dsb-sc
TRANSCRIPT
1
Double Side Band Suppressed Carrier
Professor Z Ghassemlooy
Electronics and IT DivisionSchool of Engineering
Sheffield Hallam UniversityU.K.
2
Contents
• Theory• Implementation
– Transmitter
– Detector• Synchronous
• Square
• Power analysis• Summary
3
Double Side Band Suppressed Carrier
From AM spectrum:• Carries signal c carries no information m.
• Carries signal consumes a lot of power more than 50%
c - m c c + m
Carrier
USBLSB
Single frequencyQuestion: Why transmit carrier at all?Ans:
Question: Can one suppress the carrier?
Ans.: Yes, just transmit two side bands (i.e DSB-SC)
System complexity at the receiver
But what is the penalty?
4
DSB-SC - Theory
General expression: )(cos])([)( 1 cctCtmktc
Let k1 = 1, C = 0 and c = 0, the modulated carrier signal, therefore:
ttmtc c cos)()( ttmtc c cos)()(
Information signal m(t) = Em cos mt Thus
tME
tME
ttEtc
mcc
mcc
cmm
)(cos2
)(cos2
coscos)(
tME
tME
ttEtc
mcc
mcc
cmm
)(cos2
)(cos2
coscos)(
upper side bandupper side band lower side bandlower side band
5
DSB-SC - Waveforms
B = 2m
Mixer (Multiplier)
Mixer (Multiplier)
Notice: No carrier frequency
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DSB-SC - Implementation
AMmod.AMmod.
CarrierEc cos ct
0.5 m(t)
-0.5 m(t)
++-
DSB-SCEc m(t) cos ct
+
Ec (1+ 0.5 m(t) cos ct
Ec (1- 0.5 m(t) cos ct
• Balanced modulator
• Ring modulator
• Square-law modulator
AMmod.AMmod.
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DSB-SC - Detection
• Synchronous detection
MultiplierMultiplierLow passfilter
Low passfilter Message signal
DSB-SC
Local oscillatorc(t) = cos ct
Local oscillatorc(t) = cos ct
tttmty cc cos]cos)([)(
ttmtm
ttmty
c
c
2cos)()(
]2cos1[)()(
21
21
21
)()( 21 tmtv
Condition:•Local oscillator has the same frequency and phase as that of the carrier signal at the transmitter.
m 2c+m2c-m
Low pass filter
high frequencyinformation
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DSB-SC - Synchronous Detection
• Case 1 - Phase error
MultiplierMultiplierLow passfilter
Low passfilter
Message signalDSB-SC
Local oscillatorc(t) = cos(ct+)
Local oscillatorc(t) = cos(ct+)
Condition:•Local oscillator has the same frequency but different phase compared to carrier signal at the transmitter.
m 2c+m2c-m
Low pass filter
high frequencyinformation
)(cos]cos)([)( tttmty cc
)2(cos)(cos)(
)(cos)()2(cos)()(
21
21
21
21
ttmtm
tmttmty
c
c
cos)()( 21 tmtv
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Phase Synchronisation - Costa Loop
Recoveredsignal
• When there is no phase error. The quadrature component is zero
• When 0, yip(t) decreases, while yqp(t) increases
XX
VCOVCO
DSB-SC
Ec cos (ct+)
LPFLPF
Phasediscriminator
Phasediscriminator
In-phase0.5Ec m(t) cos
Vphase(t)
yip(t)
X.X.0.5Ec m(t) sin
90o
phase shift
LPFLPF
Ec sin (ct+)
Quadrature-phaseyqp(t)
•The out put of the phase discriminator is proportional to
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DSB-SC - Synchronous Detection
• Case 1 - Frequency error
MultiplierMultiplierLow passfilter
Low passfilter
Message signalDSB-SC
Local oscillatorc(t)=Eccos(ct+)Local oscillator
c(t)=Eccos(ct+)
Condition:•Local oscillator has the same phase but different frequency compared to carrier signal at the transmitter.
m 2c+m2c-m
Low pass filter
high frequencyinformation
)(cos]cos)([)( tttmty cc
)2(cos)(cos)(
)(cos)()2(cos)()(
21
21
21
21
ttmtm
tmttmty
c
c
cos)()( 21 tmtv
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DSB-SC - Square Detection
Regenerated carrier
z(t)
Band pass filter
Band pass filter
DSB-SCSi(t) C
Squaring circuit g =x2
Squaring circuit g =x2
g(t)
by 2
by 2
MultiplierMultiplierLow pass filterLow pass filter Message
signal
y(t)
2c
g(t) = Si2(t) = B2 cos2 mt cos2 ct
= B2 (½ + ½ cos 2 mt )(½ + ½ cos 2 ct ) = B2/4 [1 + ½ cos 2(c + m)t + ½ cos 2(c - m)t + cos 2mt + cos 2 ct ]
y(t) = B2/4 cos 2wct z(t) = B2/4 cos wct
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DSB-SC - Power
• The total power (or average power):
R
ME
ME
RP
c
cSCDSBT
4
)(
2
2/2
2
2
R
ME
ME
RP
c
cSCDSBT
4
)(
2
2/2
2
2
• The maximum and peak envelop power
2)(
R
MEP c
SCDSBP
2)(
R
MEP c
SCDSBP
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DSB-SC - Summary
• Advantages:– Lower power consumption
• Disadvantage: - Complex detection
• Applications: - Analogue TV systems: to transmit colour information - For transmitting stereo information in FM sound broadcast at VHF