通信电子电路试题及答案==

、写出AM波、DSB波、SSB波的数学表达式。Р 5、画出对应的波形图和频谱图。Р 6、写出对应的频带宽度BW表达式。Р 解:Р 1、AM:U(t)= Ucm(1+macosΩt)cosωct,ma=kaUΩm/Ucm (2分) DSB:U(t)= AUΩmcos(Ωt)×Ucmcosωct=0.5 AUΩmUcm[cos(ωc+Ω)t+cos(ωc-Ω)t] (2分) SSB:U(t)= 0.5 AUΩmUcmcos(ωc+Ω)t或U(t)= 0.5 AUΩmUcmcos(ωc-Ω)t (2分)Р 2、AM:Р (1分)Р (1Р 分)Р DSB:Р (1分)Р (1分)Р SSB:Р (1分)Р (1分)Р ?c ?c?? ω? 3、BAM=2F (1分) BDSB=2F (1分)Р BSSB=F (1分)Р 六、(20分)已知高频功率放大器的某晶体管的转移特性,且知Uj=0.6V,g=10mA/V,Eb=-0.5V,Ubm=3.2V,求电流iC的IC0,ICm1,Icm2,及iC1, iC2.Р (已知α0(700)=0.253, α1(700)=0.436, α2(700)=0.267, cos700=0.344)Р (请写简单解题步骤)Р cР 解: cos??Uj?EbР Ubm?0.6?0.51.1??0.3443.23.2图1 o,??70 Icmax=gUbm(1-cos?)=10×3.2×(1-0.344)=21mA (3分) I=?0I=0.251×21=5.31mA (3分) c0cmaxIc1m=?1Icmax=0.436×21=9.16mA (3分) Ic2m=?2Icmax=0.267×21=5.61mA ic1?Ic1mcos?t?9.16cos?tmA ic2?Ic2mcos2?t?5.61cos2?tmA 3分) 4分) (4分) ( (