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<TD borderColor=#6699ff><FONT size=2>您的位置：技术园地>技术讲座-扩频通信</FONT></TD></TR>
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<DIV align=center><FONT size=4><B><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman"><FONT
color=#000080 size=5>第五讲
直接序列扩频系统(3)</FONT></SPAN></B></FONT></DIV></TD></TR>
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<P class=MsoNormal
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color=#800080><B><FONT size=4><SPAN lang=EN-US>5.3
直扩信号的发送与接收</SPAN></FONT></B></FONT></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'">在图</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">l</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">中所示出的直扩系统发送接收系统的原理方框中，在发端输入<BR>信息要经过信息调制“扩频和射频调制”，在收端接收到的信号要经过变频、<BR>解扩和信息解调。与一般模拟或数字通信系统比较，信息识别与解调、射频的上变频和下变频，情况基本相同。</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt"><O:P> </O:P></SPAN></P>
<P class=MsoNormal
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style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">直扩通信系统的主要特点在于直扩信号的产生，即扩频调制和直扩信号的接收，即相关解扩。</SPAN></P>
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lang=EN-US style="FONT-SIZE: 12pt"><O:P></O:P></SPAN></P>
<P class=MsoNormal
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lang=EN-US style="FONT-SIZE: 12pt; COLOR: #333399">5.3.1
</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'">扩频调制</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; 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'">通过对扩频信号波形与频谱关系的分析和对</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'">码序列性能的了解，来说明获得扩频信号的调制方法就比较容易了。一般说来，都是用高码率的</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'">码脉冲序列去进行调制扩展信号的频谱的。</SPAN></P>
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lang=EN-US style="FONT-SIZE: 12pt"><O:P></O:P></SPAN></P>
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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'">通常采用的调制方式为</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">BPSK</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，输入信号与</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'">码在平衡调制器调制而输出展宽的扩频信号；图</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">5-2</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">中已经表示出直扩扩频调制的原理图。图中平衡调制器的输出信号的中心频率位置决定于输入的载波频率，在这里是载频抑制的。而两个边带则为展宽的频谱，它决定于调制</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'">码脉冲的宽度。</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'">码码率越高，或脉冲宽度越窄，扩展的频谱越宽。</SPAN></P>
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<P class=MsoNormal
style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; TEXT-INDENT: 24pt; 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'">那么这一扩频调制的原理是如何具体实现的呢</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">?</SPAN></P>
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lang=EN-US style="FONT-SIZE: 12pt"></SPAN></P>
<P class=MsoNormal
style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; TEXT-INDENT: 24pt; 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'">图</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">5-7(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'">中为一常见的二极管平衡调制器。它的作用原理是：左端上面输入为正弦载波信号，下面输入的是</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'">码脉冲信号。</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">4</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">个二极管起作开关的作用。当脉冲信号为正</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">D2</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">D3</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">导通，此时输出变压器中载波信号电流是向上的。脉冲输入信号变负时，</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">Dl</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">D4</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">导通，此时输出变压器中载波电流是向下的。换句话说，随着脉冲信号极性的不同，输出载波信号的相位改变</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">180</SPAN><SPAN lang=EN-US
style="FONT-SIZE: 12pt; FONT-FAMILY: Symbol; mso-ascii-font-family: Times New Roman; mso-hansi-font-family: Times New Roman; mso-char-type: symbol; mso-symbol-font-family: Symbol">°</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">。因此，平衡调制器起到了二相相移键控</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">(BPSK)</SPAN><SPAN
style="FONT-SIZE: 12pt; FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">调制器的作用。输出正弦波相位改变的情况如图</SPAN><SPAN
lang=EN-US style="FONT-SIZE: 12pt">5</SPAN><SPAN
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