anlog communication lecture 05

8/14/2019 ANLOG COMMUNICATION Lecture 05

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Modulation Schemes

Lecture 05

EEE 352 Analog Communication Systems

Mansoor KhanEE Dept.

CIIT Islamabad Campus


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Communication Systems

Baseband Communication Systems

Baseband: band of frequencies of the signal delivered by a transducer.

Eg voice signals 0 to 3.5 kHz, TV signal occupies band from 0-4.5MHz

Short haul communication links, eg Local telephone communication,PCM(between two exchanges) etc

Carrier Communication Systems

Uses modulation to shift frequency spectra of baseband signals over

multiplexed communication channel.

Long haul effective communication


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Modulation

A process of varying parameters of one waveform by another.

In communication systems, a message signal (baseband signal),which contains the information is used to control or vary theparameters of a carrier signal, so as to impress the information ontothe carrier.

A device that performs modulation is known as modulator and adevice which performs reverse operation i.e. extracting themodulating or baseband from modulated carrier is termed asdemodulator.


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Analog and Digital Modulation

The Messages

The message or modulating signal may be either:

analog denoted by m(t)

digital denoted by d(t) i.e. sequences of 1's and 0's

The Carrier

The carrier could be a 'sine wave' or a 'pulse train'. Frequency of carrier is much higher thanthat of modulating baseband signal.

Analog Modulation: transferring analog baseband m(t) signal over a high frequency carrier.Modulation is applied continuously in response to baseband signal.

Digital Modulation: analog carrier is modulated by digital bitstream d(t) analog carrier ismodulated from finite ary of symbols.


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Change parameters of a carrier

Controlled Parameters: Ac(t)fc(t)(t)

Ac(t) : amplitude modulation AM ASK

fc(t) : frequency modulation FM FSK(t) : phase modulation PM PSK

mod cos 2 cc cv t tfA

Modulation principle

DigitalAnalog


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Amplitude Modulation Illustration


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Frequency Modulation (FM)

In FM, the amplitude of the carrier remains fixed

In FM, frequency changes according to the signal: when the signal is stronger, the carrier frequency increases slightly,

and when the signal is weaker, the carrier frequency decreases slightly

Figure (next slide) illustrates an example of FM for an info

signal

FM is more difficult to visualize because slight changes in frequency are not as clearly visible

However, one can notice that the modulated wave has higher frequencieswhen the signal used for modulation is stronger


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Frequency Modulation Illustration


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Modulation, Digital Input, And Shift Keying

How can digital input be used in modulation?

Modifications to the modulation schemes described aboveare needed: instead of modulation that is proportional to a continuous signal, digital

schemes use discrete values

To distinguish between analog and digital modulation we use the term shift keying rather than modulation

Shift keying operates similar to analog modulation Instead of a continuum of possible values, digital shift keying has a fixed set

For example, AM allows the amplitude of a carrier to vary by arbitrarily small

amounts in response to a change in the signal In contrast, amplitude shift keying uses a fixed set of possible amplitude


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Figure Illustration ofa carrier wave

a digital input signalamplitude shiftkeyingfrequency shiftkeying


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Amplitude Shift Keying (ASK)

Amplitude shift keying (ASK): is a form of modulation which

represents digital data as variations in the amplitude of a carrier

wave. Two different amplitudes of carrier frequency represent '0'

, '1'.


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Frequency Shift Keying (FSK)

In Frequency Shift Keying, the change in frequency define

different digits. Two different frequencies near carrier frequency

represent '0' ,''1'.


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Phase Shift Keying (PSK)

PSK changes the phase of the carrier wave abruptly each such change is called a phase shift

The phase of the carrier is discretely varied in relation either to a reference phaseor to the phase of the immediately preceding signal element, in accordance with

data being transmitted. Phase of carrier signal is shifted to represent '0' , '1'.


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Phase Shift Keying (PSK)

Figure illustrates how phase shifts affects a sine wave

There are three (abrupt) phase changes A phase shift is measured by the angle of the change

The left most portion of sine wave changes its phase by /2 radians

The second phase change corresponds to a 180 shift


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Antenna Height

Transmitting and receiving antenna are the important elements of long range radio communications

where communication channel is free space and signal travel in the form of electromagnetic waves,

antenna converts information signal to EM waves and are converted back to electrical signals at

receiver side. The most important design parameter is to design antenna that provides efficient radiation of EM

waves and their satisfactory reception.

The most important design parameter is the height of antenna.

A rule of thumb for proper transmission and reception of radio waves, the height of antenna should be

the order of one quarter or half wavelength of the frequency of transmitted signal.

Ifh is the height of antenna, l is the wavelength of the signal, f is the frequency of the signal, and c isthe velocity of light (3 * 10^8 m /sec) , then the required height of antenna according to rule is:

h = /4

Or

h = /2

Where = c/f

height of antenna can be expressed as a function of transmitted signals frequency as:

h(f) = c/4f

From the above result it is clear that height of antenna is inversely proportional to the frequency of

transmitted signal.


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Consider the example of a radio broadcast system that transmits voice of 300 to 3000Hz.

Required antenna height to transmit the audio in the given range is:

Maximum height = 3 x 10^8 / 4 x 300

= 250kmMinimum height = 3 x 10^8 / 4 x 3000

= 25km

Therefore to transmit a signal in audio range the required height of antenna lies between 250 km and

25 km. It is obvious that the height is not practically feasible.

The only solution to this problem lies in transmitting the signal on a carrier of frequency higher thanthat of baseband signal modulation.

Therefore a modulated signal is transmitted to keep the height of antenna in practical range.

Suppose carrier frequency for audio range broadcast is 1.5MHz, corresponding height of antenna comes

to:

h = 3x10^8 / 4 x 1.5 x 10^6 = 50m

which is in practical limits.


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Narrow banding

In all communication systems, the baseband signal occupies a rangeof frequencies i.e. has a certain bandwidth for example audiosignals lie in the range of 20 20kHz, video signals lie in the rangeof 0 to 5MHz.

In radio communication system the frequency of the signal to be

transmitted determines antenna height. If an audio signal is to be transmitted, then each frequency suggests

corresponding antenna height, if baseband signal is transmittedwithout modification then as many antennas are required as thefrequency components.

As this is not practical, solution to this problem is smaller number of

antennas or preferably single antenna with height in practical limits,which can transmit the entire baseband range.

Solution to this problem can be found by analyzing band-edge ratioof baseband signal.


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Band-edge ratio of audio signal is(20 20kHz):

20 / 20 x 10^3 = 1 : 1000

The practical significance of this ratio is to transmit a frequency component of 20kHz height of antenna

required is 1 unit, then height of antenna for 20Hz component is 1000 units. Since band edge ratio is

large the antenna height may not be practical.

Solution to this problem is to translate the baseband signal to high frequency say 1.5MHz then lowest

and highest freq components becomes:

Lowest frequency component : 1.5MHz + 20Hz = 1500020Hz

Highest frequency component: 1.5MHz + 20kHz = 1520000 Hz

The band edge ratio of modulated signal is:

1500020/1520000 = 0.9868 (approximately 1)

Therefore band edge ratio is 1:1

Thus a single antenna can be used to transmit the entire range of baseband signal. With this concept a

wideband is practically narrowed so the ratio to lowest and highest frequency component is 1:1.


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Poor Radiation and Penetration

The radiated power is directly proportional to the frequency of EM (radio)waves.

E = hf; where h is the Plancks constant

h = 6.626068 10-34 m2 kg / s

If the baseband signal is to be transmitted without modulation the

radiations will be poor and may suffer complete deterioration over largedistances due to attenuation and additive channel noise.

This may result in loss of signal at the receiver.

Therefore to make sure that the transmitted radiation has sufficient powerto reach the receiver the baseband signal needs to be translated to highfrequency carrier modulation.

Another reason to use modulation is to increase the penetration ofradiation in ionospheric layers of atmosphere in free space for effectivesatellite communication systems. Low frequency baseband signals cannotpenetrate the atmospheric layers resulting in poor reception at the end.


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Multiplexing

Consider a case of several radio stations broadcasting in the audio range

simultaneously without any modification. Since the bandwidth of

transmitted signal for each radio station is close to each other.Interference takes place over the communication channel.

This makes it impossible for several radio stations to broadcast

simultaneously.

Bandwidth of channel may be greater than that of audio range, which

results in wastage of channel resource utilization if single station transmits

at a time.

Practical solution to such problem is to modulate the transmitted signals

at same time assigning each station a frequency range far apart from one

another (preventing overlapping)which can be bandpass tuned at receiver

end.

Such technique which uses same channel to transmit several signals at the

same time assigning each signal a frequency band is known as Frequency

Division Multiplexing.

Here the bandwidth of the channel is shared by various signals without

overlapping.

anlog communication lecture 05

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