Analog Modulation AM(Amplitude Modulation) Demodulation of AM Signals Angle Modulation
Analog Modulation AM(Amplitude Modulation) Demodulation of AM Signals Angle Modulation
Why modulate Ease of radiation ■The size of antenna oc入l4=cl4f If we wish to throw a piece of paper(baseband signal),it cannot go too far by itself.But by wrapping it around a stone(carrier),it can be thrown over a longer distance Simultaneous transmission of several signals FDM(Frequency Division Modulation) Reduce the influence of interference ·Frequency Hopping Effecting the exchange of SNR with B Shannon's equation:C=Blog,(1+SNR) C is rate of information change per second(bit/s)
Why modulate ? ◼ Ease of radiation ◼ The size of antenna /4 = c/4f ◼ If we wish to throw a piece of paper(baseband signal), it cannot go too far by itself. But by wrapping it around a stone(carrier), it can be thrown over a longer distance ◼ Simultaneous transmission of several signals ◼ FDM(Frequency Division Modulation) ◼ Reduce the influence of interference ◼ Frequency Hopping ◼ Effecting the exchange of SNR with B ◼ Shannon’s equation : ◼ C is rate of information change per second (bit/s) 2 C B SNR = + log (1 )
Properties of analog modulation ■ Time domain representation of the modulated signal Frequency domain representation of the modulated signal Bandwidth of the modulated signal Power component of the modulated signal SNR after demodulation Message Modulated ■ Modulator Signal Signal (or modulating Signal)
Properties of analog modulation ◼ Time domain representation of the modulated signal ◼ Frequency domain representation of the modulated signal ◼ Bandwidth of the modulated signal ◼ Power component of the modulated signal ◼ SNR after demodulation ◼ Modulator Message Signal (or modulating Signal) Modulated Signal
AM (Amplitude modulation) Also known as "Linear modulation) Small bandwidth,Power inefficient ■Applications AM radio,TV video broadcasting(VSB),Point-to-point communications(SSB),Transmission of many telephone channels over microwave links ■ Class of AM DSB-AM(Double Side Band-AM) BW=2W=2*BW of the message signal SSB-AM(Single Side Band-AM) BW=W VSB-AM(Vestigial Side Band-AM) ■BW=W~2W
AM (Amplitude modulation) ◼ Also known as “Linear modulation) ◼ Small bandwidth, Power inefficient ◼ Applications ◼ AM radio, TV video broadcasting(VSB), Point-to-point communications(SSB), Transmission of many telephone channels over microwave links ◼ Class of AM ◼ DSB-AM(Double Side Band – AM) ◼ BW = 2W = 2 * BW of the message signal ◼ SSB-AM(Single Side Band – AM) ◼ BW = W ◼ VSB-AM(Vestigial Side Band – AM) ◼ BW = W ~ 2W
DSB-AM Amplitude of modulated signal is proportional to the message signal m(t)_ (t)=Am(t)cos(2πft) c(t)=A.cos(2πft) m(t) m(t)cos wr m() -m(t)
DSB – AM ◼ Amplitude of modulated signal is proportional to the message signal ( ) cos(2 ) c c c t A f t = ( ) ( )cos(2 ) m t( ) u t A m t f t = c c
DSB-AM at frequency domain ■Take FT =A0eo2af1=U-+冬MU+月 ■( Transmission Bandwidth:B ·B=2W DSB-AM M(f) ↑U(f) 2W A AAC/2 -W 0 W -fc fe
DSB-AM at frequency domain ◼ Take FT ◼ ◼ Transmission Bandwidth: BT ◼ BT = 2W ( ) [ ( )cos(2 )] ( ) ( ) 2 2 c c c c c c A A U f F A m t f t M f f M f f = = − + + -W 0 W f M(f) f U(f) fc -fc 2W DSB-AM A AAc2 /2
Power of modulated signal If m(t)is lowpass signal with frequency contents much less than 2f ■R=m7∫do0dh=m7」%m(0cos(2rf0h -tm0+e由 = 0 -号-i0a+ega0 A m M(f)元 2 U(f) Ac/2 A2Pm/2→ -fc fe
Power of modulated signal ◼ If m(t) is lowpass signal with frequency contents much less than 2fc ◼ / 2 / 2 2 2 2 2 / 2 / 2 / 2 2 2 / 2 2 / 2 / 2 2 2 / 2 / 2 2 1 1 lim ( ) lim ( ) cos (2 ) 1 1 cos(4 ) lim ( ) 2 1 1 {lim ( ) lim ( ) cos(4 ) } 2 2 T T u c c T T T T T c c T T T T c c T T T T c m P u t dt A m t f t dt T T f t A m t dt T A m t dt m t f t dt T T A P → → − − → − → → − − = = + = = + = 0 f U(f) fc -fc Ac/2 M(f) Pm Ac 2Pm/2
SNR for DSB-AM Equal to baseband SNR o E U()1 R(f)1 A2Pm/2 Transmit -Distortion -LOSs N(f) White Gaussian Noise N/2 WNo 2W
SNR for DSB-AM ◼ Equal to baseband SNR ◼ 0 0 ( ) S PR N N W = fc U(f) Ac 2Pm/2 Transmit -Distortion -Loss fc R(f) PR N(f) 2W N0 White Gaussian Noise /2 WN0
Demodulation of AM signals Demodulation The process of extracting the message signal from modulated signal Type of demodulation Coherent demodulation Local oscillator with same frequency and phase of the carrier at the receiver ■DSB-AM,SSB-AM Noncoherent demodulaion Envelope detector which does not require same frequency and phase of carrier Easy to implement with low cost:Conventional AM
Demodulation of AM signals ◼ Demodulation ◼ The process of extracting the message signal from modulated signal ◼ Type of demodulation ◼ Coherent demodulation ◼ Local oscillator with same frequency and phase of the carrier at the receiver ◼ DSB – AM , SSB – AM ◼ Noncoherent demodulaion ◼ Envelope detector which does not require same frequency and phase of carrier ◼ Easy to implement with low cost : Conventional AM
DSB-AM demodulation Coherent demodulation )=A,m(t)cos(2πf) Lowpass Filter m(t) 2 cos(2πft) y(t)=A,m(t)cos(2πft)cos(2πft) =子m0+子m0eo4af0 Local oscillator ■How do we generate cos(2πft)? Frequency and phase should be synchronized to incoming signal PLL or FLL
DSB – AM demodulation ◼ Coherent demodulation ◼ Local oscillator ◼ How do we generate ? ◼ Frequency and phase should be synchronized to incoming signal ◼ PLL or FLL cos(2 ) c f t ( ) ( )cos(2 ) u t A m t f t = c c Lowpass Filter ( ) ( ) cos(2 ) cos(2 ) ( ) ( ) cos(4 ) 2 2 c c c c c c y t A m t f t f t A A m t m t f t = = + ( ) 2 Ac m t cos(2 ) c f t