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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/Loudspeaker7/28/2019 55868568 DSB SC Final Project
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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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