ANLOG COMMUNICATION Lecture 05
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Transcript of 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.