dsbsc

DoubleSideBandSuppressedCarrier

1.Drawback of Amplitude Modulation or Why DSBSC

2.Time-domain and frequency domain description Power and bandwidth

3.Generation Methods Balanced modulator, Ring modulator

4.DetectionMethodsCoherent detection, Costas loop

5.Performance Efficiency and Comparisons

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Course objectives for DSBSC

1. To understand the need of double sideband suppressed carrier

2. To understand the power and bandwidth required for the transmission

3. To understand the generation methods of DSBSC

4. To understand the detection methods of DSBSC

5. To understand the performance of DSBSC with AM


Course outcomes of DSBSC

Upon successful completion of DSBSC, the student will be able to:1.Understand the need for DSBSC and the representation in time and frequency

domain

2. Design a balanced and ring modulator to generate DSBSC

3. Design a synchronous detector for detection of original signal.

4. Understand the required power and bandwidth for the transmission of DSBSC


Why DSB SC?Lecture 1 : Understand the concept of DSBSC

The carrier contains no information. So we can think of avoiding or suppressing carrier.

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TIME AND FREQUENCY DOMAIN REPRESENTATION

General expression for standard AM is

)2(cos]1)([)( ccac tftmkAts

Let k1 = 1, and c = 0, the modulated carrier signal, therefore DSBSC wave is

tftmAtS cc 2cos)()(

Information signal m(t) = Am cos 2πfmt for single toneThus

tAA

tAA

tftfAAtS

mcmc

mcmc

cmmc

)(cos2

)(cos2

2cos2cos)(

upper side bandupper side band lower side bandlower side band

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The modulated signal s(t) undergoes a phase reversal whenever the message signal m(t) crosses zero.

This is called double side-band suppressed carrier (DSB-SC) modulation.

).2cos()()( tftmAts ccDSB

1. Transmission bandwidth is same as standard AM.

2. Transmitted power is less than that used by standard AM.

DSB-SC modulation is generated by using a product modulator that simply

multiplies the message signal m(t) by the carrier wave Accos(2fct). Specifically, we write:


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Waveforms

B = 2m

Notice: No carrier frequency

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Signal Representation in time domainLecture 2 : Able to understand the required power &

bandwidth

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Spectrum of DSB SC

When m(t) is limited to the interval -W < f < W.The spectrum S(f) of the DSB-SC wave s(t)


Spectrum Representation


Spectrum of AM DSB SC

Because it doesn’t have components of the carrier, we call this kind of modulation suppressed carrier


Time domain Vs Frequency domain

Time-domain (on the left) and frequency-domain (on the right) characteristics of DSB-SC modulation produced by a sinusoidal modulating wave. (a) Modulating

wave. (b) Carrier wave. (c) DSB-SC

modulated wave. Note that = 2.


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GenerationMethodsLecture 3 : To understand the generation of DSBSC

AMmodulator AMmodulator

CarrierAc cos 2πfct

0.5 m(t)

-0.5 m(t)

++-

DSB-SCAc 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

AMmodulator AMmodulator

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Generation of DSBSC

Ring Modulator

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Waveforms of Ring modulator

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Detection MethodsLecture 4 : To understand the detection of DSBSC

• 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


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


Phase Synchronization - Costa Loop

When there is no phase error. The quadrature component is zero

For small φ, cosφ≈1 and sinφ≈ φ.

The out put of the phase discriminator is proportional to 17Rajani Devi13/08/2014

PerformanceLecture 5 : Able to understand the power& comparison with AM

• Total power in AM and DSBSC


Quadrature-Carrier Multiplexing or Quadrature-Amplitude Modulation (QAM)

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tftmAtftmAtS ccccQAM 2sin2cos)( 21

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The multiplexed signal is applied simultaneously to two separate coherent detectors that are supplied with two local carriers of the same frequency, but differing in phase by -900. To maintain this synchronization, we may use a Costas receiver

Quadrature-Amplitude Modulation (Receiver)Lecture 6 : Understand practical receiver

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Summary

Advantages:

Lower power consumption Disadvantage:

Complex detection Applications:

Analogue TV systems: to transmit color information For transmitting stereo information in FM sound broadcast at VHF

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THANK YOU


dsbsc

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