s05-03.htm

关于扩频的一些好资料

            <P class=MsoNormal 
            style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; LINE-HEIGHT: 150%"><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">相关解扩在性能上固然很好，但总是需要在接收端产生本地</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">PN</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">码。这一点有时带来许多不方便。例如，解决本地信号与接收信号的同步问题就很麻烦，还不能做到实时把有用信号检测出来。因为匹配滤波和相关检测的作用在本质上是一样的，我们可以用匹配滤波器来解扩直扩信号。</SPAN></P>
            <P class=MsoNormal 
            style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; LINE-HEIGHT: 150%"><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt"><O:P></O:P></SPAN></P>
            <P class=MsoNormal 
            style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; TEXT-INDENT: 24pt; LINE-HEIGHT: 150%; mso-char-indent-count: 2.0; mso-char-indent-size: 12.0pt"><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">所谓匹配滤波器，就是与信号相匹配的滤波器，它能在多种信号或干扰中把与之匹配的信号检测出来。这同样是一种“用相片找人”的方法。对于视频矩形脉冲序列来说，无源匹配滤波器就是抽头延迟线上加上加法累加器。有时称为横向滤波器，其结构如图</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">5</SPAN><SPAN 
            style="FONT-SIZE: 12pt; 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: 12pt">11(a)</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">所示。</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt"><O:P> </O:P></SPAN></P>
            <P class=MsoNormal 
            style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; TEXT-INDENT: 24pt; LINE-HEIGHT: 150%; mso-char-indent-count: 2.0; mso-char-indent-size: 12.0pt" 
            align=center><SPAN lang=EN-US style="FONT-SIZE: 12pt">&nbsp;<IMG 
            height=435 src="s05-03.files/s5-9.gif" width=450 border=0><O:P> 
            </O:P></SPAN></P>
            <P class=MsoNormal 
            style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; TEXT-INDENT: 24pt; LINE-HEIGHT: 150%; mso-char-indent-count: 2.0; mso-char-indent-size: 12.0pt" 
            align=center><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">图</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">5</SPAN><SPAN 
            style="FONT-SIZE: 12pt; 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: 12pt">11<O:P> </O:P></SPAN></P></DIV>
            <P class=MsoNormal 
            style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; TEXT-INDENT: 24pt; LINE-HEIGHT: 150%; mso-char-indent-count: 2.0; mso-char-indent-size: 12.0pt" 
            align=center><SPAN lang=EN-US 
            style="FONT-SIZE: 12pt"><O:P></O:P></SPAN></P>
            <P class=MsoNormal 
            style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; LINE-HEIGHT: 150%"><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt"><O:P></O:P></SPAN></P>
            <P class=MsoNormal 
            style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; TEXT-INDENT: 24pt; LINE-HEIGHT: 150%; mso-char-indent-count: 2.0; mso-char-indent-size: 12.0pt">　</P>
            <P class=MsoNormal 
            style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; TEXT-INDENT: 24pt; LINE-HEIGHT: 150%; mso-char-indent-count: 2.0; mso-char-indent-size: 12.0pt"><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">但</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">SAW</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">匹配滤波器制作有一定难度。主要是插入损耗较大，且工艺要求很严，特别是在码位长时。一般情况，根据</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">PN</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">码序列结构做成固定的抽头，它就不能适应码序列需要改变的情况。如果在输出端加上控制电路，也可做成可编程的</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">SAW</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">匹配滤波器。这样应用起来就很方便，但制作起来就更困难了，要求有</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">VLSI</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">制作艺的精密度。</SPAN></P>
            <P class=MsoNormal 
            style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; TEXT-INDENT: 24pt; LINE-HEIGHT: 150%; mso-char-indent-count: 2.0; mso-char-indent-size: 12.0pt">　</P>
            <P class=MsoNormal 
            style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; TEXT-INDENT: 24pt; LINE-HEIGHT: 150%; mso-char-indent-count: 2.0; mso-char-indent-size: 12.0pt"><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt"><O:P></O:P></SPAN></P>
            <P class=MsoNormal 
            style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; LINE-HEIGHT: 150%"><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt; COLOR: #333399">5.3.3<SPAN 
            style="mso-spacerun: yes">&nbsp; </SPAN></SPAN><SPAN 
            style="FONT-SIZE: 12pt; COLOR: #333399; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">射频系统</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt; COLOR: #333399"><O:P> 
            </O:P></SPAN></P>
            <P class=MsoNormal 
            style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; LINE-HEIGHT: 150%"><SPAN 
            lang=EN-US 
            style="FONT-SIZE: 12pt; mso-spacerun: yes">&nbsp;&nbsp;&nbsp; 
            </SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">上面详细讨论了扩频调制和相关解扩的问题。但是直扩系统总是离不开发射机把信号通过天线辐射出去，也离不开天从空间收到的信号经接收机再进行处理。射频系统就是指的发射机相接收机而言。现在的问题是常规的窄带通信系统的收发信机能不能用在直扩系统呢</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">? </SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">回答是否定的。不应忘记直扩信号是宽带信号。直扩系统就必须具有适应这种宽带</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">PN</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">码信号的特点。下面就是一些直扩射频系统的特点。</SPAN></P>
            <P class=MsoNormal 
            style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; LINE-HEIGHT: 150%"><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt"><O:P></O:P></SPAN></P>
            <P class=MsoNormal 
            style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; LINE-HEIGHT: 150%"><SPAN 
            lang=EN-US 
            style="FONT-SIZE: 12pt; mso-spacerun: yes">&nbsp;&nbsp;&nbsp; 
            </SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">直扩发射机常见时中频是</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">70MHz</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，此时调制信号的带宽不超过</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">20MHz</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">。射频频率由中频变频得到，而不用倍频。因为倍频能使相位关系产生变化，会改变或甚至完全去掉</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">DS</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">调制。对于末级功率放大器，则要求其要有足够的带宽，以允许直扩信号可以顺利的通过。保持线性放大当然是希望的，但要求并不十分严格，因相位特性非线性不致引起大的问题。</SPAN></P>
            <P class=MsoNormal 
            style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; LINE-HEIGHT: 150%"><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt"><O:P></O:P></SPAN></P>
            <P class=MsoNormal 
            style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; LINE-HEIGHT: 150%"><SPAN 
            lang=EN-US 
            style="FONT-SIZE: 12pt; mso-spacerun: yes">&nbsp;&nbsp;&nbsp; 
            </SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">射频系统阻抗匹配很重要，特别要注意使电压驻波比达到一定的要求，因为在宽带运用时频率范围很广，驻波比会随频率而变，应使阻抗在宽度范围内尽量匹配。</SPAN></P>
            <P class=MsoNormal 
            style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; LINE-HEIGHT: 150%"><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"></SPAN></P>
            <P class=MsoNormal 
            style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; LINE-HEIGHT: 150%"><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt"><O:P></O:P></SPAN></P>
            <P class=MsoNormal 
            style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; TEXT-INDENT: 24pt; LINE-HEIGHT: 150%"><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">直扩接收机的问题要复杂一些，因为除有用宽带信号外，还存在其他干扰信号。直扩系统接收机的线性很重要，限幅会引起</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">6dB</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">信噪比的损失。从接收机前端到相关器要求保持线性，不仅在信号范围内，也包含干扰。自动增益控制只能部分地解决问题。通常应尽量把相关器靠近前端，使相关器前高电平级尽量的少，这样做的结果也降低了对本振信号电平的要求。另外，一般认为接收机前端最好能复盖整个宽频带，用改变本振频率经混频得到固定的中频信号。但由于干扰信好的存在，这会导致大量的干扰信号落入中频通带内，故一般最好不用宽带放大。一个理想的直扩接收系统应使有用信号得到放大，而干扰信号被滤除。故接收机前端应调谐在</SPAN><SPAN 
            lang=EN-US style="FONT-SIZE: 12pt">PN</SPAN><SPAN 
            style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">码钟率的两倍。当然，实际上有多种接收机的结构可供我们选择。</SPAN></P></TD>
          <TD width="5%">　</TD></TR></TBODY></TABLE>
      <P>　</P>
      <P>　</P></DIV></TD></TR>
  <TR>
    <TD bgColor=#ccffff>
      <DIV align=center><B><a href="main.htm">目录</a> 
      &nbsp;&nbsp;<a href="s05-02.htm">上一页</a> 
      &nbsp;&nbsp;<a href="s05-04.htm">下一页</a></B></DIV></TD></TR></TBODY></TABLE><!-- #EndEditable -->
<P align=center><!-- #BeginLibraryItem "/Library/close.lbi" --><A 
href="javascript:window.close();">关闭窗口</A><!-- #EndLibraryItem --></P>
<TABLE width="100%" border=0>
  <TBODY>
  <TR>
    <TD>
      <DIV align=center><FONT size=2><!-- #BeginLibraryItem "/Library/copyright.lbi" --><FONT 
      size=2>版权所有：移动通信国家工程研究中心 
      2000-2001&copy;</FONT><!-- #EndLibraryItem --></FONT></DIV></TD></TR>
  <TR>
    <TD>
      <DIV align=center><FONT size=2></FONT><FONT size=2><!-- #BeginLibraryItem "/Library/tel.lbi" --><FONT 
      size=2>tel:[phone]</FONT><!-- #EndLibraryItem -->&nbsp;<!-- #BeginLibraryItem "/Library/email.lbi" --><FONT 
      size=2><A 
      href="mailto:[e-mail]">mailto:[e-mail]</A></FONT><!-- #EndLibraryItem --></FONT></DIV></TD></TR></TBODY></TABLE>
<P>　</P><!-- #EndTemplate --></BODY></HTML>