Chapter 4e_pulse Txn

8/3/2019 Chapter 4e_pulse Txn

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CHAPTER 4: PULSE TRANSMISSION


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Chapter 3/Pulse Txn Prepared By: Pn.Norliza Mohamed 2

Each analog modulation method leads to its corresponding binarymethod.

The basebands involved are all digital and the simplest of these is

the binary digit which will be used to develop the basic theory ofthe methods.

There are three basic methods, each corresponding to one of thethree analog modulation methods. They are: Amplitude Shift Keying (ASK)

Frequency Shift Keying (FSK)

Phase Shift Keying (PSK)

Nowadays, keying methods are used in many different areas andthe theoretical requirements differ in some respects between them.

INTRODUCTION


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ASK is the simplest and the oldest method but it is the main method for opticalfiber transmission.

In ASK, the strength of the carrier signal is varied to represent binary 1 or 0.Both frequency and phase remain constant while the amplitude changes.

The amplitude of a sinusoidal carrier is shifted from 0 volts for a logic 0 to somefixed value, V for a logic 1.

This method gives 100% amplitude modulation and often called on-off keying(OOK)

OOK is one of the value that represented by no voltage. The advantage of thismethod is a reduction in the amount of energy required to transmit

information.The main disadvantage of ASK is easily corrupted by unwanted noise. Theterm noise refers to unintentional voltages introduced onto a line by variousphenomena such as heat or electromagnetic induction by other sources.

AMPLITUDE SHIFT KEYING (ASK)


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ASK can be done by multiplying the sinusoidal carrier by the digital

baseband signal as shown below.

ASK GENERATION

1 0 1 1 1 0 0 1 0 1 1

Carrier signal

Baseband signal

X

ASK signal

=


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ASK EXPRESSION

ASK can be expressed as:

The Fourier series of the baseband signal is given by:

signalcarrierthe-cos

signalbasebandthe-)(

Where,

cos)()(

tE

tv

tEtvtv

cc

m

ccmASK

[

[y!

.....

)5cos513cos

31(cos2

21)( 000 ! ttttvm [[[

T


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The ASK equation is:

ASK EXPRESSION

......)3cos(3

)3cos(3

)cos()cos(cos2

.....3coscos3

2coscos

2cos

2

...

5cos513cos

31(cos2

21cos

)(cos)(

00

00

00

000

!

!

-

y!

y!

tE

tE

tE

tE

tE

ttE

ttE

tE

ttttE

tvtEtv

cc

cc

cc

cc

cc

cc

cc

cc

cc

mccASK

[[T

[[T

[[T[[T[

[[T

[[T

[

[[[T

[

[


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Frequency spectrum of ASK can be shown as follow:

FREQUENCY SPECTRUM OF ASK

f0 3f0 5f0 f

Baseband signal|V|

2/T

2/3T

2/5T

fc f

Carrier signal|V|

Ec/2

Ec /T Ec /TEc/3T Ec/3T

3 x bit rate

|V|

f

ASK signal

Ec


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Bandwidth of ASK is depends on the requirements and the

specifications required.

The BW is given by:

Example:

Obtain the bandwidth of ASK if the required signal to be sent is up

to the 3rd harmonics.

BANDWIDTH OF ASK

AMm

ASK

BWf

BW

!!

!

2

frequencymodulatingx2

0

0

0

00

222

11

ratebitx336)3(22

fff

TTf

ffffBW

bb

b

bmASK

!@!!!

!!!!!


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In FSK, the frequency of the carrier signal is varied to representbinary 0 or 1.

FSK avoids most of the noise problem of ASK. Because the

receiving device is looking for specific changes over a givennumber of periods, it can ignore voltage spikes.

There are two main areas of FSK used:

Audio frequency multiplexing on to 4kHz telephone channels(voice frequency) for teletype or data.

Radio teletype (RTTY) transmission at HF and VHF.FSK is also being used in computer modem.

FREQUENCY SHIFT KEYING (FSK)


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ASK can be obtained by the sum of two ASK signals as shown below.

FSK GENERATION

Carrier signal 1 Carrier signal 2

Baseband zeros

signal

Baseband ones

signal

X X

ASK signal

1

ASK signal

2

= =

+

=

FSK signal


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FSK can be expressed as:

Usually fc1 < fc2 and E1 = E2.

FSK EXPRESSION

)()()()()( 2211 tvtvtvtvtv cmcmFSK yy!

tEtv

tEtv

tvtv

ttttv

c

c

mm

m

222

111

12

0001

cos)(

cos)(

)(1)(

.....)5cos5

13cos

3

1(cos

2

2

1)(

Where,

[

[

[[[T

!

!

!

!


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FSK EXPRESSION

? A ? A

.....3cos3

3cos3

coscoscos2

.....3cos

3

3cos

3

coscoscos2

...5cos5

13cos

3

1cos

2

2

1cos

...5cos5

13cos

3

1cos

2

2

1cos

)(cos)(cos)(

22

22

22

22

22

11

11

11

11

11

22

11

222111

!

-

y

-

y!

yy!

tE

tE

tE

tE

tE

tE

tE

tE

tE

tE

ttttE

ttttE

tvtEtvtEtv

oo

oo

oo

oo

ooo

ooo

mmFSK

[[T

[[T

[[T

[[T

[

[[

T

[[

T

[[T

[[T

[

[[[T

[

[[[T

[

[[


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FREQUENCY SPECTRUM OF FSK

Ec/2


fc1

|V|

f

ASK signal 1

fc1 f

Carrier signal 1|V|

Ec

f0 3f0 5f0 f

Baseband signal|V|

2/T

2/3T

2/5T


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Ec/2


fc2

|V|

f

ASK signal 2

fc2 f

Carrier signal 2|V|

Ec

f0 3f0 5f0 f

Baseband signal|V|

2/T

2/3T

2/5T


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Ec/2

Ec/T Ec/T

Ec/3T

Ec/3T

fc2

|V|

f

ASK signal 2

Ec/2

Ec/T Ec/T

Ec/3T

Ec/3T

fc1

|V|

f

ASK signal 1

+

Ec/2

Ec/T Ec /TEc/3T Ec/3T

fc1

|V| E c/2

Ec /T Ec/TEc/3T Ec /3T

fc2f

=

FSK signal

Deviation, 2(f


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Bandwidth of FSK is given by:

Example:

Obtain the bandwidth of FSK if the required signal to be sent is up

to the 3rd harmonics.

BANDWIDTH OF FSK

m

FSK

ff

BW

(!

!

2

frequency)modulatingdeviation(frequencyx2

bb

mFSK

fff

f

ffffffBW

322

62

62)3(2)(2

0

0

(!(!

(!

(!(!


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The phase of the carrier is varied to represent binary 0 or 1.

Both peak amplitude and frequency amplitude remain constant asthe phase changes.

Advantages: Less susceptible to noise degradation

Less bandwidth than FSK (same as ASK)

Bandwidth reduced by multilevel scheme.

Applications very desirable method because of high bit rate and

high carrier frequency which it becomes the only method used fordigital transmission on microwave radio.

PHASE SHIFT KEYING (PSK)


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PSK can be done by multiplying the sinusoidal carrier by the

bipolar baseband signal in order the phase changing happen as

shown below.

PSK GENERATION

Carrier signal

Bipolar baseband

signal

PSK signal

x

=


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The PSK equation can be expressed as:

PSK EXPRESSION

?

!

-

!

-

!

......)3cos(3

1)3cos(

3

1

)cos()cos(2

.....5coscos5

13coscos

3

1coscos

4

...5cos

5

13cos

3

1(cos

4cos)(

00

00

000

000

tt

ttE

ttttttE

ttttEtv

cc

ccc

cccc

ccPSK

[[[[

[[[[T

[[[[[[T

[[[

T

[


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FREQUENCY SPECTRUM OF PSK

From the equation of PSK, it can be shown that the spectrum of

PSK is more or less like ASK spectrum but without the carrier.

It can be shown as follow:

2Ec /T 2Ec/T

2Ec/3T 2Ec /3T

fc3f0 fcf0 fc fc +f0 fc +3f0

|V|

f


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BANDWIDTH OF PSK

Bandwidth of PSK is given by:

Example:

Obtain the bandwidth of PSK if the required signal to be sent is up

to the 5th harmonics.

ASKm

PSK

BWf

BW

!!

!

2

frequencymodulatingx2

0

00

2ratebitx5510)5(22

ffffffBW

b

bmASK

!!!!!!


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Multiplexing is a process of combining two or more informationsignals and sending them simultaneously down the sametransmission system or communication channel.

The method used is depending on both the type of signal and thetransmission medium being used.

Two main methods are: Frequency Division Multiplexing (FDM) usually deal with analog

information.

Time Division Multiplexing (TDM) usually deal with digital information intime domain.

Advantages of multiplexing: The communication system more efficient and reliable.

Can save cost and bandwidth.

MULTIPLEXING


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Using modulated carriers, several carriers can be used at differentfrequencies.

Frequency Division Multiplexing is the process when the available

frequency range is divided among the signals.Individual signals are assigned a different frequency within acommon bandwidth.

Radio and television signals are examples of FDM.

There are limitations to the number of signals that can be crowdedinto a given frequency range Each signal requires a certain bandwidth

At the receiving end, the signals are then separated out again byfiltering

FREQUENCY DIVISION MULTIPLEXING (FDM)


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FDM EXAMPLE

If a medium between a transmitter and a receiver has a bandwidth of

at least 5 times from a bandwidth of an information signal, thus the

medium can be multiplexed for carrying 5 information signals at a

time.The FDM spectrum can be drawn as follow:

vm1(t) vm2(t) vm3(t) vm4(t) vm5(t)

v

f(Hz)

BW of the information signal

BW of the medium

Each signal will be assigned

at different carrier frequency.

They must not overlapping to

avoid crosstalk.


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FDM EXAMPLE

4 channels with 100 kHz of bandwidth are to be

multiplexed together in a link. Calculate the

minimum bandwidth of the link if the guard band of10 kHz is required to prevent interference. Draw the

FDM spectrum.


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This method is suitable when a signal is in the form of a pulse train

(as in PCM).

The pulses are made narrower and the spaces that are left between

pulses are used for pulses from another signals.

The transmission time is shared by a number of signals by

interleaving the pulse trains of various signals in a specified order

where each signal must be sampled at a Nyquist rate.

At the receiver, the pulse trains corresponding to various signals areseparated.

Digital telephone systems use TDM.

TIME DIVISION MULTIPLEXING (TDM)


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TDM EXAMPLE

Three signals have to be multiplexed together using TDM. The three

signals have the same of bandwidth, B. Draw the diagram that

showing the transmission of these signals if they are represented by

S1, S2 and S3.

S1 S1 S1 S1S3 S3 S3 S3

S2 S2 S2 S2

.. tTs = sampling time Tn = Ts/n = sampling interval

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Chapter 4e_pulse Txn