抑制载波双边带调制

*******************实践教学*******************高频电子线路课程设计题目：抑制载波双边带调制专业班级：姓名：学号：指导教师：成绩：目录摘要··································································································································1一、电路设计······················································································································11.1设计的意义···············································································································21.2原理分析·················································································································21.2.1DSB信号的调制································································································21.2.2DSB解调原理···································································································41.3电路设计·················································································································51.3.1调制电路·········································································································51.3.2解调电路········································································································61.3.3总体电路········································································································71.3.4电路分析········································································································8二、multisim软件简介·········································································································13三、仿真电路····················································································································143.1调制电路仿真·········································································································143.2解调电路仿真·········································································································163.2.1无噪声源时解调电路仿真·················································································163.2.2有噪声源时的解调电路仿真··············································································183.3仿真结果分析·········································································································18四、参考文献····················································································································19五、总结··························································································································201摘要在常规AM调幅时，由于载波分量不包含任何信息，又占整个调幅波平均功率的很大比例，造成了发射功率的极大浪费。为了提高调制效率，使总功率包含在边带中。因此，在传输前把载波抑制掉，就可以在不影响传输信息的条件下，大大节省发射机的发射功率。针对AM波的不足，产生了双边带（DSB）信号调制与解调。本次课程设计为一个DSB调制解调过程电路。DSB调幅调制过程中将载波完全抑制，它的产生原理是调制信号与载波信号直接相乘。DSB解调过程中将已调信号经过一个低通通滤波器然后与载波信号直接相乘。从而提高了调制效率。关键词：双边带、高频载波、调幅、调制信号、仿真2一、电路设计1.1设计的意义调制就是对信号源的信息进行处理，使其变为适合于信道传输的形式的过程。一般来说，信号源的信息（也称为信源）含有直流分量和频率较低的频率分量，称为基带信号。基带信号往往不能作为传输信号，因此必须把基带信号转变为一个相对基带频率而言频率非常高的信号以适合于信道传输。这个信号叫做已调信号，而基带信号叫做调制信号。调制是通过改变高频载波的幅度、相位或者频率，使其随着基带信号幅度的变化而变化来实现的。而解调则是将基带信号从载波中提取出来以便预定的接收者（也称为信宿）处理和理解的过程。调制在整个通信过程中是最基本、最重要的一个处理信号方法，在通信系统中，对模拟基带信号进行调制的目的就是为了让多个基带信号经过调制后在有线信道上同时传输，同时也适合于在无线信道中实现频带信号的传输；并且还能增强信号的抗噪声能力。因此，调制的意义可概括为减小干扰，提高系统抗干扰能力，同时还可实现传输带宽与信噪比之间的互换。1.2原理分析DSB(DoubleSideBand)信号是抑制了载波的双边带信号，发送时，不发送载波信号，对于DSB信号，其包洛的变化反映了调制信号绝对值的变化情况，当调制信号过零点时，有相位的突变。DSB信号的解调就是从它的幅度变化上提取原调制信号的过程。DSB的调制与解调原理图如图下所示。图1DSB的调制与解调原理图1.2.1DSB信号的调制针对AM波的不足，DSB条幅调制过程中将载波完全抑制，它的产生原理是调制信号与载波信号直接相乘。原理图如下图所示。3图2抑制载波双边带调幅调制原理框图（1）式中K为一常数，与实现DSB波的电路有关。抑制载波的双边带调幅波的时域和频域波形如图所示。DSB波的包络不再与调制信号成正比关系，当改变正负符号时，DSB相应的跳转180度，如图所示。图3DSB波时域波形与频域谱DSB信号的频谱有如下特点：（1）上、下边带均包含调制信号的全部信息。（2）幅度减半，带宽加倍。（3）线性调制。41.2.2DSB解调原理由抑制载波双边带调幅信号的频谱可知，如果将已调信号的频谱搬回到原点位置，即可得到原始的调制信号频谱，从而恢复出原始信号。解调中的频谱搬移同样可用调制时的相乘运算来实现。图4抑制载波双边带调制的相干解调将已调信号乘上一个同频同相的载波，如图4所示，可得到（2）用一个带通滤波器可以将上式中第1项和第2项分离，无失真地恢复出原始的调制信号。这种调制方法又称为同步解调或相干解调。相干解调的关键是必须产生一个同频同相的载波。如果同频同相的条件得不到保证，则会破坏原始信号的恢复。同步检波器用于对载波被抑止的双边带或单这带信号进行解调。它的特点是必须外加一个频率和相位部与被抑止的载波相同的电压。同步检波的名称即由比而来，外加载波信号电压加入周步检波器可以有两种方式：一种是将它与接收信号在检波器中相乘，经低通滤波器后，检出原调制信号，另一种是权巳与接收信号相加，经包络检波器后取出原调制信号。51.3电路设计1.3.1调制电路图5调制电路图调制方式电力线载波机采用的调制方式主要有双边带幅度调制、单边带幅度调制和频率调制三种。双边带幅度调制双边带幅度调制(AM)也称为双边带调幅。在这种调制方式中，频率稳定的载频被音频信号所调制，使其幅度与调制信号的幅度成正比变化。调制后的信号频谱包含原来的载频和上下两个边带。两个边带的频率等于载频与调制音频的和(上边带)或差(下边带)。双边带调幅占用的频带宽度为调制信号频带宽度的两倍。这种方法所用电路简单，早期应用普遍。它有以下缺点:①接收频带宽，对噪声及干扰敏感。②电力线载波通道日益增多，频率十分拥挤，而这种方式占用频带宽，故不能充分利用电力线载波频谱。③必须发送对传输信息无用的载频.传输功率利用不充分，对其他电路形成干扰。中国目前在35kV及以下的电力网中还有应用这种调制方式的载波机，工厂也有少量生产。61.3.2解调电路R9500R10500R11500Q1BJT_NPN_4T_VIRTUALQ22N6772Q32N6772Q42N6772Q52N6772Q62N6772Q72N6772Q82N677232282726R13kR21kR31kR451kR51kR63kR710kR81kR12100R133kR1420C11uFC20.005uFC30.1uFC40.1uFC50.1uFC61uFC70.005uFC80.005uF3130VCC12V0205142500VCC70XSC1ABGT89003D6RGP10A610图6解调电路图71.3.3总体电路图7调制解调总电路图81.3.4电路分析（1）调制与解调电路分析调制信号：频率400Hz振幅40mV载波信号：频率40KHz振幅40Mv调制电路：设载波为:uc(t)=Ucmcosωct（3）单频调制信号为:uΩ(t)=UωmcosΩt(Ω〈〈ωc)（4）则双边带调幅信号为:uDSB(t)=kuΩ(t)uc(t)=kUΩmUcmcosΩtcosωct［cos(ωc+Ω)t+cos(ωc-Ω)t］（5）其中k为比例系数。可见双边带调幅信号中仅包含两个边频,无载频分量,其频带宽度仍为调制信号带宽的两倍。图8显示了单频调制双边带调幅信号的有关波形与频谱图。图8双边带调幅波形与频谱图需要注意的是,双边带调幅信号不仅其包络已不再反映调制信号波形的变化,而且在调制信号波形过零点处的高频相位有180°的突变。可以看出,在调制信号正半周,cosΩt为正值,双边带调幅信号uDSB(t)与载波信号uc(t)同相；