5 DSB-SC-Demodulation.pdf

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1. DSB-SC Demodulation
Prof.Dr.G.Aarthi,AssociateProfessor,VIT

2. Demodulation of DSBSC-AMwave
•Coherent detection/Synchronousdetection
•Costasreceiver
•Squaringloop
TheprocessofextractinganoriginalmessagesignalfromDSBSCwaveisknownas
detectionordemodulationofDSBSC.Thefollowingdemodulators(detectors)areused
fordemodulatingDSBSCwave.

3. CoherentDetection
•In this process, the message signal can be extracted from DSBSC wave by multiplying it
with a local carrier.
•Local oscillator signal is exactly coherent (both frequency and phase) with the carrier
signal used in DSBSC modulationprocess.

4. CoherentDetection
•The resulting signal is then passed through a Low Pass Filter. Output of this
filter is the desired message signal.

5. CoherentDetection
•The local oscillator signal is givenas
V
c
cos(2f
ct)(1)
Where,ϕisthephasedifferencebetweenthelocaloscillatorsignalandthe
carriersignal,whichisusedforDSBSCmodulation.
s(t)V
ccos(2f
ct)m(t)(3)
TheDSB-SCwaveisgivenas

6. CoherentDetection
•The local oscillator signal is givenas
V
c
cos(2f
ct)(1)
•The output of product modulator is givenas

7. CoherentDetection
•The local oscillator signal is givenas
V
c
cos(2f
ct)(1)
•The output of product modulator is givenas
v(t)V
c
cos(2f
ct)s(t)(2)
s(t)V
ccos(2f
ct)m(t)(3)
•Sub Eq.(3) inEq.(2)

8. CoherentDetection
•The local oscillator signal is givenas
V
c
cos(2f
ct)(1)
•The output of product modulator is givenas
v(t)V
c
cos(2f
ct)s(t)(2)
s(t)V
ccos(2f
ct)m(t)(3)
•Sub Eq.(3) inEq.(2)
v(t)V
cV
c
cos(2f
ct)cos(2f
ct)m(t)

9. CoherentDetection
•The local oscillator signal is givenas
V
c
cos(2f
ct)(1)
•The output of product modulator is givenas
v(t)V
c
cos(2f
ct)s(t)(2)
s(t)V
ccos(2f
ct)m(t)(3)
•Sub Eq.(3) inEq.(2)
v(t)V
cV
c
cos(2f
ct)cos(2f
ct)m(t)
2 2
cc c cc
v(t)
1
VVcos(4ft)m(t)
1
VVcos()m(t)(5)
In the above equation, the second term is the scaled version of the message
signal. It can be extracted by passing the above signal through a low pass filter.

10. CoherentDetection
•After passing toLPF,
•The local oscillator signal is givenas
V
c
cos(2f
ct)(1)
•The output of product modulator is givenas
v(t)V
c
cos(2f
ct)s(t)(2)
s(t)V
ccos(2f
ct)m(t)(3)
•Sub Eq.(3) inEq.(2)
v(t)V
cV
c
cos(2f
ct)cos(2f
ct)m(t)
2 2
cc c cc
v(t)
1
VVcos(4ft)m(t)
1
VVcos()m(t)(5)
1
2
0
VVcos()m(t)v(t)
cc
 (6)

11. CoherentDetection
•The amplitude of the demodulated signal is maximum whenФ=0(That’s
why the local oscillator signal and the carrier signal should be in phase, i.e., there
should not be any phase difference between these two signals)
•The amplitude of the demodulated signal is minimum when Ф=+/-π/2
•Zero demodulated signal occurs when Ф=+/-π/2 –The effect is called as
Quadrature Nulleffect
2
0 cc
v(t)
1
VVcos()m(t)(6)

12. CostasLoop
•Costas loop is used to make both the carrier signal (used for
DSBSC modulation) and the locally generated signal in phase.

13. CostasLoop
Costasloopconsistsoftwoproductmodulatorswithcommoninputs(t),whichisDSBSCwave.The
otherinputforbothproductmodulatorsistakenfromVoltageControlledOscillator(VCO)
with−90
o
phaseshifttooneoftheproductmodulatorasshowninfigure.

14. CostasLoop
•Upper path –Inphase coherent detection –I-channel
•Lower path –Quadrature phase coherent detection –Q-channel
•The P.D and VCO is used to correct the phaseerrors

15. CostasLoop
Let the output of VCO be
ThisoutputofVCOisappliedasthecarrierinputoftheupperproductmodulator.
Hence,theoutputoftheupperproductmodulatoris
Substitute,s(t)andc1(t)valuesintheaboveequation.
We know that the equation of DSBSC wave is

16. CostasLoop
Aftersimplifying,wewillgetv1(t)as
This signal is applied as an input of the upper low pass filter. The output of this low pass filter is
Therefore,theoutputofthislowpassfilteristhescaledversionofthemodulatingsignal.
Theoutputof−90
o
phaseshifteris

17. CostasLoop
Thissignalisappliedasthecarrierinputofthelowerproductmodulator.Theoutputofthelower
productmodulatoris
Substitute,s(t)andc2(t)valuesintheaboveequation.
Aftersimplifying,wewillgetv2(t)as
This signal is applied as an input of the lower low pass filter. The output of this low pass filter is
TheoutputofthisLowpassfilterhas−90
o
phasedifferencewiththeoutputoftheupperlowpassfilter.

18. CostasLoop
Theoutputsofthesetwolowpassfiltersareappliedasinputsofthephasediscriminator.
Basedonthephasedifferencebetweenthesetwosignals,thephasediscriminatorproducesa
DCcontrolsignal
ThissignalisappliedasaninputofVCOtocorrectthephaseerrorinVCOoutput.
WhentheP.Doutputiszero,thereisnoneedtocorrecttheL.O.
Therefore,thecarriersignal(usedforDSBSCmodulation)andthelocallygeneratedsignal
(VCOoutput)areinphase.

19. SquaringLoop
•It is used to recover the carrier signal from DSBSC signal (carrierrecovery)
•The recovered carrier signal is used in the coherent detectionprocess

20. SquaringLoop
•The output of the squareris
y(t) s
2
(t)[Acos(2ft)m(t)]
2
c c

21. SquaringLoop
•The output of the squareris
y(t) s
2
(t)[Acos(2ft)m(t)]
2
c c
c
A
2
ccos
2
(2ft)m
2
(t)

22. SquaringLoop
•Theoutputofsquarerisgiventhenarrowbandfilterwhichiscenteredat
+/-4πf
c
•The output of filteris
•The output of the squareris
y(t) s
2
(t)[Acos(2ft)m(t)]
2
c c
c
A
2
c cos
2
(2f t)m
2
(t)
2
2
cft)]m(t)[1cos(4
2

A c

23. SquaringLoop
•Theoutputofsquarerisgiventhenarrowbandfilterwhichiscenteredat
+/-4πf
c
•The output of filteris
•TheoutputoffilterisgiventoPLLtoprovideconstantfrequencysignal
cos(4πf
ct)
•Any drift in frequency is corrected by the error signal e(t) generated byLPF
•The output of VCO is connected to frequency divider(/2) to getcos(2πf
ct)
•The output of the squareris
y(t) s
2
(t)[Acos(2ft)m(t)]
2
c c
c
A
2
c cos
2
(2f t)m
2
(t)
2
2
cft)]m(t)[1cos(4
2

Ac
2
2
c
A
2
c
ft)v(t) m(t)cos(4