通信系统第二章Continuous-Wave Modulation.ppt

东南大学移动通信国家重点实验室“通信系统通信系统(Communication Systems)”Communication Systems)”课件课件1Ch.2 Continuous-Wave Modulation2.1 Introduction2.2 Amplitude Modulation2.3 Linear Modulation Schemes 2.4 Frequency Translation2.5 Frequency-Division Multiplexing2.6 Angle Modulation2.7 Frequency Modulation2.8 Nonlinear Effects in FM Systems2.9 Superheterodyne Receiver2.10 Noise in CW Modulation Systems2.11 Noise in Linear Receivers Using Coherent Detection2.12 Noise in AM Receivers Using Envelope Detection 2.13 Noise in FM Receivers 2.14 Computer Experiments:Phase Locked Loop东南大学移动通信国家重点实验室“通信系统通信系统(Communication Systems)”Communication Systems)”课件课件22.1 IntroductionSome basic conceptsBaseband signals:information-bearing signalsModulation:the process by which some characteristic of a carrier is varied in accordance with a modulating wave(signal)Continuous-Wave(CW)modulation:the carrier is a sinusoidal waveThe baseband signal modulating waveResult of the modulation process modulated waveModulation and demodulation东南大学移动通信国家重点实验室“通信系统通信系统(Communication Systems)”Communication Systems)”课件课件3Two Families of CW Modulation SystemsAmplitude modulationAngle modulationPhase modulationFrequency modulation东南大学移动通信国家重点实验室“通信系统通信系统(Communication Systems)”Communication Systems)”课件课件4Two Families of CW Modulation Systems(Contd)Fig.2.2 Illustrating AM and FM signals produced by a signal tone.(a)Carrier wave(b)Sinusoidal modulating signal(c)Amplitude-modulated signal(d)Frequency-modulated signal东南大学移动通信国家重点实验室“通信系统通信系统(Communication Systems)”Communication Systems)”课件课件52.2 Amplitude ModulationSinusoidal carrier waveCarrier amplitude(volt)Carrier frequencyAmplitude-modulated waveAmplitude sensitivity(Constant,volt-1)Modulating signal(volt)东南大学移动通信国家重点实验室“通信系统通信系统(Communication Systems)”Communication Systems)”课件课件6Amplitude Modulation(Contd)Fig.2.3 Illustrating the amplitude modulation process.(a)m(t);(b)AM wave for|kam(t)|1 for some t.The envelope of s(t)has the same shape as m(t)provided that:(1)|kam(t)|WKam(t)100%:percentage modulation|kam(t)|1:overmodulation东南大学移动通信国家重点实验室“通信系统通信系统(Communication Systems)”Communication Systems)”课件课件7Amplitude Modulation(Contd)Fig.2.4 (a)Spectrum of baseband signal;(b)Spectrum of AM wave.Fourier transform of AM modulated signal东南大学移动通信国家重点实验室“通信系统通信系统(Communication Systems)”Communication Systems)”课件课件8Amplitude Modulation(Contd)Spectrum of an AM modulated signal consists of 2 delta functions at fc and 2 versions of baseband spectrum shifted by fcThe negative frequencies become visibleUpper sideband and lower sidebandThe transmission bandwidth is twice the message bandwidth东南大学移动通信国家重点实验室“通信系统通信系统(Communication Systems)”Communication Systems)”课件课件9Virtues and LimitationsVirtuesSimple in both transmitter and receiver designLimitationsWasteful of power(due to the existence of carrier)Wasteful of bandwidth(due to the symmetry of upper and lower sidebands)东南大学移动通信国家重点实验室“通信系统通信系统(Communication Systems)”Communication Systems)”课件课件102.3 Linear Modulation SchemesDefinition of linear modulationThree types of linear modulationDouble sideband-suppressed carrier(DSB-SC)Single sideband(SSB)Vestigial sideband(VSB)sI(t)and sQ(t)are linearly related to the message signal m(t).东南大学移动通信国家重点实验室“通信系统通信系统(Communication Systems)”Communication Systems)”课件课件11DSB-SC Modulation Fig.2.5 (a)Block diagram of product modulation;(b)Baseband signal;(c)DSB-SC modulation.东南大学移动通信国家重点实验室“通信系统通信系统(Communication Systems)”Communication Systems)”课件课件12DSB-SC Modulation(Contd)Fourier transform of DSB-SC modulated signalFig.2.6(a)Spectrum of baseband signal;(b)Spectrum of DSB-SC modulated wave.东南大学移动通信国家重点实验室“通信系统通信系统(Communication Systems)”Communication Systems)”课件课件13DSB-SC Modulation(Contd)Fig.2.7 Coherent detector for demodulating DSB-SC modulated wave.东南大学移动通信国家重点实验室“通信系统通信系统(Communication Systems)”Communication Systems)”课件课件14Costas ReceiverAn synchronization methodConsists of 2 coherent detectors with 90 phase difference in the local oscillatorsPhase error of the Voltage-Controlled Oscillator(VCO)is compensated by output of the phase discriminator,which varies with the phase error东南大学移动通信国家重点实验室“通信系统通信系统(Communication Systems)”Communication Systems)”课件课件15Costas Receiver(Contd)Fig.2.9 Costas receiver.东南大学移动通信国家重点实验室“通信系统通信系统(Communication Systems)”Communication Systems)”课件课件16Quadrature-Carrier MultiplexingFig.2.10 Quadrature-carrier multiplexing system.(a)Transmitter;(b)Receiver.(QAM)东南大学移动通信国家重点实验室“通信系统通信系统(Communication Systems)”Communication Systems)”课件课件17SSB ModulationOnly the upper or lower sideband is transmittedTwo stagesGenerating DSB-SC modulated wavePassing one of the sidebands and suppressing the otherRequires highly selective filters东南大学移动通信国家重点实验室“通信系统通信系统(Communication Systems)”Communication Systems)”课件课件18SSB Modulation(Contd)Fig.2.11(a)Spectrum of a message signal m(t)with an energy gap of width 2fa centered on the origin;(b)Spectrum of corresponding SSB signal containing the upper sideband.A spectrum gap around the origin is desire。