通信电子线路侯丽敏版第六章.ppt

第六章,School of Communication and Information Engineering,第六章 角度的调制与解调,School of Communication and Information Engineering,6.1 调角波的性质,调频：FM 高频振荡的频率受调制信号的控制,调相：PM 高频振荡的相位受调制信号的控制,等幅波：v(t)=Vmcos (t)= Vmcos(t)+0,初始相位,School of Communication and Information Engineering,调频波、调相波：,1.调频波,(t) = C + Kfv(t),若设调制信号 v(t) = Vmcost,(t) = C + KfVm cost,m,(最大角频率偏移与Vm成正比，与无关),School of Communication and Information Engineering,最大相位偏移，与成反比 调频指数,表达式：v(t) = Vmcos(Ct + mf sint),图,School of Communication and Information Engineering,2. 调相波,(t) = Ct + KPV(t),设调制信号：v(t) = Vmcost,(t) = Ct + KPVmcost,mP,最大相位偏移，与Vm成正比、与无关 调相指数,(t) = C mP sint,m,与成正比,v(t) = Vmcos( Ct + mPcost ),图,School of Communication and Information Engineering,v(t) = Vmcos( Ct + mPcost ) PM,v(t) = Vmcos( Ct + mfsint ) FM,设调制信号 v(t) = Vmcost 情况下：,当 恒定时，调频波,当 恒定时，调相波,School of Communication and Information Engineering,设有一组正弦调制信号，信号频率最低为 最高为 。调制信号为等幅振荡。调频时，最 大频偏 ；调相时，最大相偏 试求调频时调制指数 的变化范围及调相时最大频偏 的范围。,例1.,School of Communication and Information Engineering,1. 调频时:,School of Communication and Information Engineering,2. 调相时:,School of Communication and Information Engineering,调制信号v(t) 为周期重复的三角波。 画出调频波和调相波.,例2.,调频9-1图,调9-2相图,School of Communication and Information Engineering,二. 调角信号的频谱,FM、PM波的数学表示式是相似的，统一写成调角波表示式,利用角函数展开：,在贝塞尔函数理论中，已经证明存在下列关系：,School of Communication and Information Engineering,式中 是以m为宗数的n阶第一 类贝塞尔函数。代入上式，整理得：,School of Communication and Information Engineering,调角的频谱为：,无限多对上、下边频分量 当n为偶数时，上、下边频分量相加 当n为奇数时，上、下边频分量相减,School of Communication and Information Engineering,当调制指数 m 增大时，则具有较大 振幅的边频分量也增加。,当m为某些特定值时，如m=2.405、 5.520、8.653、载波分量成 分 。用来测量频偏 和调制指数 。,School of Communication and Information Engineering,第一类贝塞尔函数,School of Communication and Information Engineering,三、调角信号的频谱宽度,m1,窄带调制,m很小，近似：,调角信号的频谱与AM波频谱相似。由 构成。差别就是下边频的相位相反。频谱宽度为 rad/s. 它广泛运用于移动通信中。,School of Communication and Information Engineering,2) 宽带调制 m1,频谱是无限宽的，认为当边频幅度小于载频的10%时，即忽略这些边频。因此，实际调角信号所占的有效频谱宽度是有限的。当m1时，m+1 以上各阶频谱的幅度均小于载波幅度10%。因而可以忽略。,School of Communication and Information Engineering,有效频谱宽度为：,以上讨论的是单音调制情况。,实际上，调制信号中包括很多频率分量，设最高频率 Fmax,School of Communication and Information Engineering,四. 各种调制方式的比较：,1. 抗干扰能力：,调角波的幅度不带有调制信号的信息, 抗干扰能力强.,2.信号频谱及带宽：,设调制信号的频率F：（m1）,School of Communication and Information Engineering,FM与PM的比较：,但当增加时, 调频:,调相:,School of Communication and Information Engineering,3. 设备的利用率,AM:,FM、PM：,调角波振幅不变，工作在最大工作 状态.,功率：,School of Communication and Information Engineering,根据第一类贝塞尔函数特性：,上式表明，当 一定时，不论 为何值，调频波的平均功率恒为定值，并且等于未调制的载波频率。换句话说， 改变仅会引起载波分量和各边带分量之间功率的重新分配，但不会引起总功率的改变。,School of Communication and Information Engineering,例3.,已知,求(1)此时FM波和PM波的带宽? (2)若 不变,F 增大一倍两种调制信号的 带宽如何? (3)若F 不变, 增大一倍两种调制信号的 带宽如何? (4)若 、F都增大一倍两种调制信号的带 宽如何?,School of Communication and Information Engineering,School of Communication and Information Engineering,School of Communication and Information Engineering,6.2 调频的方法,间接调频： 对调制信号先积分后调相 ( fC 较稳定，但是频偏小 ),直接调频： 对振荡器直接进行频率调制 ( 可获大频偏，但 fC 不稳定 ),School of Communication and Information Engineering,1. 调制特性,要线性,调频电路输出电压的频率偏移与调制电压的关系（ fV ）,一. 调频电路的指标：,School of Communication and Information Engineering,4. 1% 大频偏 1% 小频偏,单位调制电压变化所产生的频率偏移,3. 中心频率 ( 载波频率 ),要稳定,2. 调制灵敏度：,School of Communication and Information Engineering,利用它的结电容受反向外加电压控制而变化来实现调频。,1. 变容二极管,Cj0 反向电压v = 0 时的结电容 VD PN结内建电位差( 很小) 电容的变化指数,二. 变容二极管直接调频电路：,School of Communication and Information Engineering,CjQ,当调制信号为：,=,School of Communication and Information Engineering,School of Communication and Information Engineering,静态时的结电容：,结电容的调制度,School of Communication and Information Engineering,2. 变容管作振荡回路的总电容 ,C1 隔直作用 C2 高频滤波电容 ( 对高频短路，对开路 ) L1 高频扼流圈,大频偏直接调制电路,应满足： | VQ | Vm,图,School of Communication and Information Engineering,瞬时振荡角频率为:,School of Communication and Information Engineering,线性调制,最大角频偏,中心频率偏移,基波分量,两次谐波分量,调制特性分析:,这时，调频波获得最大角频偏为,School of Communication and Information Engineering,最大角频偏,School of Communication and Information Engineering,例 调频振荡回路有电感L和变容二极管组成。L=2mH,变容二极管的参数为：,(1)载波fc,(2)由调制信号引起的载波漂移fc,(3)最大频偏fm,(4)调频系数kf,(5)二阶失真系数,School of Communication and Information Engineering,School of Communication and Information Engineering,School of Communication and Information Engineering,School of Communication and Information Engineering,140MHz的变容管直接调频电路,150pF,VQ,p型滤波,School of Communication and Information Engineering,L1与变容管Cj构成振荡回路并与晶体管Q1接成电感三点式 振荡电路。,变容管的直流电压偏置从正电源稳压电路中通过两个470W 电位器取出一部分提供，作为VQ。,调制信号通过1.7mH的高频扼流圈L2和两个150pF电容 C1、C2接成的p型滤波网络加到变容管上。,School of Communication and Information Engineering,C3为高频滤波电容,小频偏的直接调频电路,3. 变容管部分接入振荡回路 ,图,School of Communication and Information Engineering,展开成幂级数，只考虑二次方项：,School of Communication and Information Engineering,C1、C2 的引入，使Cj 对回路总电容的影响减小， 从而c 的稳定性提高,P ,m ,讨论： C2 P1 C1 P2,School of Communication and Information Engineering,试指出各元器件的作用；画出高频等效电路；计算最大频偏fm=？,School of Communication and Information Engineering,等效电路图,克拉泼振荡器,各元器件的作用略。,School of Communication and Information Engineering,最大频偏：,School of Communication and Information Engineering,例：对题图所示的变容管直接调频电路。 （1）说明电路中各元件的作用； （2）画出该调频振荡器的高频通路、变容管的直流通路和音频通 路。 （3）若已知 ，变容管结电容 , 调频指数mf =5，当vW=0时的振荡频率为 fc=90MHz，试求变 容管所需的直流偏置电压VQ，最大频偏fm以及调制信号的 振幅VWm 。,School of Communication and Information Engineering,(1)R1、R2、R3、R4为晶体管提供直流偏置和直流通路，R4、R5、R6、R7为 变容管提供直流反偏置和直流通路；1000pF，0.001mF、0.002mF均为高频 旁路电容，即高频交流通路；47mH为高频扼流图，对高频近似开路，对直 流、低频近似短路；47 mF为隔直电容，对低频近似短路；C1、C2、C3、 C4、C5、Cj和电感L1与晶体管Q1构成电容三点式振荡器； L1、L2互感 耦合输出vFM。,School of Communication and Information Engineering,(2)高频通路,School of Communication and Information Engineering,(3),School of Communication and Information Engineering,School of Communication and Information Engineering,School of Communication and Information Engineering,一. 间接调频的原理：用调相电路得到调频波,满足：RC 1,6.3 间接调频电路,先将调制信号积分, (例如RC积分电路),再 进行调相, 所得结果就 是调频波. 这里, 采用 高稳定的晶振作为主 振级, 调相是在后级进 行, 对晶振频率无影响.,School of Communication and Information Engineering,用 进行调相,产生的相位偏移为,相当于调频时:,调相电路可以间接取得调频波。,School of Communication and Information Engineering,二. 调相电路,School of Communication and Information Engineering,1. 变容管调相器,School of Communication and Information Engineering,若加在变容二极管的电压：,则有：,School of Communication and Information Engineering,图,School of Communication and Information Engineering,调相电路便转为间接调频电路。,根据上述原理构成的电路:,图,School of Communication and Information Engineering,见书P168(b)三级单回路,上式表明最大频偏m 与c 无关。,School of Communication and Information Engineering,2. 可变时延法调相电路,School of Communication and Information Engineering,图,m为最大时延差，n为谐波次数,理论上m=0.5Tc，而实际上限制m 0.4Tc 。,可见，脉冲调相电路有线性相移较大的优点，用于调频广播发射机。,School of Communication and Information Engineering,三、间接调频与直接调频电路在性能上的差别,1、直接调频,P = 1,P = (1+P1)(1+P2+P1P2),在直接调频电路中，最大频偏 m与c 成正比， 而在间接调频电路中，最大频偏 m与c 无关。,School of Communication and Information Engineering,2、间接调频,如果调制信号中包含min、max众多频率分量，则当m一定时（Vm一定时）,越小,mf就越大。,School of Communication and Information Engineering,在间接调频电路中，最大频偏 必发生在最低调制信号频率上，m=mf min,例如 F : 1003400Hz, 这时Fmin=100Hz,当采用变容二极管调相时，,当采用三级变容管调相时，,当采用脉冲调相时，,School of Communication and Information Engineering,四、扩展线性频偏的方法,方法1: 倍频,方法2：混频 + 倍频,图,图,School of Communication and Information Engineering,调角波在产生过程中或多或少附带寄生调幅，当传输到接收端时，也会在调频波上出现寄生调幅。通常用限幅器来消除调频波的寄生调幅。,限幅器的组成：,非线性元件 + 谐振回路,常用限幅器：,晶体二极管限幅器 晶体三极管限幅器 差分对限幅器,6.4 限幅器,School of Communication and Information Engineering,鉴频方法：,1、斜率鉴频器，相位鉴频器，比例鉴频器,2、移相正交鉴频器,3、脉冲计数式鉴频器,6.5 鉴频器,图,School of Communication and Information Engineering,3. 非线性失真,1. 灵敏度 SD ,fc处的鉴频特性曲线的斜率,(要求大),2. 线性范围： BPP 2fm,(要求大),(要求小),一. 主要指标,鉴频特性曲线：,School of Communication and Information Engineering,(变换器) FM FM、AM,1. 单失谐的斜率鉴频器,实现框图：,二. 斜率鉴频器,School of Communication and Information Engineering,vR(t)变为幅度反映瞬时频率变化的调频波，称其为调频-调幅 (FM-AM)波。而后通过包络检波器完成鉴频功能。,School of Communication and Information Engineering,School of Communication and Information Engineering,其中,School of Communication and Information Engineering,在 处，,取得最大值，即,可得最大鉴频灵敏度,School of Communication and Information Engineering,取较小的对称区域作为频偏，取,School of Communication and Information Engineering,L1,2. 平衡斜率鉴频器,实际上，单调谐回路的谐振曲线，其倾斜部分的线性范围 是很小的，为了扩展鉴频特性的线性范围，实用的斜率鉴 频器都是采用两个单失谐回路构成的平衡电路。,School of Communication and Information Engineering,成对称失谐：f01 fC = fC f02,设包络检波效率均为kd则:,f 不能过大，也不能过小，,则鉴频特性的线性范围达到最大,School of Communication and Information Engineering,平衡斜率鉴频器 主要工作波形图,School of Communication and Information Engineering,三. 相位鉴频器,相位鉴频器是利用耦合谐振回路次级和初级间的 相移随频率变化的特性来实现鉴频的。,下面介绍利用互感耦合相位鉴频器的原理。,School of Communication and Information Engineering,2.平衡型叠加鉴相器,图,School of Communication and Information Engineering,(b)相位鉴频器检波回路等效电路,(b)图中，加在二级管D1和D2两端的高频电压为：,School of Communication and Information Engineering,1. 频相转换网络,(a)初、次级回路,（b）次级回路,假设，图(a)中，初次级回路参数完全相同，即 ，则得初、,次级回路的阻抗为：,School of Communication and Information Engineering,初级回路 通过互感在次级产生的感应电势为：,School of Communication and Information Engineering,School of Communication and Information Engineering,2. 矢量图：,f =fC,VO = 0,f fC,VO 0,VO 0,f fC,School of Communication and Information Engineering,(fc),综上所述，当调频信号的瞬时频率以 为中心而按调制信号的规律变化时，以及 和 间的相位差， 和 的幅值等，都随之化，从而形成了调幅调频波。经过检波后的输出电压即为调制电压。,School of Communication and Information Engineering,对V0作精确推导：,图,School of Communication and Information Engineering,特点： （1）f ( , )是 的奇函数；,(2)每条曲线在= 时取得最大值，而=- 出现最小值， 所以正负峰值之间距离为2，,(3)一般取 =1.5 3.,School of Communication and Information Engineering,School of Communication and Information Engineering,2、,School of Communication and Inf