1. Introduction & Amplitude Modulation
18년 1월 3일 10시 50분 49초
Introduction & Amplitude Modulation
Dr. Monir Hossen
ECE, KUET
Department of Electronics and Communication Engineering, KUET
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2. Here, ‘Tele’ Means Long Distance
Introduction (1/2)
Long distance communication is called Telecommunication
Technically Communication is the summation of:
Transmission
Reception and
Processing of information by electric means
Radio communication was greatly improved during 2nd world war
Here, ‘Tele’ Means Long Distance
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3. Department of Electronics and Communication Engineering, KUET 3
Introduction (2/2)
Communication is widely using after invention of :
Transistor
Integrated circuit
Other semiconductor devices
Satellite and
Fiber optics
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4. Examples of Communication System
Telephony and Telegraphy
Radio Broadcasting (Example?)
Point-to-point & Mobile Communication
Computer Communication
Radar (Applications?)
Television Broadcasting
Radio Telemetry
Radio aids to Navigation
Radio aids to Aircraft Landing etc.
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5. Block Diagram of General Communication System
Transmitter
Information & Message ??
Receiver
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6. Open System Interconnections (OSI) Model
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7. Need For Using High Carrier Frequency
Radiation efficiency ( Increase/ decrease?)
Size of antenna ( Large/ small?)
Range of Communication (Short/ long?)
Easy of selection of radio signal in the receiver ( Why?)
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8. Classification of RF Spectrum (1/2)
λ= v/f = 3*108/f
Carrier Frequency
Free space wave-length (m)
Class
Propagation Characteristic
Service
10 – 30 KHz 3*
VLF
Attenuation: always is low
Long distance P2P Commn
30 – 300 KHz LF
absorption :
Day time > VLF
Night time = VLF
& navigation
300 – 3000 KHz MF
Attenuation:
Day time is high Night time is low
Broadcasting: Ship to shore
3 – 30 MHz HF
Ionospheric propagation
Broadcasting and P2P T&T
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9. Classification of RF Spectrum (2/2)
Carrier Frequency
Free space wave-length (m)
Class
Propagation Characteristic
Service
30 – 300 MHz
10 - 1
VHF
Tropospheric propagation
Radar, TV, FM broadcast, Short distance Commn
300 – 3000 MHz
UHF
Facsimile, TV relay, Air navigation
3000 – 30,000 MHz
SHF
Radar navigation, Radio relay
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10. Department of Electronics and Communication Engineering, KUET 10
Modulation (1/2)
Fulbarigate
Dhaka X
KUET
Fulbarigate
Only a Person Cannot go
Only a Person Can go
Fulbarigate
Dhaka
Person using bus Can go
Modulation
Information
Carrier
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11. Information Signal (What?)
Modulation (2/2)
Amplitude
Frequency
Phase
Definition:
It is a process of vary some characteristics of the Carrier Voltage in accordance with the Modulating Voltage
High Frequency Signal,
Information Signal (What?)
Amplitude of the carrier
Freq. of the carrier
Phase of the carrier
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12. Applications & Types of Modulation
Application Examples
Broadcasting of both audio and video signals.
Mobile radio communications, such as cell phone.
Types of Modulation:
Amplitude Modulation
Amplitude of carrier is varied with respect to modulating signal
Frequency Modulation
Frequency of carrier is varied with respect to modulating signal
Phase Modulation
Phase angle of carrier is varied with respect to modulating signal
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13. We know energy in a signal is proportional to its frequency
Need for Modulation
1) The modulating signal produced by microphone or camera is very weak and very low frequency (Why?)
So this signal can not travel long distance. That is why, weak signal requires high frequency carrier.
2) Height of antenna needed
3) Modulation permits the transmission without wire.
We know energy in a signal is proportional to its frequency
For best result antenna height = the wavelength
we know
Example: if signal is 20KHz
Then antenna height = v/f = m ( Practical?)
V= velocity of light (3*108)
f = frequency
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14. Need for Modulation
4. Modulation makes the message signal more robust against the noise.
5. Modulation is required to match the signal frequency with the bandwidth of the channel.
For example: Microwave communication required frequency of microwave range, Satellite communication requires frequency of satellite communication range, FM radio requires FM frequency range, Optical fiber communication required frequency of optical communication range.
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15. Complete Classification of Modulation Processes (1/2)
Amplitude Modulation (AM)
i) Single sideband AM (SSBAM)
ii) Double sideband AM (DSBAM)
iii) Frequency division multiplexing (FDM)
Frequency Modulation (FM)
Phase Modulation
Pulse Modulation
Pulse amplitude modulation (PAM)
Time Division multiplexing (TDM)
Pulse time modulation (PTM)
Pulse division multiplexing
Pulse code modulation (PCM)
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16. Complete Classification of Modulation Processes (2/2)
Digital modulation (DM)
Differential PCM (DPCM)
Adaptive PCM (ADPCM)
Delta modulation (DM)
Adaptive delta modulation (ADM)
Multiplexing
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17. Amplitude Modulation (AM)
Amplitude of the carrier is varied according to amplitude of the modulating signal
Modulator
Baseband Signal
with frequency fm
(Modulating Signal)
Bandpass Signal fc
(Modulated Signal)
Channel
fc >> fm
Voice: Hz
Source
Demodulator
Original Signal
with frequency fm
Sink
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18. Modulation Index or Factor (m) (1/3)
it is the ratio of maximum value of the modulating signal to the maximum amplitude of the carrier
m = Em / Ec
(2) It is the ratio of change of amplitude of the carrier to its original amplitude
m = ∆Ec / Ec
(3) m = 100* (∆Ec / Ec ) Em
Amp. t Ec
Amp. t
Upper envelope
Amp. t Ec
Ec + Em
Ec - Em
Lower envelope
Modulated wave
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19. Modulation Index or Factor (m) (2/3)
(4) It is the ratio of minimum amplitude to the maximum amplitude of the modulated wave
Proof: Emax = Ec + mEc (1)
Emin = Ec – mEc (2)
Amp. t Ec
Emax = Ec + Em
= Ec +mEm
Emin = Ec – Em
=Ec - mEc
Em = mEc
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20. Modulation Index or Factor (m) (3/3)
Adding Equations (1) and (2) we have:
Subtracting Equations (1) and (2) we have:
Now:
Note: 1) The value of m lies between 0 & 1
2) It is also depends on Amplitude of modulating signal and carrier
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21. Mathematical Analysis of Amplitude Modulated Wave (1/3)
Let, the carrier signal,
the modulating signal,
the amplitude of modulated wave,
Let, the voltage equation of modulated wave be
[ here, Em = mEc ]
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22. Mathematical Analysis of Amplitude Modulated Wave (2/3)
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23. Mathematical Analysis of Amplitude Modulated Wave (3/3)
Frequency spectrum of the above modulated wave:
Carrier, A=1.
Amplitude
mEc/ 2
mEc/ 2
frequency fc
fc+fm
fc-fm
Lower sideband
Upper sideband
Bandwidth = USB – LSB
= (fc + fm ) - (fc - fm )
= 2fm
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24. Practical Amplitude Modulator (1/4 ) Simple Transistor modulator
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25. Practical Amplitude Modulator (2/4)
Simple Transistor Modulator:
Transistor modulation consists of a resistive mixing network, a transistor, and an LC tuned circuit.
The emitter-base junction of the transistor serves as a diode and nonlinear device.
Modulation and amplification occur as base current controls a larger collector current.
The LC tuned circuit oscillates (rings) to generate the missing half cycle.
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26. Practical Amplitude Modulator (3/4)
Series modulator
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27. Practical Amplitude Modulator (4/4)
Series Modulator:
A series modulator produces high-level modulation without a large and expensive modulation transformer used in collector modulators
It improves frequency response
It is, however, very inefficient
A series modulator replaces the modulation transformer with an emitter follower
The modulating signal is applied to the emitter follower
The emitter follower is in series with the collector supply voltage
The collector voltage changes with variations in the amplified audio modulating signal
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28. Example of Amplitude Modulation
The information signal is usually not a single frequency but a range of frequencies (i.e., band). For example, frequencies from 20Hz to 15KHz. If we use a carrier of 1.4MHz, what will be the AM spectrum?
Soln: In frequency domain the AM waveform are the lower-side band frequency (fc - fm), the carrier frequency fc, the upper-side frequency/band (fc + fm). Bandwidth: 2×(15K-20)Hz.
1.4 MHz
frequency fc
fc-fm =1,385,000Hz to 1,399,980Hz
fc+fm =1,400,020Hz to 1,415,000Hz
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29. Modulation Index of AM (1/3)
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30. Modulation Index of AM (2/3)
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31. Modulation Index of AM (3/3)
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32. Power Distribution in the AM Wave (1/4)
We know power in a voltage wave
Total power in an AM wave, PT = P(LSB) + PC + P(USB)
Power contained in carrier is:
[ Ec = Maxm value ]
Power contained in LSB is:
Power contained in USB is:
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33. Power Distribution in the AM Wave (2/4)
Total power in AM wave is:
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34. Power Distribution in the AM Wave (3/4)
Power in both side bands is:
Ratio of sideband power to total power is:
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35. Power Distribution in the AM Wave (4/4)
For 100% modulation m = 1
So the side bands contain 1/3rd (33%) of the total power of the AM wave
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36. Example
1) A transmitter supplies 10Kw power to an aerial when unmodulated. Determines the power radiated when modulated to 30%.
Sol: Here, Pc = 10Kw
m = 0.3
PT = ?
We know, PT = Pc(1+m2/2)
= 10[1+(0.3)2/2]
=10.45 Kw
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37. Example
2) A 100v, 10KHz carrier is modulated with the help of a 5v, 50Hz signal. Calculate-
modulation factor(m)
Amplitude of each sideband
Frequency of each sideband
Bandwidth of modulated wave
Sol: (a)
(b) Amplitude of each sideband
(c) Frequency of sidebands, USB = fc+fm = =10050Hz
LSB = fc- fm = =9950Hz
(d) Bandwidth BW=USB-LSB = =100Hz
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38. Example
3) A 100v, 100KHz carrier is modulated with the help of a 10v, 1KHz signal to the extent of 50%. Write down the equation for the AM wave.
Sol: Here, m=0.5, Ec = 100v, Em = 10v, fc= 100KHz, fm= 1KHz
We know AM wave is:
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39. Example
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40. Self Study: All related examples & Problems of GK Mithal
4) An AM wave is represented by the equation.
Determine (1) m, (2) Ec, (3) fm, (4) fc, (5) Emax, (6) Emin, (7) BW
Sol: We know standard AM wave equation
Comparing this equation with the above equation. We get
(1) m = 0.7
(2) Ec = 20
(3) fm = 1KHz (How?)
(4) fc = 100KHz (How?)
(5) Emax = Ec + m Ec =34v
(6) Emin = Ec - m Ec =6v
(7) BW = 2fm =2KHz
Self Study: All related examples & Problems of GK Mithal 40
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41. Example
5) Determine the percentage of the ratio of the sidebands power to the total output power of the AM wave for m = 0.5 and m = 0.3.
Soln: for m=0.5
m = 0.3
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42. Limitations of The AM
The useful power is contained in the sidebands & even at 100% modulation the sidebands contain only 33% of the total power & hence the modulation efficiency is poor.
Due to poor efficiency transmitter has very poor range.
The reception in this modulation is noisy.
The audio reception is poor quality due to noisy reception.
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43. Modulation Used in Various Communications
Radio Broadcasting:
Frequency range: 20Hz to 30MHz
Modulation type: Amplitude modulation
TV Broadcasting:
Frequency range: VHF and UHF
Modulation type: For picture => AM
For Sound => FM
Satellite Broadcasting:
Frequency range: Up to 30GHz
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44. Modulation Used in Various Communications
Telephony, Telex, Telegraphy:
Frequency range: Up to 30GHz
Modulation type: FM
Mobile System :
Frequency range: Below 30MHz => AM
Above 30MHz => FM
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45. Department of Electronics and Communication Engineering, KUET
Thanks for Your Kind
Attention
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