55868568 DSB SC Final Project

7/28/2019 55868568 DSB SC Final Project

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American University Of Science and Technology

CCE 401

Communication Systems

DSB-SC AM

Prepared By

Christopher A. Mansour

Submitted to:

Dr Roger Achkar

Spring 2009-2010


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I- Objectives:

The objective from this project is to design a double side band suppressed

carrier amplitude modulation (DSB-SC AM) using a very important well

known tool the Simulink in MATLAB ; this project contains

information about AM DSB-SC modulation in many ways and it was

simulated using the simulink and presented to my colleagues.

II-Introduction:

Amplitude modulation (AM) is a technique used in electronic

communication, most commonly for transmitting information via a radio

carrier wave. AM works by varying the strength of the transmitted signal

in relation to the information being sent. For example, changes in the

signal strength can be used to specify the sounds to be reproduced by a

loudspeaker, or the light intensity of television pixels.

In amplitude modulation the message signal m(t) is impressed on the

amplitude of the carrier signal c(t) = Ac cos (2fct). This results in a

sinusoidal signal whose amplitude is function of the message m(t). There

exists many different ways in amplitude modulation but my project will

handle DSB-SC AM.
http://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Carrier_wavehttp://en.wikipedia.org/wiki/Loudspeakerhttp://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Carrier_wavehttp://en.wikipedia.org/wiki/Loudspeaker


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III- DSB-SC AM Modulation:

DSB-SC: stands for Double Side Band Suppressed carrier . This

signal is obtained by multiplying the message m(t) with the carrier

signal m(t). What happens is that the slowly varying message for

example would change to rapidly varying message containing higher

frequency components retaining at the same time the main

characteristics of the original message signal therefore it can be used

to retrieve the message signal at the receiver end.

DEMONSTRATION:

FIGURE REPRESENTING THE MESSAGE SIGNAL


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FIGURE REPRESENTING THE CARRIER SIGNAL

FIGURE REPRESENTING THE AM MODULATOR

FIGURE REPRESENTING THE SIGNAL AFTER MODULATION


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This modulated signal is double side banded since it contains frequency

contents in the frequency band less than fc and in that greater than fc.

Furthermore the bandwidth of the modulated signal is twice that of the

original signal. The other characteristic of the modulated signal is that it

does not contain a carrier component that is all the transmitted power is in

the message signal. This is the main reason for calling this modulation as

suppressed carrier.

IV- DSB-SC AM Demodulation:

The demodulation of the DSB-SC amplitude modulated signal is

achieved with 2 main steps:

1- Multiplying the received signal by locally generated signal

2- Pass the resulted signal through a low pass filter with the same

bandwidth of m(t).


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DEMONSTRATION:

FIGURE REPRESENTING THE RECOVERED SIGNAL COMPARED TO

THE MESSAGE ONE.

Note that here we considered to be zero since we would use in real time a

kind of synchronization known as phase-coherent or what is known as

synchronous demodulator.


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V-Special cases:

1- Case:1

Sometimes the maximum amplitude of the locally generated signal used

to modulate the received signal is different than 1 and in this case we

cannot recover the message signal without the usage of a certain gain

therefore the following example illustrates:

Suppose the signal has the following equation:

Therefore the amplitude is different than one so if we dont use gain the

recovered signal would look as follows:


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So if we apply the gain then we would retrieve the same message:


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The modulation and demodulation circuit would look as follows:


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2- Case 2:

In real life the communication channel across which the message signal

traverses is full of noise especially man made noise so as engineers we

have to examine these noise and their effect on our transmission and on

our message signal.

The following figure illustrates the addition of noise to our channel and

their effect on our message signal:

The message signal that would be obtained is destroyed because of noise,

the following figure illustrates:


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Therefore we as engineers must do the best to avoid this noise and to

reconstruct the original message signal with no errors.

3- Case3:

When is different from zero the amplitude of the desired signal is

reduced by

The proceeding discussion demonstrates the need for a phase-coherent

for recovering the message signal m(t) from the received signal.


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A sinusoid that is phase locked to the phase of the received carrier can be

generated at the receiver in one of two ways.One method is to add acarrier component into the transmitted signal. Thus the signal is DSB but

not suppressed carrier. At the receiver a narrow band filter tuned to fc

filters out the pilot tone signal component and its output is used to

multiply the signal. We would obtain a DC component that should be

subtracted out in order to recover the signal m(t).

What proceeded is known as the Pilot Tone.

VI- Conclusion:

Even if DSB-SC AM modulation is not used nowadays but one cant

neglect its importance and its usage in the past and how human could

have ever thought about finding a way in order to communicate with

others in from far distances using the AM modulation.

One can benefit a lot from this project because in this project one can

learn about AM modulation especially DSB-SC and how everybody can

create a design for this modulation. One can learn how do noise affects

this kind of transmission and how to reconstruct or recover the message

signal being transmitted.

One can learn from this project how to use a very important tool in

engineering domain which is the MATLAB and the Simulink.


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Documents

55868568 DSB SC Final Project