AM (Amplitude Modulation)

8/11/2019 AM (Amplitude Modulation)

http://slidepdf.com/reader/full/am-amplitude-modulation 1/24

AMPLITUDE MODULATION





Two major Disadvantages/Drawbacks

of DSBFC

◦ Large power consumption, where carrier power constitutes >2/3transmitted power.{remember:carrier does not contain anyinformation}

◦ Large bandwidth utilized.

Thus, DSBFC is both power and bandwidth inefficient



Single Side Band Full Carrier (SSB-

FC)Frequency spectrum:

Bandwidth=fm(max)

Total Power=Pcarrier +Pusb f cf c+f m

f c-f m





Single Side band SuppressCarrier (SSB-SC)

Frequency spectrum:

Bandwidth=fm(max)

Total Power=+Pusb f cf c+f m

f c-f m



Cont’d…

◦ A form of amplitude modulation in which the carrier is totallysuppressed and one of the sidebands removed.

◦ Therefore, SSBSC requires half as much bandwidth as

conventional DSB AM and considerably less transmitted power



Comparison of time domain representation ofthree common AM transmission systems:

Tomasi



Example 1

For an AM DSCFC wave with a peak unmodulatedcarrier voltage Vc = 10 Vp,frequency of 100kHz, aload resistor of RL = 10 , frequency of modulating

signal of 10kHz and m = 1, determine the following

i) Powers of the carrier and the upper and lower sidebands.

ii) Total power of the modulated wave.

iii) Bandwidth of the transmitted wave.

iv) Draw the power and frequency spectrum.



Example 1..cont’d

◦ For the same given values, determine questions (ii)-(iv) for a AMDSB-SC, AM SSB-FC and AM SSB-SC systems. Determine also thepercentage of power saved in each of the system design.



Example 1..cont’d Solution for DSBFC;i)

ii)

iii) Bandwidth=2xfmmax=2(10kHz)=20kHz

W

P m

P m

P P ccct

5.7)5(4

1)5(

4

15

4422

22

W

P m

P P

W R

V

R

V P

c

lsbu sb

cc

c

25.14

5102

)10(

2

)2/(

2

222



Example 1..cont’d

◦

Solution:For DSB-SCii)

iii)Bandwidth=2xfmmax=2(10kz)=20kHz

iv)

%67.66

%1005.7

5%

5

5.25.7

xW

W Power

W

W W Power

sa ved

sa ved

W

P m

P m

P cct

5.2)5(

4

1)5(

4

1

4422

22

110kHz

90kHz





Example 1..cont’d

◦ Solution:For SSB-sC

ii)

iii)Bandwidth=fmmax=10kHz

iv)

%33.83

%1005.7

25.6%

25.6

25.15.7

xW

W Power

W

W W Power

sa ved

sa ved

W

P m

P ct

25.1)5(4

1

42

2

f c

110kHz f c-f m

M th d f G ti SSB



Methods of Generating SSB

i) Filtering method

◦ A filter removes the undesiredsideband producing SSB.

◦ Balanced modulators is used to

suppress the unwanted carrier andfilters to suppress the unwantedsidebands

◦ Quartz crystal filters are the most

widely used sideband filters since theyare very selective and inexpensive.



Block diagram of filtering

method

Filterresponse

curve

Sidebandfilter

Balanced

modulator

Carrier

oscillator

Microphone Audio

amplifier

Linear

amplifier

Antenna

Upper

sidebands

DSB

signal

SSB

signal

Lower

sidebands





Block diagram of phasing

methodBalanced

Modulator 1

Balanced

Modulator 2

Phase shifter

Phase shifter

+

Information signal

Carrier signal

Output Signal, aot

Am cos wmt

Am cos (wmt + 90)

Ac cos (wct + 90)

A2(t)

A1(t)



Mathematical analysis of

phasing method

)2()90cos()90cos(21

cos*)90cos()(

)1()()()(

00

0

1

210

t wt w A At wt w A A

t w At w At a

t at at a

mcmcmcmc

mmcc

)3()90cos()90cos(21

)90cos(*)cos()(

00

0

2

t wt w A At wt w A A

t w At w At a

mcmcmcmc

mmcc

)90cos(

)3()2()(0

0

t wt w A A

t a

mcmc



VESTIGIAL SIDEBAND (VSB)

Also called asymmetric sideband system.

Compromise between DSB & SSB.

Easy to generate.

Bandwidth is only ~ 25% greater than SSBsignals.

Derived by filtering DSB, one pass band ispassed almost completely while just a traceor vestige of the other sideband is included.



Cont’d

AM wave is applied to a vestigial sideband filter, producing amodulation scheme – VSB + C

Mainly used for television video transmission.



Cont’d…vsb

VSB Frequency Spectrum

fc fc

LSB MSB

Carrier

VSB



Advantages/Benefits of SSB

◦ Power consumption

◦ Bandwidth conservation

◦ Selective fading

◦ Noise reduction



Disadvantages of SSB

◦ Complex receivers

◦ Tuning difficulties

Documents

AM (Amplitude Modulation)