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