CS_lec5.ppt

Communication Systems
Week-6
Lecture -5

2
Types of Amplitude Modulation
1.Single Sideband Modulation
2.Vestigial side band Modulation

3
Amplitude Modulation (Single Sideband SSB)
• The DSB spectrum has two sidebands: USB and LSB
• Both USB and LSB contain complete information of the baseband
signal.
• A scheme in which only one sideband is transmitted is known as
single-sideband ( SSB) transmission.
• In SSB transmission the required bandwidth is half compared to DSB
signal.
• An SSB signal can be coherently (synchronously) demodulated. E.g.
For example multiplying USB signal by shifts its spectrum to
the left and right by wc
t
wc
cos

5
Single Sideband SSB (cont..)
Low pass filtering will give the required baseband signal at the
receiver.

12
Single Sideband SSB (cont..)
Time domain representation of SSB signals:
t
w
t
m
t
w
t
m
t c
h
c
SSB sin
)
(
cos
)
(
)
( 


Where minus sign applies to USB and the plus sign applies to LSB
Hilbert Transform of m(t)
and delays the phase of each component by
)
(t
mh
2


13
Example 4.7 p-174
Tone Modulation:
Find for a simple case of tone modulation, that is, when the
modulating signal is a sinusoid
)
(t
SSB

t
w
t
m m
cos
)
( 
Solution:
t
w
t
m
t
w
t
m
t c
h
c
SSB sin
)
(
cos
)
(
)
( 



16
Generation of SSB Signals
Two methods are generally used to generate SSB signals.
1) Sharp cutoff filters
2) Phase shifting networks
Selective Filtering Method:
• In this method the DSB-SC signal is passed through a sharp cutoff
filter to eliminate the undesired sideband.
• To obtain USB , the filter should pass all components above wc,
attenuated and completely suppress all components below wc
• Such an operation requires an ideal filter that is practically not
possible.

17
Generation of SSB Signals
• This method of generating SSB signal can be used when there is
some separation between the pass band and stop band.
• In some application this can be achieved e.g. voice signals
Voice signals spectrum shows little power content at the
origin. Thus filtering the unwanted sideband is relatively
easy.
Tests have shown that frequency components
below 300Hz are not important.
600Hz transition region around the cutoff
frequency wc , makes filtering easy and
minimize the channel interference

22
Demodulation of SSB Signals

Envelope Detection of SSB Signals with
a Carrier(SSC+C)
23

Telephone Channel Multiplexing
24
Almost all long-haul telephone channels were multiplexed
by FDM using SSB signals.

SSB vs. AM
• Since the carrier is not transmitted, there is a reduction by
67%of the transmitted power.-In AM @100% modulation:
2/3 of the power is comprised of the carrier; with the
remaining (1/3) power in both sidebands.
• Because in SSB, only one sideband is transmitted, there is
a further reduction by 50% in transmitted power
• Finally, because only one sideband is received, the
receiver's needed bandwidth is reduced by one half--thus
effectively reducing the required power by the transmitter
another 50%
• Relative expensive receiver
26

Vestigial Sideband
• A compromise between the DSB-SC modulation
and the SSB modulation is known as Vestigial
Side Band (VSB) modulation.
• This type of modulation is generated using a
similar system as that of the selective–filtering
system for SSB modulation.
28

Problems
• 4.2-1,4.2-4,4.2-5,4.2-8,4.3-1,4.3-2,4.5-8
31

Quiz#02
Q.1 Prove that
Q.2 Show the amplitude modulation and
demodulation of signal given below in time domain
as well as in frequency domain . [5]
32
dt
t
x
t
g
E
c
t
t
x


2
1
)
(
)
(
1 [5]
Q.3 Show the demodulation of Am signal by
means of envelop detector method. [5]

Lab-#1 Assignment
• Write down a Matlab code for the
demodulation of AM signal.[5]
33

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