DSB-SC & SSB
DSB-SC MODULATION
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A DSB-SC signal is an AM signal of the form:
S(t)=Ac m(t) coswct
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Normal AM signal :
S(t)=Ac[1+um(t)] coswct
=Ac coswct +Ac u m(t) coswct
FREQUENCY SPECTRUM OF DSB-SC
SIGNAL
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Let M(f) be the Fourier transform of m(t), then
the fourier transform of the DSB-SC is:
S(f)= Ac/2 M(f-fc)+Ac/2 M(f+fc)
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Notes:
No impulses are present at ±fc; no carrier
transmitted.
The transmission bandwidth is 2W (same as
normal AM).
Power efficiency= (Power in the sideband/ total
transmitted power) * 100% = 100%
SSB MODULATION
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Here only one of the two sidebands of a DSB-SC signal
is retained while the other sideband is suppressed.
This means that the bandwidth requirement is half
that for DSB-SC.
This saving in B.W comes at the expense of increasing
modulation and demodulation complexity.
In SSB modulation we eliminate the carrier and one
sideband, thus less transmitted power is needed, and
approximately 83% power efficiency is achieved.
GENERATION OF SSB SIGNAL
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I.
II.
o
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A SSB signal can be generated either by:
Filtering Method.
Phase discrimination method.
Filtering Method:
The SSB signal is obtained by selecting either the
upper- side band(USB) or the lower-Sideband (LSB)
of the DSB-SCby means of a suitable band pass
filter.
A band pass filter with appropriate BW and center
frequency is used to pass the desired side band only.
Phase Discrimination method:
 This method is based on the time representation
of the SSB signal:
S(t)=Acm(t)cos wct ±Ac m*(t) sin wct
(-) : upper side band is retained.
m*(t): the hilbert transform of m(t).
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•
Notes:
TP5 represents (DSB-SC)Q
quadrature component:
which
is
the
(DSB-SC)Q = cos2π(fc-fm)t - cos2π(fc+fm)t
•
TP6 represents the (DSB SC)I which is the in
phase componen:
(DSB-SC)I = cos2π(fc-fm)t+ cos2π(fc+fm)t
•
To generate the SSB signal, an adder is used to
obtain the desired side band:
- Lower side-band:
LSSB= (DSB-SC)I+ (DSB-SC)Q= 2cos2π(fc-fm)t
- Upper side-bad:
USSB= (DSB-SC)I- (DSB-SC)Q= 2cos2π(fc+fm)t
NORMAL AM, DSB-SC, AND SSB
SIGNALS:
DSB-SC AND SSB DEMODULATION
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In this experiment , a coherent detector is used to
recover the message signal.
S(t) is multiplied by a locally generated signal at
the receive which has the same frequency and
phase as c(t) at the transmitter, then it passes
through a low-pass filter.
EXAMPLE ON DSB-SC DEMODULATION:
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Consider s(t) represents a DSB-SC signal, after
the multiplication process the signal will be:
V(t)= s(t) Ac* cos wct
= Ac Ac*/2 m(t)+ Ac Ac*/2 m(t) cos 2wct
The high frequency term can be eliminated using
a low pass filter.