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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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