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بسم ا الرمحن الرحيم
LecturerAhmed H. Hadi
Training package inAmplitude Modulation
For students of second class
Ministry of Higher Education and Scientific ResearchFoundation of Technical Education
Technical College / Al-Najaf
1 / B –Rationale :-
Having studied amplitude modulation (AM) theory,students will be able to appreciate that an amplitude-modulated wave is made up of a number of sinusoidalcomponents having a specific relation to one another.They will be able to visualize the AM wave andcalculate the frequency present in it , as well as theirpower or current relations to each other.
/ Over view1/ Over view1
1 / A –Target population :-For students of second class inCommunications Techniques Engineering Department
1 / D –Objectives:-
• Describe the AM process• Compute the modulation index.• Solve problems• Draw a representation of an AM sine wave.• Analyze and determine through computation the carrier powerand sidebands power in AM
1 / C –Central Idea :-1) DSBLC(AM), DSBSC, and SSB signals2) Generation and detection DSBLC(AM), DSBSC, and SSB
signals
2/ Pre test :-2/ Pre test :-
Over modulation causes
(a) distorted output at the AM receiver
(b) excessive transmission bandwidth.
(c) interference with other transmission.
(d) higher carrier power requirement.
Over modulation causes
(a) distorted output at the AM receiver
(b) excessive transmission bandwidth.
(c) interference with other transmission.
(d) higher carrier power requirement.
Multiple Choice Questions With Answer
١١ The maximum efficiency transmission in AM with largecarrier is
(a) 100% (b) 50% (c) 66% (d) 33%
The maximum efficiency transmission in AM with largecarrier is
(a) 100% (b) 50% (c) 66% (d) 33%
٢٢
٣٣ Pilot carrier in SSB is provided for having(a) Low power consumption (b) Better noise immunity
(c) frequency stability purpose (d) None of these
Pilot carrier in SSB is provided for having(a) Low power consumption (b) Better noise immunity
(c) frequency stability purpose (d) None of these
٤٤ Mark the wrong statement. The advantage of SSB overDSB/AM is That
(a)more channel space is available
(b) transmitter must be stable giving better reception
(c) the signal noise resistant.
(d) Much less power is required for the same signalstrength
Mark the wrong statement. The advantage of SSB overDSB/AM is That
(a)more channel space is available
(b) transmitter must be stable giving better reception
(c) the signal noise resistant.
(d) Much less power is required for the same signalstrength
3/ Performance Objectives :-3/ Performance Objectives :-
Modulation is defined as the process by which somecharacteristics of a signal called carrier is varied inaccordance with instantaneous value of another signalcalled modulating signal.
Modulation is defined as the process by which somecharacteristics of a signal called carrier is varied inaccordance with instantaneous value of another signalcalled modulating signal.
The two type of analog modulation are
a) Amplitude modulationb) Angle modulation
The two type of analog modulation are
a) Amplitude modulationb) Angle modulation
AMPLITUDE MODULATEDAmplitude modulation is the process in whichinstantaneous amplitude of the carrier signal is variedaccording to the amplitude of the modulating signalkeeping the frequency and phase of the carrier signalconstant.
AMPLITUDE MODULATEDAmplitude modulation is the process in whichinstantaneous amplitude of the carrier signal is variedaccording to the amplitude of the modulating signalkeeping the frequency and phase of the carrier signalconstant.
Let the instantaneous carrier current be
The modulating signal
Let the instantaneous carrier current be
The modulating signal
The modulation index isThe modulation index is
The AM signalThe AM signal
Figure: Amplitude modulated wave: the carrierfrequency remains sinusoidal at wc while the envelopevaries at frequency ws
Figure : Frequency spectrum of the AM wave of Figure in timedomain. Note that there are three distinct frequencies present.
Figure : Frequency spectrum of the AM wave when the singlefrequency modulating signal is replaced by a band of audiofrequencies. Note that the information in the signal resides only inthe sidebands.
Generation of Normal AM Signals :
Square-law Modulator
THE AMPLITUDE MODULATEDRADIO RECEIVER
• The two basic functions of the radio receiverare:
1) to separate the signal induced in the antennaby the transmission which we wish toreceive from all the other signals present,
2) to recover the ‘‘message’’ signal which wasused to modulate the transmitter carrier.
The envelope detector circuit. The diode ‘‘half-wave’’rectifies the AM wave and the RC time-constant ‘‘follows’’the envelope with a slight ripple.
The input signal to the envelope detector.
The output signal of the envelope detector. Note that (1) when the voltageis rising the ripple is larger than when the voltage is falling. A longer timeconstant will help reduce the ripple; however, it will also increase thelikelihood that the output voltage will not follow the envelope when thevoltage is falling causing ‘diagonal clipping’. (2) In practice, the carrierfrequency is much higher than the modulating frequency, hence the ripple ismuch smaller than shown.
• y(t) = f(t) cos(2π fc t)fc is the carrier frequency (frequency of radio
station)
f(t)
cos(2πfct)
y(t)
Amplitude Modulation :Suppressed Carrierby Cosine Function
00
0
21
cos
FY
ttfty
Amplitude Modulation :Suppressed Carrierby Cosine Function
• Multiplication in time is convolution in Fourierdomain
• Sifting property of the Dirac delta functional
• Fourier transform property for modulation by acosine
000 ttxdtxttttx
txdtxttx
00 21
21
FFY
Amplitude Modulation :Suppressed Carrierby Cosine Function
• Example: φ(t) = f(t) cos(w0 t)– f(t) is an ideal lowpass signal– Assume ω1 << ω0
– φ(ω) is real-valued if F(ω) is real-valued
• Demodulation is modulation then lowpass filtering• Similar derivation for modulation with sin(w0t)
0
1
ω1-ω1w
F(w)
w0
½
-ω0 - ω1 -ω0 + ω1−ω0
ω0 - ω1 ω0 + ω1ω0
½F(w-w0)½F(w + w0)
00 21
21
FFY
Y(w)
00
0
21
sin
jFY
ttfty
Amplitude Modulation :Suppressed Carrierby Sine Function
• Multiplication in time is convolution in Fourier domain
• Sifting property of the Dirac delta functional
• Fourier transform property for modulation by a sine
000 ttxdtxttttx
txdtxttx
00 22 FjFjY
Amplitude Modulation :Suppressed Carrierby Sine Function
• Example: y(t) = f(t) sin(ω0 t)– f(t) is an ideal lowpass signal– Assume ω1 << ω0
– Y(ω) is imaginary-valued ifF(ω) is real-valued
• Demodulation is modulation then lowpass filtering
0
1
ω1-ω1w
F(w)
w
j ½
-ω0 - ω1 -ω0 + ω1-w0
ω0 - ω1 ω0 + ω1
w0
-j ½F(w-w0)j ½F(w + w0)
-j ½
00 22 FjFjY
Y(w)
The single-balanced modulator
The single-balanced shunt-bridge diode modulator.
The double-balanced ring modulator
Demodulation of DSB Signals :
Synchronous detector
Generation of SSB Signals :
1- Filter Method :
Demodulation of SSB Signals :
Synchronous detector
Anther Circuit to GenerateAmplitude Modulation
Amplitude Modulation Reception
Frequency Division Multiplexing (FDM)
)(3 tf
x
1w
)(1 tf
x
2w
x
3w
+)(2 tf
Transmitter
Antenna
١١
٢٢
Give the mathematical expression for the modulation index of anamplitude modulated wave.
Quiz /Quiz /
Name any three-generation modulates of DSBSCsignals.Name any three-generation modulates of DSBSCsignals.
(i) Chopper type modulator.(ii) Balanced modulator.(iii) Ring modulator.
(i) Chopper type modulator.(ii) Balanced modulator.(iii) Ring modulator.
minmax
minmax
EEEE
ma
-/ Post test :5
1. The highest modulation frequency typically used in AMbroadcast is
(a) 5 kHz (b) 10 kHz (c) 15 kHz (d) 25 kHz
1. The highest modulation frequency typically used in AMbroadcast is
(a) 5 kHz (b) 10 kHz (c) 15 kHz (d) 25 kHz
2. The TV broadcast Uses(a) amplitude modulation for both picture and sound.(b) frequency modulation for picture and amplitude
modulation for sound.(c) amplitude modulation for picture and frequency
modulation for sound
2. The TV broadcast Uses(a) amplitude modulation for both picture and sound.(b) frequency modulation for picture and amplitude
modulation for sound.(c) amplitude modulation for picture and frequency
modulation for sound
3. The household receiver are
(a) Synchronous detectors b) radio receivers
(c) envelope detectors
3. The household receiver are
(a) Synchronous detectors b) radio receivers
(c) envelope detectors
4.The purpose of balance modulator is toeliminate
(a) the carrier (b) Upper sideband
(c) lower sideband (d) base bandsignal
4.The purpose of balance modulator is toeliminate
(a) the carrier (b) Upper sideband
(c) lower sideband (d) base bandsignal
References:References:
11
2
T. R. Ganesh Babu, and G. Srinivasan:“ Communication Theory and systems”, 2006.
Sanjay Sharma: “Communication Systems(Analog and Digital) ”Sanjay Sharma: “Communication Systems(Analog and Digital) ”
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