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Analog Communications. Amplitude Modulation. Double Side Band Suppressed Carrier. Contents. Theory Implementation Transmitter Detector Synchronous Square Power analysis Summary. Carrier. USB. LSB. c - m c c + m. Single frequency. - PowerPoint PPT Presentation
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April 19, 2023
T.Srinivasa Rao
Analog Communications
Amplitude Modulation
April 19, 2023
T.Srinivasa Rao
Double Side Band Suppressed Carrier
April 19, 2023
T.Srinivasa Rao
Contents
• Theory• Implementation
– Transmitter
– Detector• Synchronous
• Square
• Power analysis• Summary
April 19, 2023
T.Srinivasa Rao
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?
April 19, 2023
T.Srinivasa Rao
DSB-SC - Theory
General expression: )2(cos])([)( ccac tfCtmkAts
Let k1 = 1, C = 0 and c = 0, the modulated carrier signal, therefore:
tftmAtS cc 2cos)()( tftmAtS cc 2cos)()(
Information signal m(t) = Am cos 2πfmt Thus
tAA
tAA
tftfAAtS
mcmc
mcmc
cmmc
)(cos2
)(cos2
2cos2cos)(
tAA
tAA
tftfAAtS
mcmc
mcmc
cmmc
)(cos2
)(cos2
2cos2cos)(
upper side bandupper side band lower side bandlower side band
April 19, 2023
T.Srinivasa Rao
DSB-SC - Waveforms
B = 2m
Mixer (Multiplier)
Mixer (Multiplier)
Notice: No carrier frequency
April 19, 2023
T.Srinivasa Rao
DSB-SC - Implementation
AMmodulator AMmodulator
CarrierAc cos 2πfct
0.5 m(t)
-0.5 m(t)
++-
DSB-SC
Ac m(t) cos 2 π fct
+
Ac (1+ 0.5 m(t) cos 2 π fct
Ac (1- 0.5 m(t) cos 2 π fct
• Balanced modulator
• Ring modulator
• Square-law modulator
AMmodulator AMmodulator
April 19, 2023
T.Srinivasa Rao
DSB-SC - Detection
• Synchronous detection
MultiplierMultiplierLow passfilter
Low passfilter Message signal
DSB-SC
Local oscillatorc(t) = cos 2πfctLocal oscillatorc(t) = cos 2πfct
tttmty cc f2cos]f2cos)([)(
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
April 19, 2023
T.Srinivasa Rao
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
April 19, 2023
T.Srinivasa Rao
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
April 19, 2023
T.Srinivasa Rao
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
April 19, 2023
T.Srinivasa Rao
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
April 19, 2023
T.Srinivasa Rao
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
April 19, 2023
T.Srinivasa Rao
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