●●● 2.DSB-SC Modulation ●●●● ●●●●@ ●●●● DSB m(t)） L丛 v;(t) v() S Ni N So,N。 IF Baseband "o/2 filter filter (AWGN) DSB modulator Channel and receiver DSB demodulator Double-sideband suppressed carrier (DSB-SC) 双边带抑制载波 A A ,()= oso coS(@IF+@m)t+n(t) √2 So S SSB So= S S 4 S。-S N No N,= N No No 4 17
2. DSB-SC Modulation Double-sideband suppressed carrier (DSB-SC) 双边带抑制载波 cos( ) ( ) 2 cos( ) 2 ( ) t n t A t A v t i IF m IF m i i N S N S 2 0 0 4 2 0 0 i i N N S S 4 4 : 0 0 i i N N S SSB S i i N S N S 0 0 17
●●● ●●●● Conclusion:the DSB-SC demodulator improves the input signal-to-noise ratio by a factor of two? 双边带抑制载波解调器把输入的信噪比改善了两倍 FSSB-2FDSB SSB S-S,/4-S=S ButN-2*2。*空2 N。N,/4 Ni nofu (SSB:N,=2/2 =nfw） DSB:S=5.12=25.-s No N:14 Ni nofu Conclusion:The coherent SSB and DSB-SC demodulators have the same SNR performance. 相干的SSB和DSB-SC解调器有同样的信噪比性能。 18
Conclusion: the DSB-SC demodulator improves the input signal-to-noise ratio by a factor of two. 双边带抑制载波解调器把输入的信噪比改善了两倍 FSSB=2FDSB But Conclusion: The coherent SSB and DSB-SC demodulators have the same SNR performance. 相干的SSB和DSB-SC 解调器有同样的信噪比性能。 18 0 0 2*2 * 2 2 iM M n N f n f 0 SSB 2 0 2 iM M n （ ：N f nf ） 00 0 /2 2 : / 4 i ii i iM SS S S DSB N N N nf 0 0 0 / 4 : / 4 i ii i iM SS S S SSB N N N nf
●●● ●●●● 3.DSB-LC Modulation ●●●● ●●●● ●●●● ●●●● Double-Sideband Large-Carrier(DSB-LC) ● 双边带大载波 Advantage:the carrier signal can be used as a reference signal to phase-look the local oscillator to synchronization with the incoming signal. 载波信号可以用作锁相本振的参考源，从而实现与输入信号的相参。 Disadvantage:the carrier power increases the total input power but does not contain any modulation information. 载波信号增加了总的输入功率，但并不包含任何调制信息。 19
3. DSB-LC Modulation Advantage: the carrier signal can be used as a reference signal to phase-look the local oscillator to synchronization with the incoming signal. 载波信号可以用作锁相本振的参考源，从而实现与输入信号的相参。 Double-Sideband Large-Carrier (DSB-LC) 双边带大载波 Disadvantage: the carrier power increases the total input power but does not contain any modulation information. 载波信号增加了总的输入功率，但并不包含任何调制信息。 19
●●】 a(1)=[A+f(t)]cos@ot ●●●● ●●●●● ●●●0 ●●●● v;(t)=A[1+m(t)]cos@t+n(t) ●●●● ●●●● ●● m(t)=mcoso,t m:modulation index调制指数 SoS,2m2 S m2 N。N,2+m 2 nofu 2+m2 N:=2nofu m=1 implies a reduction in SNR of 4.8dB. 意味着信噪比减小了4.8dB IF m>>1,DSB-LC>DSB-SC DSB-SC:S-25 SSB:So=S N。N, 20
m: modulation index调制指数 IF m>>1, DSB-LC DSB-SC m=1 implies a reduction in SNR of 4.8dB. 意味着信噪比减小了4.8dB 20 0 0 ( ) [ ( )]cos AM t A ft t 0 m AM A A ( ) [1 ( )]cos ( ) ( ) cos i IF m v t A mt t nt mt m t 2 2 0 2 2 0 0 2 2 2 i i i M S Sm S m N N m nf m 0 2 N n i M f 0 0 : 2 i i S S DSB SC N N 0 0 : i i S S SSB N N
●●● 4.Envelope Detection of ●●●● ●●●●● ●●●0 ●●●●0 DSB-LC Modulation ●●●● ●●●● 双边带大载波调制的包络检波 ●● Advantage of DSB-LC:can be detected by using an envelope detector,without a LO and mixer,results in a much simple receiver circuit,such as broadcast AM radio. 它可以用包络检波进行检测，不必采用本振和混频器，因而接收机电 路简单，常用在广播调幅。 noncoherent demodulation非相千解调 v() v,() cos @jF r() Envelope Carrier 21
4. Envelope Detection of DSB-LC Modulation 双边带大载波调制的包络检波 Advantage of DSB-LC: can be detected by using an envelope detector, without a LO and mixer, results in a much simple receiver circuit, such as broadcast AM radio. 它可以用包络检波进行检测，不必采用本振和混频器，因而接收机电 路简单，常用在广播调幅。 noncoherent demodulation非相干解调 21