Principles of Communications 2.1

8/11/2019 Principles of Communications 2.1

1/20

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.

Engr. Alma Christine C. Danzalan 1

SIDEBAND SYSTEMS


2/20

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


Suppressed-Carrier AM


3/20


Suppressed-Carrier Signal


4/20

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


SIDEBAND SYSTEMS


5/20

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


DSB and SSB AM


6/20


DSBSC and SSB

Transmission


7/20

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.


3-5: Single-Sideband Modulation


8/20

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.




9/20

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:


Power in Suppressed-Carrier Signals

RL

VPEP

p

2

2


10/20

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



11/20

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)


INDEPENDENT SIDEBAND


12/20



13/20

Form of amplitude modulation in which

the carrier and one complete sideband

are transmitted, but only part of thesecond sideband is transmitted.


VESTIGIAL SIDEBAND


14/20

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.




15/20

Audio carrier


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


16/20

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


3-6: Classification of

Radio Emissions


17/20

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


18/20


3-6: Classification of Radio Emissions

Figure 3-19: Radio emission code designations.


19/20

A typical SSB AM transmitter block diagram is illustratedbelow:


Single-Sideband AM Transmitters


20/20

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


Assignment:

Date post:	02-Jun-2018
Category:	Documents
Upload:	argie-omaga-huinda
View:	233 times
Download:	0 times

Principles of Communications 2.1

Documents