CHETTINAD COLLEGE OF ENGINEERING & TECHNOLOGY

NH-67, TRICHY MAIN ROAD, PULIYUR, C.F. – 639 114, KARUR DT.

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

COURSE MATERIAL

Subject Name: ANALOG AND DIGITAL COMMUNICATION Class / Sem: BE (CSE) / III

Subject Code: CS2204 Staff Name: SUGANYA. J

UNIT I FUNDAMENTALS OF ANALOG COMMUNICATION

Syllabus

Principles of amplitude modulation, AM envelope, frequency spectrum and bandwidth, modulation index and

percent modulation, AM Voltage distribution, AM power distribution, Angle modulation - FM and PM waveforms,

phase deviation and modulation index, frequency deviation and percent modulation, Frequency analysis of angle

modulated waves. Bandwidth requirements for Angle modulated waves.

Objectives

• To study the principles of amplitude modulation.

• To define the modulation index and percent modulation.

• To learn how to draw the frequency spectrum, power spectrum and output envelope of an AM DSB FC

wave for different values of modulation index.

• To know how to calculate the bandwidth, power and voltages of the modulated carrier, upper side band and

the lower side band of an AM DSB SC wave by solving problems.

HISTORY OF COMMUNICATION SYSTEMS

1831 Samuel Morse invents the first repeater and the telegraph is born

1837 Charles Wheatstone patents "electric telegraph"

1849 England to France telegraph cable goes into service -- and fails after 8 days.

1850 Morse patents "clicking" telegraph.

1851 England-France commercial telegraph service begins. This one uses gutta-percha, and survives.

1858 August 18 - First transatlantic telegraph messages sent by the Atlantic Telegraph Co. The cable

deteriorated quickly, and failed after 3 weeks.

1861 The first transcontinental telegraph line is completed

1865 The first trans-Atlantic cable goes in service

1868 First commercially successful transatlantic telegraph cable completed between UK and Canada, with

land extension to USA. The message rate is 2 words per minute.

1870 The trans-Atlantic message rate is increased to 20 words per minute.

1874 Baudot invents a practical Time Division Multiplexing scheme for telegraph. Uses 5-bit codes & 6 time

slots -- 90 bps max. rate. Both Western Union and Murray would use this as the basis of multiplex telegraph

systems.

1875 Typewriter invented.

1876 Alexander Graham Bell and Elisa Grey independently invent the telephone (although it may have been

invented by Antonio Meucci as early as 1857)

1877 Bell attempts to use telephone over the Atlantic telegraph cable. The attempt fails.

1880 Oliver Heaviside's analysis shows that a uniform addition of inductance into a cable would produce

distortion less transmission.

1883 Test calls placed over five miles of under-water cable.

1884 - San Francisco-Oakland gutta-percha cable begins telephone service.

1885 Alexander Graham Bell incorporated AT&T

1885 James Clerk Maxwell predicts the existence of radio waves

1887 Heinrich Hertz verifies the existence of radio waves

1889 Almon Brown Strowger invents the first automated telephone switch

1895 Gugliemo Marconi invents the first radio transmitter/receiver

1901 Gugliemo Marconi transmits the first radio signal across the Atlantic

1901 Donald Murray links typewriter to high-speed multiplex system, later used by Western Union

1905 The first audio broadcast is made

1910 Chesapeake Bay cable is first to use loading coils underwater

1911 The first broadcast license is issued in the US

1912 Hundreds on the Titanic were saved due to wireless

1915 USA transcontinental telephone service begins (NY-San Francisco).

1924 The first video signal is broadcast

1927 First commercial transatlantic radiotelephone service begins

1929 The CRT display tube is invented

1935 Edwin Armstrong invents FM

1939 The blitzkrieg and WW II are made possible by wireless

1946 The first mobile radio system goes into service in St. Louis

1948 The transistor is invented

BASICS OF COMMUNICATION SYSTEMS

Communication

Communication is defined as the transmission of useful information from one point to another through a

communication channel.

The processes involved in the communication are:

Message signal generation

Encoding of message symbols

Transmission of encoded symbols

Decoding and reproduction of original symbols in the receiver

Recreation of original message signal

There are two major types of communication namely wired communication and the wireless

communication.

The wired communication is the communication done through a physical channel which exists

between the transmitter and the receiver. Eg.: Twisted pair cable, co-axial cable, optical fiber cable.

The wireless communication is the communication in which there is no physical medium

between the transmitter and the receiver. Eg.: Mobile communication, satellite communication,

transmission by microwaves.

Communication System

A communications subsystem is a functional unit or operational assembly that is smaller than the larger

assembly under consideration.

Communication subsystem basically consists of a receiver, frequency translator and a transmitter. It also

contains transponders and other transponders in it and communication satellite communication system

receives signals from the antenna subsystem.

Basic Communication System

The objective of designing a communication system is to reproduce the electrical signal at the receiving

end with minimal distortion.

The block diagram of a generic communication system is shown in Figure 1.1. The information source

produces symbols (such as English letters, speech, video, etc.) that are sent through the transmission

medium by the transmitter.

The communication medium introduces noise, and so errors are introduced in the transmitted data.

At the receiving end, the receiver decodes the data and gives it to the end user.

COMPONENTS OF A CONTINUOUS WAVE MODULATION

(a)Transmitter (b)Receiver

Modulator:

Modulation is a process of transforming the signal so that the signal can be transmitted

through the medium. It is a process by changing the characteristics of the carrier wave with respect

to the instantaneous value of the amplitude of the modulating signal. The characteristic of the

carrier wave includes amplitude, phase and frequency. The device used for modulation process is

called Modulator.

Demodulator:

The demodulator performs the inverse operation of the modulator.

Modulation and Demodulation

Modulation can be defined as superimposition of the signal containing the information on a high-

frequency carrier.

Example: If we have to transmit voice that contains frequency components up to 4kHz, we superimpose

the voice signal on a carrier of, say, 140MHz. The input voice signal is called the modulating signal. The

transformation of superimposition is called the modulation. The hardware that carries out this

transformation is called the modulator. The output of the modulator is called the modulated signal.

The modulated carrier is sent through the transmission medium, carrying out any other operations

required on the modulated signal such as filtering.

At the receiving end, the modulated signal is passed through a demodulator, which does the reverse

operation of the modulator and gives out the modulating signal, which contains the original information.

The modulating signal is also called the baseband signal.

In a communication system, both ends should have the capability to transmit and receive, and therefore

the modulator and the demodulator should be present at both ends. The modulator and demodulator

together are called the modem.

Carrier modulation can be broadly divided into two categories

Analog (Continuous Wave) modulation

Digital modulation

The various analog modulation techniques are:

Amplitude modulation (AM)

Frequency modulation (FM)

Phase modulation (PM) Angle modulation

The various digital modulation techniques are:

Amplitude shift keying (ASK)

Frequency shift keying (FSK)

Phase shift keying (PSK)

Types of Analog modulation

(i) Amplitude modulation

(ii) Frequency modulation

(iii) Phase modulation

Principles of Amplitude modulation

Amplitude modulation (AM) is a technique used in electronic communication, most commonly for

transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted

signal in relation to the information being sent.

For example, changes in the signal strength can be used to specify the sounds to be reproduced by a

loudspeaker, or the light intensity of television pixels. (Contrast this with frequency modulation, also

commonly used for sound transmissions, in which the frequency is varied; and phase modulation, often

used in remote controls, in which the phase is varied.

AM Envelope

AM is also called as AMDSBFC( Double side band full carrier)

Carrier Vcsinct

Moulating signalVm sinmt

Modulated signal Vam(t)

When a single frequency modulating signal acts on a high frequency carrier signal. The output waveform

contains all the frequency. The fig shows the modulated wave and the shape is called an envelope.

When a modulating signal is applied the amplitude of the output wave varies in accordance with the

modulating modulating signal.

The repition rate of the envelope is equal to the frequency of the modulating signal and the shape of the

envelope is identical to the shape of the modulating signal.

AM frequency spectrum and Bandwidth

AM modulator is a nonlinear device. The nonlinear mixing of the message and carrier gives an envelope ( i.e.

complex wave made up of dc voltage the carrier frequency and the sum and difference frequencies)

An AM signal spectrum contains frequency components spaced fm Hz on either side of the carrier.

Modulating frequency Modulated frequency

fccarrier frequency

fc-fm(max) lower sideband

fc+fm(max)upper sideband

The Envelope: The AM Signal

ttmkAts cc cos1

tmkAts c 1

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-5

-4

-3

-2

-1

0

1

2

3

4

5

Frequency spectrum of AMDSBFC

Bandwidth

The bandwidth of an AMDSBFC wave is equal to the difference between the highest upper side frequency

and the lowest lower side frequency.

Twice the highest modulating frequency is defined as bandwidth.

B=2fm(max)

Modulation index and percentage of modulation

Modulation index is otherwise called as co-efficient modulation which is represented by m.

Modulation index describes the amount of amplitude change present in AM wave (or) ratio between

maximum amplitude of modulating signal to the carrier signal

m= modulation co-efficient(unitless)

Em= Peak change in the amplitude of the output waveform (v)

Ec=peak amplitude of the unmodulted carrier.(V)

freq fc-fm(max) fc+fm(max) fc

LSB

USF

USB

LSF

Amplitude

carrier

c

m

EEm

It is not practically possible to construct a circuit producing a 100% modulation so in most of the applications

the under modulation is used. Over Modulation results in the intersection of the two side band signals, so it is

not possible to reconstruct the same original modulating signal in the demodulator.

Percentage modulation

The co-efficient of modulation in percentage is called percentage modulation which is represented by M

100c

m

EEM

Relation among Em,Ec and m( refer diagram P.no 142- ECS-wayne tomasi)

The percentage modulation is derived by

100

21212121

minmax

minmax

minmax

minmax

minmax

minmax

vvvvM

vv

vvM

vvE

vvE

c

m

Vmax=Ec+Em

Vmin=Ec-Em

The peak change in the amplitude of the output wave is the sum of the voltage from the upper and

lower side frequencies.

Em=EUSf+ELSf

EUSf=ELSf=

minmax

minmax

41

221

2

vv

vvEm

Eusf=peak amplitude of the upper side frequencies(V)

ELsf=peak amplitude of the lower side frequencies(V)

AM voltage Distribution

The unmodulated carrier is given by VC(t)=EC sin 2fCt

VC(t)=time varying voltage waveform or the carrier

EC=Peak carrier amplitude(Volts)

Fc=carrier frequency(Hertz)

The maximum amplitude of the modulated wave is equal to EC+Em. The instantaneous amplitude of the

modulated wave can be expressed as

Vam(t)=[ EC+Emsin 2fmt][sin2fct]

EC+Emsin 2fmt= amplitude of the modulated wave

Em=peak change in the amplitude of the envelope(volts)

fm= frequemcy of the modulating signal(Hz)

Vam(t)= [1+ m sin 2fmt] [Ec sin2fct]

[1+ m sin 2fmt]= constant + modulating signal

Ec sin2fct =unmodulated carrier

The constant component produces the carrier component in the modulated wave and the sinusoidal

component produces the side frequencies.

Vam(t)= Ec sin 2fct + [m Ec sin 2fmt][Ec sin2fct]

Vam(t)= Ec sin2fct - ])(2[cos2

])(2[cos2

tffmEtffmEmc

cmc

c

The amplitude of the carrier in the first term is not changed and the amplitude of the upper and lower

sideband is depends on both the carrier amplitude and the co-efficient of modulation.

For 100 percent modulation m=1, and the amplitude of upper and lower side frequencies are equal to one half

of the amplitude of the carrier.

ccc

c EEEEV 222max

VEEEV ccc 0

22min

The maximum peak amplitude of an AM envelope Vmax=2Ec

The maximum peak amplitude of an AM envelope Vmin=0 V.

AM power Distribution

The power dissipated is equal to the voltage squared divided by the resistance.

The Average power dissipated in a load by an unmodulated carrier is equal to the RMS carrier voltage

squared divided by the load resistance.

Power in unmodulated carrier is

RE

P cc

2)707.0(

=R

Ec

2

2

Pc= carrier Power in Watts

Ec=Peak carrier voltage (volts)

R= load ressitance.

The upper and lower sideband powers are expressed mathematically as

Pusb=PLsb=(mEc)2/2R

Pusb= upper sideband power ( watts)

PLsb= lowe sideband power (watts)

Pusb= PLsb=

R

Em c

24

22

Pusb= PLsb= 4

2c

Pm

When modulation coefficient m=0, the power in the upper and lower sideband is zero, and the total

transmitted power is imply the carrier power.

The total power in an amplitude modulated wave is equal to the sum of the powers of the carrier , the upper

sideband and the lower sideband.

Pt=pc_+ Pusb+PLsb

Pt=Pc+ 2

2cPm

Pt= cPm

21

2

Advantages of AM

AM is relatively inexpensive.

AM is a low quality form of modulation used for many commercial applications.

Modulation and demodulation are easily done by a non-linear device like switching modulator in the

transmitting side and the envelope detector in the receiving side.

The demodulated output is same as the incoming AM wave when M<100%. So it is easy to

reconstruct the original waveform in the receiver which is same as that of the input modulating signal.

Disadvantages of AM

It has poor performance in the presence of noise and interference.

Waste of Power: AM has inefficient use ot transmitter power. The two-third of the power is consumed

by the carrier for its transmission, so it is wasted.

Waste of Bandwidth: It requires twice the bandwidth of the message signal, but both usb and lsb

contains the same information. So it is enough to use only one side band to retrieve the original

message signal.

Applications of AM

It is used for commercial broadcasting of both audio and video signals.

It is also used for two-way mobile radio communication (citizen band radio).

It is used in the broadcasting in the medium and high frequency bands.

It is also used in the aircraft communications in the VHF frequency range.

Angle modulation

Frequency modulation

The classic definition of FM is that the instantaneous output frequency of a transmitter is varied in

accordance with the modulating signal. An equation for a sine wave is

e(t) = EP sin(ωt + φ)

While amplitude modulation is achieved by varying EP, frequency modulation is realized by varying ω in

accordance with the modulating signal or message. Notice that one can also vary φ to obtain another form

of angle modulation known as phase modulation (PM).

An important concept in the understanding of FM is that of frequency deviation. The amount of frequency

deviation a signal experiences is a measure of the change in transmitter output frequency from the rest

frequency of the transmitter.

The rest frequency of a transmitter is defined as the output frequency with no modulating signal applied.

For a transmitter with linear modulation characteristics, the frequency deviation of the carrier is directly

proportional to the amplitude of the applied modulating signal.

An FM transmitter is said to have a modulation sensitivity, represented by a constant, kf, of so many kHz/V,

kf = frequency deviation/V = kf kHz/V

For a single modulating tone of

eM(t) = eM sin(ωMt),

The amount of frequency deviation is given by

δ(t) = kf * eM(t)

Where δ(t) is the instantaneous frequency deviation and eM(t) represents the Modulating signal. The peak

deviation is given by δ = kf * EM

Where both δ and EM are peak values.

Time display of a Typical FM signal

PHASE MODULATION

Frequency modulation requires the oscillator frequency to deviate both above and below the carrier frequency.

During the process of frequency modulation, the peaks of each successive cycle in the modulated waveform

occur at times other than they would if the carrier were unmodulated.

This is actually an incidental phase shift that takes place along with the frequency shift in FM. Just the

opposite action takes place in phase modulation.

The audio frequency signal is applied to a PHASE MODULATOR in pm.

The resultant wave from the phase modulator shifts in phase, the time

period of each successive cycle varies in the modulated wave according to

the audio-wave variation. Since frequency is a function of time period per

cycle, a phase shift in the carrier will cause its frequency to change.

The frequency change in fm is vital, but in pm it is merely incidental. The

amount of frequency change has nothing to do with the resultant

modulated wave shape in PM

compare the three voltages (A, B, and C). Since voltage A begins its cycle and reaches its peak before voltage B,

it is said to lead voltage B. Voltage C, on the other hand, lags voltage B by 30 degrees.

In phase modulation the phase of the carrier is caused to shift at the rate of the AF modulating signal. In figure

note that the unmodulated carrier has constant phase, amplitude, and frequency.

The dotted wave shape represents the modulated carrier. Notice that the phase on the second peak leads the

phase of the unmodulated carrier. On the third peak the shift is even greater; however, on-the fourth peak, the

peaks begin to realign phase with each other.

These relationships represent the effect of 1/2 cycle of an AF modulating signal. On the negative alternation of

the AF intelligence, the phase of the carrier would lag and the peaks would occur at times later than they

would in the unmodulated carrier

FM and PM Waveforms

The Figure below includes both frequency and phase modulations of a sinusoidal carrier by a single

frequency modulating signal.

Both FM and PM waveforms are identical except for their time relationship (phase).

It is impossible to distinguish FM waveform from a PM waveform without knowing the dynamic

characteristics of the modulating signal.

With FM, the maximum frequency deviation occurs during maximum positive or maximum negative peaks

depending upon the type of modulator employed. The frequency deviation is directly proportional to the

amplitude of the modulating signal.

With PM, the maximum frequency deviation occurs during zero crossings of the modulating signal. Here

the frequency deviation is proportional to the slope or first derivative of the modulating signal.

For both FM and PM, the rate at which the frequency changes occur is equal to the modulating signal

frequency.

We know that m(t) = VC Cos [ωCt + θ(t)]. This equation doesn’t mean whether it is FM or PM. The correct

identification is done from the modulating signal.

If θ(t) = K Vm(t), it is Phase Modulation. If θ(t) = K1 Vm(t), then it is Frequency Modulation. Whenever the

instantaneous frequency of the carrier is directly proportional to the modulating signal, it is called FM.

When the instantaneous phase is directly proportional to the amplitude of the modulating signal, it is called

PM.

Equations for Phase and Frequency Modulated Waves

Type of Modulation Modulating Signal Angle Modulated Wave

a) PM vm(t) VC Cos (ωCt + K vm(t))

b) FM vm(t) VC Cos (ωCt + K1 ∫ vm(t) dt)

c) PM Vm Cos (ωmt) VC Cos (ωCt + K Vm Cos (ωmt))

d) FM Vm Cos (ωmt) VC Cos (ωCt + K1Vm/ωm) Cos (ωmt))

FM and PM Waveforms

Frequency deviation and percentage modulation

Frequency deviation is the change in frequency that occurs in the carrier when it is acted on by a

modulating signal frequency.

It is given as a peak frequency shift in HZ.(f ). The peak to peak frequency deviation 2f is also

called carrier swing.

The deviation sensitivity of FM is given in Hz/V. The peak frequency deviation is the product of the

deviation sensitivity and the peak modulating signal voltages and it is given by

HzVKf m1

Modulation Index

tmtVtm

tfftVtm

tfVK

tVtm

ffm

mcC

mm

cC

mm

mcC

m

sincos)(

sincos)(

sincos)( 1

In PM both the magnitude index and the peak phase deviation are directly proportional to the amplitude of

the modulating signal and unaffected by its frequency.

In FM both the modulation index and frequency deviation are directly proportional to the amplitude of the

modulating signal and the modulation index is inversely proportional to its frequency.

fm

m

FM

PM

m Vs freq Deviation

Vm

m m

FM & PM

m Vs Amplitude

FM & PM

Vm

PM

Phase deviation Vs Amplitude

f

FM

Frequency deviation Vs amplitude

Vm

Phase deviation and modulation Index

The modulating signal m(t) is given by

tmtVtm mcC coscos)( -----------------(1)

tm mcos instantaneous phase deviation (t)

m peak phase deviation in rad

Peak phase deviation is also called modulation index. In PM the modulation index is proportional to the

amplitude of the modulating signal and independent of its frequency.

The modulation index of the phase carrier is given by

m=KVm rad

m= Modulation index and peak phase deviation(,rad)

K= deviation sensitivity(rad/V)

Vm=peak modulating signal amplitude (volts)

tkVtVtm mmcC coscos)(

ttVtm mcC coscos)(

tmtVtm mcC coscos)(

In Frequency modulated carrier the modulation index is directly proportional to the amplitude of the

modulating signal and inversely proportional to the frequency of the modulating signal.

)(1 unitlessVKmm

m

K1 Deviation sensitivity rad/v

mradian frequency( rad/sec)

Deviation sensitivity is expressed in Hz/V which is given by

m

m

fVKm 1

fm cyclic frequency(Hz/S)

Frequency Analysis of Angle modulated Waves

The frequency component of the modulated wave is very complex. The single modulating frequency can

generate an infinite number of sideband which gives an infinite bandwidth.

Modulation by a single frequency sinusoidal

Frequency analysis of an angle modulated wave by a single carrier frequency sinusoidal produces a

peak phase deviation of m radians.

tmtVtm mcC coscos)(

The Bessel function is given by

nn

nnmjm2

cos)(coscos

Jn=Bessel function of the first kind of the nth order with argument m

nmcnc

ntntmjVtm2

cos)()(

For the first four term

.........)(.......2cos)(

2cos)(2

cos)(2

cos)(cos)()(

2

211

mJtmJ

tmJtmJtmJtmJVtm

nmc

mcmcmccoc

m(t) angle Modulated wave

mModulation index

Vc=peak amplitude of the unmodulated carrier.

J0(m)carrier component

J1(m)First set side frequency displaced from the carrier by m

J2(m) second set side frequency displaced from the carrier by 2m

Jn(m)nth set side frequency displaced from the carrier by nm

A single frequency modulating signal produces an infinite number of sets, of side frequencies each displaced

from the carrier by an integral multiple of the modulating signal frequency.

A side band set includes an upper and lower side frequency.

Successive set of sidebands are called fist – order side bands, second – order side bands, and so on,

and their magnitudes are determined by the coefficients J1 (m), J2 (m), and so on, respectively.

Amplitude of the side frequencies, Jn,

Table shows the Bessel Functions of the First Kind for Several Values of the Modulation Index.

A modulation index of 0 produces zero side frequencies, and the larger the modulation index, the more set of

side frequencies produced.

The values shown for Jn are relative to the amplitude of the un modulated carrier.

The amplitude of the higher order frequencies rapidly becomes insignificant as the modulation index decreases

unity.

For larger values of m , the value of Jn(m) starts to decrease rapidly as soon as n=m.

As the modulation index increases from zero, the magnitude of the carrier J0(m) decreases.

When m is equal to approximately 2.4, J0(m) = 0 and the carrier component go to zero (first carrier null).

This property is often used to determine the modulation index or set the deviation sensitivity of an FM

modulator.

The carrier reappears as m increases beyond 2.4.

When m reaches approximately 5.4, the carrier component once again disappears (second carrier null).

Further increases in the modulation index will produce additional carrier nulls at periodic intervals.

The above figure shows the curves for the relative amplitudes of the carrier and several sets of side frequencies

for values of m.

It can be seen that the amplitudes of both the carrier and the side frequencies vary at a periodic rate that

resembles a damped sine wave.

The negative values for J(m) indicate the relative phase of that side frequency set.

A side frequency is not considered significant unless it has amplitude equal to or greater than 1% of the un

modulated carrier amplitude.

As m increases the number of significant side frequencies increase.

Consequently, the bandwidth of an angle- modulated wave is a function of the modulation index.

Bandwidth requirements of angle modulated waves

The Bandwidth of an angle modulated wave is a function of the modulating signal frequency and the

modulation index.

The bandwidth is significantly wider because of the infinite number of sidebands.

Angle modulation is classified as Low, Medium or High index

In low index m<1

In high index m>10

In low index the signal information is carried by the first set of sideband and the minimum bandwidth

required is approximately equal to twice the highest modulating signal frequency so it is some times called as

narrow band FM.

In high modulation index the bandwidth is determined by quasi-stationary approach. In this approach it is

assumed that the modulating signal is changing very slowly.

The minimum bandwidth required to propagate a frequency modulated wave is approximately the peak to

peak frequency deviation 2f

For low index the frequency spectrum resembles DSBAM and the minimum bandwidth is B=2fm.

The bandwidth required to pass all significant sidebands for an angle modulated wave is equal to two times

the product of the highest modulating frequency and the number of significant sidebands.

HzfnB m )(2

n= number of significant sideband

fm= Modulating signal frequency(Hz)

Carson’s Rule (regardless of modulation index)

Bandwidth requires to transmit an angle modulated wave is twice the sum of the peak frequency deviation

and the highest modulating signal frequency.

HzffB m )(2

f Peak frequency deviation(Hz)

fm Modulating signal frequency (Hz)

Low modulation fm>> f

High modulation indices f>>fm

Carson’s rule gives slightly narrow bandwidth than Bessel function. It includes 98% of the total power in the

modulated wave.

Questions

Objective type Questions

1. a) Amplitude modulation is a __________ modulation.

i) Linear ii) Non-linear iii) Active iv) Passive

b) Amplitude modulators are __________ devices.

i) Linear ii) Non-linear iii) Active iv) Passive

2. a) When there is no modulating signal, the output waveform of the Amplitude modulator is ___________

signal.

i) Modulated ii) Modulating iii) Carrier iv) Unmodulated

b) For 0% modulation, the total transmitted power is equal to ____________.

i) Signal power ii) Half of carrier power

iii) Half of signal power iv) Carrier power

3. a) Direct FM is also called _______ PM and Direct PM is also called as _______ FM.

i) Direct, Direct ii) Indirect, Indirect

iii) Direct, Indirect iv) Indirect, Direct

b) Low modulation index FM is also called as __________ FM.

i) Narrowband ii) Wideband iii) Broadband iv) Low

4. a) The unit of deviation sensitivity is _________.

i) Hz/Volt ii) Radian/Volt iii) Hz/Sec iv) Both a and b

b) ________________ is also called as carrier swing.

i) Peak phase deviation ii) Peak frequency deviation

iii) Peak-to-peak frequency deviation .iv) Peak-to-peak frequency deviation

5. a) Angle modulation produces _________ bandwidth.

i) Infinite ii) Finite iii) Low iv) Medium

TWO MARKS

1. Define amplitude modulation.

2. Define bandwidth.

3. Define coefficient of modulation.

4. Draw the AM frequency spectrum.

5. Calculate the total co-efficient of modulation for the modulating signals with co-efficient of modulation m1=0.2,

m2=0.7 and m3=0.5.

6. Define Frequency Deviation and Carrier Swing.

7. How can you calculate the bandwidth using Carson’s rule and Bessel Table?

8. What is meant by Digital Modulation?

9. Define information capacity.

10. For any FM transmitter with an 80 KHz carrier swing determine frequency deviation if the amplitude of the

modulating signal decreases by a factor of 2, determine the new frequency deviation.

DETAIL

1. Explain the AM generation with the waveforms

2. Explain the power distribution for an AM wave with power spectrum diagram.

3. Explain the AM current calculation.

4. Explain the voltage distribution for an Am wave

5. Explain the frequency analysis of angle modulated waves using Bessel identity.

6. Explain the bandwidth requirements of angle –modulated waves in detail.