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Major Disadvantages of DSBFC
Power is wasted in the transmitted signal. Most
of the transmitted power is in the carrier, which
does not contribute to the transmitted
intelligence. Remember that the carrier
contains no intelligence.
The transmitted signal requires twice the
bandwidth of the transmitted intelligence.
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SIDEBAND SYSTEMS
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Full-carrier AM is simple but not efficient
Removing the carrier before power amplification allows
full transmitter power to be applied to the sidebands
Removing the carrier from a fully modulated AM systems
results in a double-sideband suppressed-carrier
transmission
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Suppressed-Carrier AM
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Suppressed-Carrier Signal
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Eliminating the Carrier
Suppressed-carrier modulation
Carrier suppression can be accomplished using the
balanced modulator.
Balanced Modulator Circuit
A non-linear device that generates the sum and
difference of the input frequencies Also known as product modulators
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SIDEBAND SYSTEMS
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The two sidebands of an AM signal are mirror images of
one another
As a result, one of the sidebands is redundant
Using single-sideband suppressed-carrier transmissionresults in reduced bandwidth and therefore twice as many
signals may be transmitted in the same spectrumallotment
Typically, a 3dB improvement in signal-to-noise ratio isachieved as a result of SSBSC
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DSB and SSB AM
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DSBSC and SSB
Transmission
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SSB Signals
SSB signals offer four major benefits:
1. Spectrum space is conserved and allows more signals
to be transmitted in the same frequency range.2. All power is channeled into a single sideband. This
produces a stronger signal that will carry farther andwill be more reliably received at greater distances.
3. Occupied bandwidth space is narrower and noise in
the signal is reduced.
4. There is less selective fading over long distances.
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3-5: Single-Sideband Modulation
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Disadvantages of DSB and SSB
Single and double-sideband are not widely used
because the signals are difficult to recover (i.e.
demodulate) at the receiver. A low power, pilot carrieris sometimes
transmitted along with sidebands in order to more
easily recover the signal at the receiver.
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3-5: Single-Sideband Modulation
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Carrier power is useless as a measure of power in a DSBSC
or SSBSC signal Instead, the peak envelopepoweris used
The peak power envelope is simply the power at
modulation peaks, calculated thus:
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Power in Suppressed-Carrier Signals
RL
VPEP
p
2
2
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Single-Sideband Full Carrier (SSBFC)
Single-Sideband Suppressed Carrier (SSBSC)
Single-Sideband Reduced Carrier (SSBRC)
Independent Sideband (ISB)
Vestigial Sideband (VSB)
TYPES OF AM SSB SYSTEM
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Form of amplitude modulation in which a
single carrier frequency is independently
modulated by two different modulating
signals.
It is a type of hybrid between double sideband
(DSB) and single sideband (SSB)
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INDEPENDENT SIDEBAND
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Form of amplitude modulation in which
the carrier and one complete sideband
are transmitted, but only part of thesecond sideband is transmitted.
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VESTIGIAL SIDEBAND
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Signal Power Considerations
In SSB, the transmitter output is expressed in terms
of peak envelope power (PEP), the maximum
power produced on voice amplitude peaks.
Applications of DSB and SSB
A vestigial sideband signal (VSB)is produced bypartially suppressing the lower sideband. This kind
of signal is used in TV transmission.
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3-5: Single-Sideband Modulation
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Audio carrier
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Fig. Vestigial sideband transmission of a TV picture
signal
Total TV signal bandwidth = 6 MHz
fc0.75 MHz fc fc+ 4.2 MHz
Upper video bands
Picture carrier
fc+ 4.5 MHz
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The International Telecommunications Union
(ITU),a standards organization, uses a code to
describe signals.
Examples are:
A3F amplitude-modulated analog TV
J3E SSB voice
F2D FSK data
G7E phase-modulated voice, multiple
signals
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3-6: Classification of
Radio Emissions
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A code is used to designate the types of
signals that can be transmitted by radio and
wire.
The code is made up of a capital letter and a
number.
Lowercase subscript letters are used for more
specific definition.
Examples of codes:
DSB two sidebands, full carrier = A3
DSB two sidebands, suppressed carrier = A3bEngr. Alma Christine C. Danzalan 17
3-6: Classification of
Radio Emissions
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3-6: Classification of Radio Emissions
Figure 3-19: Radio emission code designations.
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A typical SSB AM transmitter block diagram is illustratedbelow:
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Single-Sideband AM Transmitters
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Discuss the ff. topics: Engineering lettering
1.Methods of generating SSB
1. Filter method
2. Phase shift method
3. Weaver method
2. Difference of Low level modulation and high level modulation
3. Low level modulator
Diode modulator
Transistor modulator4. High level modulator
Collector modulator
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Assignment: