电压的<span lang="EN-US">50%</span>，最小延时发生在当输出第一个越过给定的阈值<span lang="EN-US">(threshold)</span>，最大延时发生在当输出最后一个越过电压阈值<span lang="EN-US">(threshold) </span>，测量所有这些情况。<span lang="EN-US"><br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span>12</span>、什么是偏移<span lang="EN-US">(skew)? <br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span></span>信号的偏移是对于同一个网络到达不同的接收器端之间的时间偏差。偏移还被用于在逻辑门上时钟和数据达到的时间偏差。<span lang="EN-US"><br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span>13</span>、什么是斜率<span lang="EN-US">(slew rate)? <br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span>Slew rate</span>就是边沿斜率<span lang="EN-US">(</span>一个信号的电压有关的时间改变的比率<span lang="EN-US">)</span>。<span lang="EN-US">I/O </span>的技术规范<span lang="EN-US"> (</span>如<span lang="EN-US">PCI)</span>状态在两个电压之间，这就是斜率<span lang="EN-US">(slew rate)</span>，它是可以测量的。<span lang="EN-US"> <br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span>14</span>、什么是静态线<span lang="EN-US">(quiescent line)? <br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span></span>在当前的时钟周期内它不出现切换。另外也被称为<span lang="EN-US"> "stuck-at" </span>线或<span lang="EN-US">static</span>线。串扰<span lang="EN-US">(Crosstalk)</span>能够引起一个静态线在时钟周期内出现切换。<span lang="EN-US"> <br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span>15</span>、什么是假时钟<span lang="EN-US">(false clocking)? <br/><span style="mso-spacerun: yes">&nbsp;&nbsp;&nbsp; </span></span>假时钟是指时钟越过阈值<span lang="EN-US">(threshold)</span>无意识地改变了状态<span lang="EN-US">(</span>有时在<span lang="EN-US">VIL </span>或<span lang="EN-US">VIH</span>之间<span lang="EN-US">)</span>。通常由于过分的下冲<span lang="EN-US">(undershoot)</span>或串扰<span lang="EN-US">(crosstalk)</span>引起。</span><span lang="EN-US" style="FONT-FAMILY: 宋体"><o:p></o:p></span></p>