信号与系统奥本海姆原版第八章课件.ppt

1、 8 Communication systems8.1 Complex Exponential and Sinusoidal Amplitude ModulationModulating system model:8.Communication Systemsx(t)c(t)y(t)=x(t)c(t)x(t)-modulating signalc(t)-Carrier signal 8 Communication systems8.1.1 Amplitude Modulation with Complex Exponential CarrierExponential carrier:For c

2、onvenience,let c=0,so)()(cctjetctjcetc)(Output signal(modulated signal):tjcetxty)()()()()(2)()(*)(21)(ccjjXjYjCjCjXjY(1)Modulation Theory 8 Communication systems 8 Communication systemsttjxttxetxtycctjcsin)(cos)()()(1)Implementation 8 Communication systems8.1.2 Amplitude Modulation with Sinusoidal s

3、ignalFor convenience,choose c=0,sottxtctxtyccos)()()()()(21)(21)()()()()(*)(21)(ccccjjXjjXjYjCjCjXjY 8 Communication systems 8 Communication systems 8 Communication systems8.2 Demodulation for Sinusoidal AM8.2.1 Synchronous demodulation(1)Demodulation process 8 Communication systems)2(41)2(41)(21)(c

4、cjjXjjXjXjWIn frequency domain:ttxtxttxtctytwcc2cos)(21)(21cos)()()()(2In time domain:Expected signal:)()(jWortx 8 Communication systems 8 Communication systems(2)Synchronous problem 8 Communication systemsTime domain:)2cos()(21)cos()(21)cos()cos()()cos()()(cccccccccccttxtxtttxttytwThe output of lowpass filter:)cos()(cctxIdeal output:x(t)(desiredisccWhen ,it is referred to as synchronous demodulation.cc 8 Communication systems8.2.2 Asynchronous demodulationAmplitude-modulated signal:tAtxtyccos)()(8 Communication systems 8 Communication systemsProblem:8.1 8.3 8.22 8.28