UNIT-I

ANALOG COMMUNICATION SYSTEMS

Introduction to Communication Engineering Communication is the transfer of information from one place to another. This should be done

efficiently as possible,as much fidelity/reliability as possible and securely as possible.

For example:

Short distance: speech and graphical symbols

Long distance: smoke signals, light beams, carrier pigeons, letter, telephone, e-mail, radio,

tv, fax.

Communication System: Components/subsystems act together to accomplish information

transfer/exchange.

Elements of a Communication System

The elements of a communication system

Input Transducer

The message produced by a source must be converted by a transducer to a form suitable for

the particular type of communication system.

Example: In electrical communications, speech waves are converted by a microphone to voltage

variation.

Transmitter

The transmitter processes the input signal to produce a transmitted signal suited to the

characteristics of the transmission channel.

Signal processing for transmission almost always involves modulation and may also

include coding.

In addition to modulation, other functions performed by the transmitter are amplification,

filtering and coupling the modulated signal to the channel.

Channel

The channel can have different forms: the atmosphere (or free space), coaxial cable, fiber

optic, waveguide, etc.

The signal undergoes some amount of degradation from noise, interference and distortion

(resulting from band limiting and nonlinearities).

Output

message

Input

message Input

Transducer Transmitter Channel Receiver Output

Transducer

Receiver

The receiver function is to extract the desired signal from the received signal at the channel

output and to convert it to a form suitable for the output transducer.

Other functions performed by the receiver: amplification (the received signal may be

extremely weak), demodulation and filtering.

Output Transducer

It converts the electrical signal at its input into the form desired by system user.

Example: Loudspeaker, personal computer (PC), tape recorders.

ANALOG COMMUNICATION SYSTEM

There are many kinds of information sources, which can be categorized into two distinct

message categories, analog and digital. This distinction determines the criterion for

successful communication.

An analog message is a physical quantity that varies with time, usually in a smooth and

continuous fashion. Examples of analog messages are the acoustic pressure produced

when you speak, the angular position of an aircraft gyro, or the light intensity at some

point in a television image. Since information resides in a time-varying waveform, an

analog communication system should deliver this waveform with a specified degree of

fidelity.

A digital message is an ordered sequence of symbols selected from a finite set of discrete

elements. Examples of digital messages are the letter printed on this page, a listing of hourly

temperature readings or the keys you press on a computer keyboard. Since the information

resides in discrete symbols, a digital communication system should deliver these symbols with a

specified degree of accuracy in a specified amount of time

Comparisons of Digital and Analog Communication Systems

Analog Communication System Digital Communication System

Disadvantages :

expensive analog components : L&C

no privacy

can not merge data from diff. sources

Advantage :

inexpensive digital circuits

privacy preserved (data encryption)

can merge diff. data (voice, video and

data) and transmit over a common

no error correction capability

digital transmission system

error correction by coding

Advantages :

smaller bandwidth

synchronization problem is relatively

easier

Disadvantages :

larger bandwidth

synchronization problem is relatively

difficult

TIME/FREQUENCY DOMAIN REPRESENTATION OF SIGNALS

Electrical signals have both time and frequency domain representations. In the time

domain, voltage or current is expressed as a function of time as illustrated in Figure . Most

people are relatively comfortable with time domain representations of signals. Signals measured

on an oscilloscope are displayed in the time domain and digital information is often conveyed by

a voltage as a function of time.

The process of (electronic) communication involves the generation, transmission and

reception of various types of signals. The communication process becomes fairly difficult,

because:

a) the transmitted signals may have to travel long distances (there by undergoing severe

attenuation) before they can reach the destination i.e., the receiver.

b) of imperfections of the channel over which the signals have to travel

c) of interference due to other signals sharing the same channel and

d) of noise at the receiver input1 .

In quite a few situations, the desired signal strength at the receiver input may not be

significantly stronger than the disturbance component present at that point in the communication

chain. (But for the above causes, the process of communication would have been quite easy, if

not trivial). In order to come up with appropriate signal processing techniques, which enable us

to extract the desired signal from a distorted and noisy version of the transmitted signal, we must

clearly understand the nature and properties of the desired and undesired signals present at

various stages of a communication system. In this lesson, we begin our study of this aspect of

communication theory. Signals physically exist in the time domain and are usually expressed as

a function of the time parameter1 . Because of this feature, it is not too difficult, at least in the

majority of the situations of interest to us, to visualize the signal behavior in the Time Domain.

In fact, it may even be possible to view the signals on an oscilloscope. But equally important is

the characterization of the signals in the Frequency Domain or Spectral Domain. That is, we

characterize the signal in terms of its various frequency components (or its spectrum). Fourier

analysis (Fourier Series and Fourier Transform) helps us in arriving at the spectral description of

the pertinent signals.

SIGNALS

• The main function of the physical layer is moving information in the form of

electromagnetic signals across a transmission media.

• Information can be in the form of data, voice, picture, and so on.

• Generally the information usable to a person or application is not in a form that can be

transmitted over a network. The information must be converted into a form that

transmission media can accept.

• Transmission media work by conducting energy along a physical path.

• So a data stream of 1s and 0s must be turned into energy in the form of electromagnetic

signals.

• A signal is the physical representation of a certain information.

CLASSIFICATION OF SIGNAL

• Signals and data are classified as analog or digital.

• Analog refers to something that is continuous- a set of data and all possible points

between.

• An example of analog data is the human voice.

• Digital refers to something that is discrete –a set of specific points of data with no other

points in between.

• An example of digital data is data stored in the memory of a computer in the form of 0s

and 1s.

Signals can be analog or digital. Analog signals can have an infinite number of values

in a range; digital signals can have only a limited number of values

TIME AND FREQUENCY DOMAINS

Time-domain representation

Frequency-domain representation

• The frequency spectrum of a signal is the collection of all the component frequencies it

contains and is shown using a frequency-domain graph.

• The bandwidth of a signal is the width of the frequency spectrum, i.e., bandwidth refers

to the range of component frequencies.

• To compute the bandwidth, subtract the lowest frequency from the highest frequency of

the range.

DIGITAL SIGNALS

• Most of the digital signals are a periodic and, thus period or frequency is not appropriate.

• Bit interval ( instead of period) and Bit rate (instead of frequency)

• The bit interval is the time required to send one single bit.

•

The bit rate is the number of bit intervals in one second, usually expressed in bits per second

(bps).

TIME DOMAIN

Time domain is the analysis of mathematical functions, physical signals or time

series of economic or environmental data, with respect to time. In the time domain, the signal or

function's value is known for all real numbers, for the case of continuous time, or at various

separate instants in the case of discrete time. An oscilloscope is a tool commonly used to

visualize real-world signals in the time domain. A time-domain graph shows how a signal

changes with time, whereas a frequency-domain graph shows how much of the signal lies within

each given frequency band over a range of frequencies.

FREQUENCY DOMAIN

In electronics, control systems engineering, and statistics, the frequency domain refers to the

analysis of mathematical functions or signals with respect to frequency, rather than time. Put

simply, a time-domain graph shows how a signal changes over time, whereas a frequency-

domain graph shows how much of the signal lies within each given frequency band over a range

of frequencies.

A frequency-domain representation can also include information on the phase shift that must be

applied to each sinusoid in order to be able to recombine the frequency components to recover

the original time signal. A given function or signal can be converted between the time and

frequency domains with a pair of mathematical operators called a transform. An example is

the Fourier transform, which converts the time function into a sum of sine waves of different

frequencies, each of which represents a frequency component. The 'spectrum' of frequency

components is the frequency domain representation of the signal. The inverse Fourier

transform converts the frequency domain function back to a time function. A spectrum

analyzer is the tool commonly used to visualize real-world signals in the frequency domain.

Some specialized signal processing techniques use transforms that result in a joint time-

frequency domain, with the instantaneous frequency being a key link between the time domain

and the frequency domain

DIFFERENT FREQUENCY DOMAINS

Although "the" frequency domain is spoken of in the singular, there are a number of different

mathematical transforms which are used to analyze time functions and are referred to as

"frequency domain" methods. These are the most common transforms, and the fields in which

they are used:

Fourier series – repetitive signals, oscillating systems

Fourier transform – non repetitive signals, transients

Laplace transform – electronic circuits and control systems

Z transform – discrete signals, digital signal processing

Wavelet transform - image analysis, data compression

More generally, one can speak of the transform domain with respect to any transform. The

above transforms can be interpreted as capturing some form of frequency, and hence the

transform domain is referred to as a frequency domain.

FREQUENCY DOMAIN REPRESENTATION OF SINUSOIDS:

CONTINUOUS TIME

Consider a

sinusoid in

continuous

time:

Fourier Series and Fourier Coefficients

Fourier Series:

Fourier Coefficients:

MODULATION

In modulation, a message signal, which contains the information is used to control the

parameters of a carrier signal, so as to impress the information onto the carrier.

The message or modulating signal may be either:

k

tkFj

keatx 02)(

2/

2/

2

0

0

0

0)(1

T

T

tkFj

k dtetxT

a

tFjjtFjj eeA

eeA

tFAtx

00 22

0

22

)2cos()(

analogue – denoted by m(t)

digital – denoted by d(t) – i.e. sequences of 1's and 0's

The message signal could also be a multilevel signal, rather than binary; this is not .

The carrier could be a 'sine wave' or a 'pulse train'.

Consider a 'sine wave' carrier:

• If the message signal m(t) controls amplitude – gives AMPLITUDE MODULATION

AM

• If the message signal m(t) controls frequency – gives FREQUENCY MODULATION

FM

• If the message signal m(t) controls phase- gives PHASE MODULATION PM or M

Modulation is the process of putting information onto a high frequency carrier for

transmission (frequency translation),

The transmission takes place at the high frequency carrier, which has been modified to carry

the lower frequency information.

Once this information is received, the lower frequency information must be removed from

the high-frequency carrier.

NEED FOR MODULATION

The frequency of the human voice range from about 20 to 30 kHz. If every one transmitted

those frequencies directly as radio waves interference would cause them to be inefficient.

To overcome hardware limitations

Transmitting such lower frequencies require antennas with miles in wavelength.

cccc φ+tωV=tv cos

Modulation to reduce noise which result in optimization of Signal to Noise ratio(S/N)

For multiplexing and frequency assignment

For efficient radio transmission

Frequency Translation

Translation of the signal from one region in the frequency domain to another region.

Advantage of frequency translation

Frequency division multiplexing (To support multiple transmissions via single channel) enable

two or more base band information signals to be transmitted through a single common

transmission channel(sharing) by assigning information signals to different designated frequency

slots, i.e., Frequency division multiplexing.

BASIC ANALOG MODULATION METHODS

Consider the carrier signal below:

Sc(t ) = Ac(t) cos( 2fc t + )

where

Ac(t) : carrier amplitude

fc : carrier frequency

: carrier phase angle

The above parameters may be varied for the purpose of transmitting information giving

respectively the modulation methods ; namely

Amplitude Modulation (AM),

Frequency Modulation (FM),

Phase Modulation (PM)

1. Changing of the amplitude produces Amplitude Modulation signal

(i.e. the amplitude of the carrier waveform varies with the information signal.)

2. Changing of the frequency produces Frequency Modulation signal

3. Changing of the phase produces Phase Modulation signal

DEMODULATION

Demodulation is the reverse process (to modulation) to recover the message signal

m(t) or d(t) at the receiver.

Amplitude Modulation:

Amplitude modulation is defined as the modulation in which amplitude of carrier signal is varied

in accordance with instantaneous amplitude of modulating signal keeping frequency of

modulated carrier constant.

Let the carrier voltage be vc =Vc sin ωct and modulating voltage be vm =Vm sin ωmt

From definition we can write amplitude Vc of unmodulated carrier can be made proportional to

instantaneous modulating voltage vm =Vm sin ωmt when carrier is amplitude modulated Vc α Vm

sin ωmt

Frequency spectrum of AM

The frequencies present in AM wave are the carrier frequency and the firdt pair of

sideband frequencies where side band is defined by fsb =fc± n fm and in first pair n=1.

Amplitude of AM wave

From the figure we can write the amplitude of amplitude modulated wave A =Vc+vm

= Vc+Vm sin ωmt

We know that modulation index is given by m= Vm / Vc rewriting the equation Vm=m Vc

substituting in above equation we get

A= Vc+ m Vc sin ωmt

= Vc(1+ m sin ωmt)

The instantaneous voltage of resulting amplitude wave is given by v=A sin θ

v=A sin ωct

Substituting the value of A in other equation we get

v= Vc(1+ m sin ωmt) sin ωct

v= Vc(sin ωct + m sin ωmt. sin ωct)

= Vcsin ωct +(mVc/2 )cos(ωc- ωm)t.-(mVc/2 ) cos (ωc - ωm)t)

The above equation contain three terms the first term is identical and it is a unmodulated

carrier the other two additional terms produced are two side band the frequency of upper

sideband (fc+fm )and lower sideband (fc-fm ).the band width required is twice that of highest

modulating frequency.

When a carrier is first amplitude modulated, proportionality is made equal to unity and

instantaneous modulating voltage variations are superimposed on to the carrier amplitude.

With single tone (one frequency) signals

With complex signals (voice or music) or multi-tone signals. When modulating signal (message

signal) is multi-tone, AM signal becomes a band of frequencies. It is illustrated in the following

figure.

The frequency spectrum of AM waveform contains three parts:

A component at the carrier frequency ( cf )

An upper sideband(USB), whose highest frequency component is at mc ff

A lower sideband(LSB), whose highest frequency component is at mc ff

mA

AMPLITUDE

MODULATION cA Amplitude

f

Amplitude

2

mAc

f

2

mAc

Lower side Upper side frequency Carrier frequency

(LSF) (USF)

)( fSm )( fS

2B

cf mc f-f mc ff

B

mf

AMPLITUDE

MODULATION

Amplitude

f

Amplitude

f

Lower side Upper side

band Carrier band

(LSB) (USB) )( fSm )( fS

where B = Bandwidth

The bandwidth of the modulated waveform is twice the information signal bandwidth.

Because of two sidebands in frequency spectrum its often called Double Sideband with

Large Carrier.(DSB-LC)

Several baseband signals may be transmitted simultaneously on different carrier

frequencies (multiplexing) provided the sidebands do no overlap.

The information in baseband (information) signal is duplicated in LSB and USB and

carrier conveys no information.

Modulation Index ( m )

In preceding section, m is merely defined as a parameter, which determines amount of

modulation. However, to establish a desirable AM communication link the depth of modulation

should be less than 1.

%)100(0.1m .

This is important as to ensure successful retrieval of original transmitted information at

receiver end. Note that by performing the demodulation process (reverse of modulation) the

message signal is simply being traced out from the envelope of the modulated signal.

Thus, %)100(0.1m , envelope distortion will occur and waveform is said to be over

modulated. Under this circumstances, cA is large enough, resulting the non-proportionality of

)( to)( tsts m ----hence distortion of the desire message signal

Figures below show resulting AM signals when )1( 5.0 mm , 1m and )1( 5.1 mm

If the amplitude of the modulating signal is higher than the carrier amplitude, which in turn

implies the modulation index %)100(0.1m . This will cause severe distortion to the

modulated signal.

By ensuring the amplitude of )( tsm to be less than the carrier amplitude, message signal can

comfortably be retrieved from the envelope waveform of )(ts . The ideal condition for

amplitude modulation (AM) is when 1m also meanscm AA ; this will give rise to the

generation of the maximum message signal outputs at the receiver without distortion.

The modulation index can be determined by measuring the actual values of the modulation

voltage and the carrier voltage and computing the ratio.

In practice, the modulation index of an AM signal can be computed from Amax and Amin.

REPRESENTATION OF AM

Amplitude modulated signal (DSB-LC)

The above figure is amplitude modulated wave for one cycle of modulating sine wave

given by the relation A=Vc + Vm sin ωmt. The negative amplitude is given by -A= - (Vc + Vm sin

ωmt). The modulated wave extends between these two envelopes and has repetition rate equal to

unmodulated carrier frequency.

Vmax : is half the peak-to-peak value of the AM signal Vmax(pk-pk) /2

Vmin : is half the peak-to-peak value of the AM signal Vmin(pk-pk) /2

Vm : is half the difference of Vmax and Vmin .

Vc : is half the sum of Vmax and Vmin.

From the figure

max

2

minm

V VV

and ………………………………….(1)

max

max maxmax . 2

2 2

c m

min min

V V V

V V V VV

The values for Vmax and Vmin can be obtained directly from the oscilloscope. The evaluation of

the modulation index m can be achieved by invoking the following expression:

Vmm

Vc

V Vmax min

V Vmax min

POWER DISTRIBUTION IN (FULL) AM

The power in a sinusoidal signal is proportional to the square of its amplitude. The total

transmitted power is the sum of the carrier power (PC) and the power in the sidebands (PUSB and

PLSB). The total power in modulated wave will be

P P P P (rms)c USB LSBtotal

Carrier power

2VcarrPc R

2 2(V / 2) Vc cR 2R

Sideband power:

2VSBP P

USB LSB R2

mVc2

2 2 2m VcR 8R

22 Vm c.4 2R

Total transmitted power:

P P P P (rms)total c USB LSB

2 2 2 2 2V m V m Vc c c . + . + .2R 4 2R 4 2R

2 2 2 2 2V m m V mc c 1 12R 4 4 2R 2

2mP 1c

2

The equation relates total power in amplitude modulated wave to unmodulated carrier

power. It is used to determine the modulation index. The maximum power in AM wave is Pt= 1.5

Pc when m =1. This is because it is the maximum power that relevant amplifier must be capable

of handling without distortion.

Current calculations

Let Ic be the unmodulated carrier current and it be total or modulated current of

AMtransmitter both being rms values. If R is the resistance in which these current flow, then

2 2 2

2 2

2

2

12

12

12

t t t

c c c

t

c

t c

P I R I m

P I R I

I m

I

morI I

Modulation by several sine waves

Modulation of a carrier by several sine waves simultaneously is exceptional case there

are 2 methods to calculate modulation index

1. Let V1, V2, V3 etc …be simultaneous modulation voltages. Then total modulating

voltage Vt will be equal to squre root of sum of squares of

1 2 3 .......tV V V V

Dividing both sides by VC, we get

2 2 2

1 2 3

22 2

31 2

2 2 2

2 2 2

1 2 3

.......

..........

....

t

c c

c c c

t

V V VV

V V

VV V

V V V

thatis

m m m m

2. To emphazise the total power in an am wave consists of carrier power and sideband

power this yields 2

2 Pm cP P 1 Pt c c2

P Pc SB

m

2

where PSB

is the total sideband power given by 2P m

cPSB 2

If several sine waves simultaneously modulate the carrier ,carrier power will be

unaffected , but total sideband power will be sum of individual powers

T 1 2 3SB SB SB SBP P P P ........

.Substituting gives 2 2 2 2

t 1 2 3P P P Pc c c cm m m m

...........2 2 2 2

2 2 2

1 2 3 ....tm m m m

To calculate the modulation index ,take the square root of sum of squares of individual

modulation indices.

Every transmitter is limited in the amount of power it can be used (more power means

larger devices, too much power causes also interference with nearby stations). The receiver

extracts the original information from the signal power that it receives. Greater the received

power, easier it is to recover the desired signal.

The power in sidebands depends upon the value of modulation index. Greater percentage of

modulation will yield a higher sideband power. Maximum power appears in the sidebands which

is when the carrier is 100% (m=1) modulated. The power in each respective sideband, is given

by

cUSB LSB

PP P

4

This indicates that the power in each sideband is one-fourth, or 25 percent, of the carrier

power. Since there are two sidebands, their power put together to give 50 percent of the carrier

power.

AMPLITUDE DEMODULATION

Amplitude modulation, AM, is one of the most straightforward ways of modulating a radio

signal or carrier. The process of demodulation, where the audio signal is removed from the radio

carrier in the receiver is also quite simple as well. The easiest method of achieving amplitude

demodulation is to use a simple diode detector. This consists of just a handful of components:- a

diode, resistor and a capacitor.

Fig. 5 AM Diode Detector

In this circuit, the diode rectifies the signal, allowing only half of the alternating waveform

through. The capacitor is used to store the charge and provide a smoothed output from the

detector, and also to remove any unwanted radio frequency components. The resistor is used to

enable the capacitor to discharge. If it were not there and no other load was present, then the

charge on the capacitor would not leak away, and the circuit would reach a peak and remain

there.

ADVANTAGES OF AMPLITUDE MODULATION:

There are several advantages of amplitude modulation, and some of these reasons have meant

that it is still in widespread use today:

It is simple to implement

it can be demodulated using a circuit consisting of very few components

AM receivers are very cheap as no specialized components are needed.

DISADVANTAGES OF AMPLITUDE MODULATION

Amplitude modulation is a very basic form of modulation, and although its simplicity is one

of its major advantages, other more sophisticated systems provide a number of advantages.

Accordingly it is worth looking at some of the disadvantages of amplitude modulation.

It is not efficient in terms of its power usage

It is not efficient in terms of its use of bandwidth, requiring a bandwidth equal to twice

that of the highest audio frequency

It is prone to high levels of noise because most noise is amplitude based and obviously

AM detectors are sensitive to it.

Thus, AM has advantages of simplicity, but it is not the most efficient mode to use, both in terms

of the amount of space or spectrum it takes up, and the way in which it uses the power that is

transmitted. This is the reason why it is not widely used these days both for broadcasting and for

two way radio communication. Even the long, medium and short wave broadcasts will ultimately

change because of the fact that amplitude modulation, AM, is subject to much higher levels of

noise than are other modes. For the moment, its simplicity, and its wide usage, mean that it will

be difficult to change quickly, and it will be in use for many years to come.

A modulated wave has more power than had by the carrier wave before modulating. The total

Frequency Spectrum of AM Wave

Lower side frequency – (wc – wm)/2

Upper side frequency – (wc +wm)/2

The frequency components present in the AM wave are represented by vertical lines

approximately located along the frequency axis. The height of each vertical line is drawn in

proportion to its amplitude. Since the angular velocity of the carrier is greater than the angular

velocity of the modulating signal, the amplitude of side band frequencies can never exceed half

of the carrier amplitude.

Thus there will not be any change in the original frequency, but the side band frequencies (wc –

wm)/2 and (wc +wm)/2 will be changed. The former is called the upper side band (USB)

frequency and the later is known as lower side band (LSB) frequency. Since the signal

frequency wm/2 is present in the side bands, it is clear that the carrier voltage component does

not transmit any information. Two side banded frequencies will be produced when a carrier is

amplitude modulated by a single frequency. That is, an AM wave has a band width from (wc –

wm)/2 to (wc +wm)/2 , that is, 2wm/2 or twice the signal frequency is produced. When a

modulating signal has more than one frequency, two side band frequencies are produced by

every frequency. Similarly for two frequencies of the modulating signal 2 LSB’s and 2 USB’s

frequencies will be produced.

The side bands of frequencies present above the carrier frequency will be same as the ones

present below. The side band frequencies present above the carrier frequency is known to be the

upper side band and all those below the carrier frequency belong to the lower side band. The

USB frequencies represent the some of the individual modulating frequencies and the LSB

frequencies represent the difference between the modulating frequency and the carrier

frequency. The total bandwidth is represented in terms of the higher modulating frequency and

is equal to twice this frequency.

Limitations of Amplitude Modulation

1. Low Efficiency- Since the useful power that lies in the small bands is quite small, so the

efficiency of AM system is low.

2. Limited Operating Range – The range of operation is small due to low efficiency. Thus,

transmission of signals is difficult.

3. Noise in Reception – As the radio receiver finds it difficult to distinguish between the

amplitude variations that represent noise and those with the signals, heavy noise is prone to

occur in its reception.

4. Poor Audio Quality – To obtain high fidelity reception, all audio frequencies till 15

Kilohertz must be reproduced and this necessitates the bandwidth of 10 Kilohertz to

minimise the interference from the adjacent broadcasting stations. Therefore in AM

broadcasting stations audio quality is known to be poor.

SINGLE SIDEBAND MODULATION

Single-sideband modulation (SSB) is a refinement of amplitude modulation that more efficiently

uses electrical power and bandwidth. It is closely related to vestigial sideband modulation (VSB)

(Amplitude modulation produces a modulated output signal that has twice the bandwidth of the

original baseband signal. Single-sideband modulation avoids this bandwidth doubling, and the

power wasted on a carrier, at the cost of somewhat increased device complexity.

SSB was also used over long distance telephone lines, as part of a technique known as

frequency-division multiplexing (FDM). FDM was pioneered by telephone companies in the

1930s. This enabled many voice channels to be sent down a single physical circuit, for example

in L-carrier. SSB allowed channels to be spaced (usually) just 4,000 Hz apart, while offering a

speech bandwidth of nominally 300–3,400 Hz. Single-sideband suppressed-carrier (SSB-SC) is a

telecommunication technique, which belongs to amplitude modulation class.

The information represented by the modulating signal is contained in both the upper and

the lower sidebands. Since each modulating frequency fc produces corresponding upper and

lower side-frequencies fc + fi and fc − fi

It is not necessary to transmit both side-bands. Either one can be suppressed at the

transmitter without any loss of information.

Advantages

Less transmitter power.

Less bandwidth, one-half that of Double-Sideband (DSB).

Less noise at the receiver.

Size, weight and peak antenna voltage of single-sideband (SSB) transmitters is

significantly less than that of a standard AM transmitter.

Single-sideband Modulation, SSB

When one (upper or lower) of the two sidebands of DSB-SC signal is removed before

transmission, we get SSB modulation.

By transmitting only either upper side band or lower side band, the original baseband

message can still be recovered

SSB = DSB - SB

Power transmitted, mcSCDSBtt PPPP2

1

2

1)(

Note that conventional amplitude modulation (Full AM) and DSB-SC modulation require a

transmission bandwidth equal to twice the information signal bandwidth (B = 2W). One half the

transmission bandwidths is occupied by the upper sideband of the modulated signal. Whereas the

other half is occupied by the lower sideband, the basic information is transmitted twice, once in

each sideband. Since the sidebands are the sum and difference of the carrier and modulating

signals, the information must be contained in both of them. There is absolutely no reason to

transmit both sidebands in order to convey the information. One sideband may be suppressed.

The remaining sideband is called a single-sideband suppressed carrier (SSSC or SSB) signal.

Advantages of SSB

i) The spectrum space occupied by the SSB signal is only half that of AM and DSB

signals. This greatly conserves spectrum space and allows more signals to be

transmitted in the same frequency range. It also means there should be less

interference between signals.

ii) The second benefit is that all the power previously devoted to the carrier and other

sideband can be channeled into the single sideband, thereby producing a stronger

signal that should carry farther and be more reliably received at greater distances.

f fc 0 -fc

VcM(0)/2

fc + fm

BW of SSB signal = fm

upper

sideband

f

M(f)=F[m(t)]

0

SSB

modulation M(0)

fm - fm

BW of baseband signal = fm

SUPPRESSION OF UNWANTED SIDEBAND

The filter system

The filter system is the simplest system of the three – after the balanced modulator the

unwanted sideband is removed (actually heavily attenuated) by a filter. The filter may be LC,

crystal, ceramic or mechanical, depending on the carrier frequency and other requirements.

The key circuits in this transmitter are the balanced modulator and the sideband-

suppression filter. Such a filter must have a flat band pass and extremely high attenuation outside

the band pass. There is no limit on this; the higher the attenuation, the better. In radio

communication systems, the frequency range used for voice is 300 to about 2800 Hz in most

cases. If it is required to suppress the lower sideband and if the transmitting frequency is f, then

the lowest frequency that this filter must pass without attenuation is f+300Hz, whereas the

highest frequency that must be fully attenuated is f-300Hz.

In other words, filter response must change from zero attenuation to full attenuation over

a range of only 600 hertz. If the transmitting frequency is much above 10 MHz, this is not

applicable. The situation become even worse if lower modulating frequencies are employed,

such as the 50-Hz minimum in AM broadcasting. In order to obtain a filter response curve with

skirts as steep as those suggested above, the Q of the tuned circuits used must be very high. As

the transmitting frequency is raised, a situation is reached where necessary Q is so high that there

is no practicable method of achieving it.

SSB signal generation using filtering method

DSB-SC signal

fc

sideband filter, H(f)

f

Mechanical filters have been used at frequencies up to 500 kHz, and crystal or ceramic

filters up to about 20 MHz. The mechanical filter seem to be best because

Small size

Good bandpass

Very good attenuation

Adequate upper frequency limit

Crystal or ceramic filters may be cheaper but preferable for above 1 MHz. The disadvantages of

these filters are their operating frequency is below usual transmitting frequencies. This is a

reason for using a balanced mixer. In this mixer, frequency of crystal oscillator or synthesizer is

added to SSB signal from the filter, frequency thus being raised to the value desired for

transmission. If transmitting frequency is much higher than operating frequency of sideband

filter, two stages of mixing will be required. It becomes too difficult to filter out unwanted

frequencies in the output of mixer.

The mixer is followed by a linear amplifier because the amplitude of SSB signal is

variable. A class B RF amplifier is used because it is more efficient than class A amplifier.

DOUBLE-SIDEBAND SUPPRESSED-CARRIER (DSB-SC)

As noted early that carrier component in full AM or DSB-LC does no convey any information, it

may be removed or suppressed during the modulation process to attain a higher power

efficiency, hence Double Side Band Suppressed Carrier (DSB-SC) Modulation.

Consider the carrier

ccccc fwtwAts 2 where)cos()(

Modulated by a single sinusoidal signal

mmmm ftwAts 2 w wherecos)( m

The modulated signal is simply the product of these two

LSB

mccm

USB

mccm

mmcc

twwAA

twwAA

twAtwAts

)cos(2

)cos(2

)cos()cos()(

twAts mmm cos)(

twAts mmm cos)(

twAts ccc cos)(

DSB-SC modulator & signal

The amplitude varies between the limits )( cm AA

Frequency Spectrum of DSB-SC

Frequency spectrum of DSB-SC

- All the transmitted power is contained in the two sidebands(no carrier present)

- The bandwidth is twice the modulating signal bandwidth.

- USB displays positive components of )(tsm and LSB displays negative components of )(tsm

.

The simplest method of generating a DSB-SC signal is merely to filter out the carrier portion of a

full AM (or DSB-LC) waveform. Given carrier reference, modulation and demodulation

(detection) can be implemented using product devices or balanced modulators.

Double-sideband suppressed-carrier transmission (DSB-SC) is transmission in which

frequencies produced by amplitude modulation (AM) are symmetrically spaced above and below

the carrier frequency and the carrier level is reduced to the lowest practical level, ideally being

completely suppressed.

X )cos()cos()( twAtwAts mmcc

2B

cf mc f-f mc ff

B

mf

DSB-SC

MODULATION

Amplitude

f

Amplitude

f

)( fSm )( fS

where B = Bandwidth

Lower side Upper side

band Carrier band

(LSB) (USB)

In the DSB-SC modulation, unlike in AM, the wave carrier is not transmitted; thus, much of the

power is distributed between the sidebands, which implies an increase of the cover in DSB-SC,

compared to AM, for the same power used.

DSB-SC transmission is a special case of double-sideband reduced carrier transmission. It is

used for radio data systems.

SPECTRUM

DSB-SC is basically an amplitude modulation wave without the carrier, therefore reducing

power waste, giving it a 50% efficiency. This is an increase compared to normal AM

transmission (DSB), which has a maximum efficiency of 33.333%, since 2/3 of the power is in

the carrier which carries no intelligence, and each sideband carries the same information. Single

Side Band (SSB) Suppressed Carrier is 100% efficient.

FIG: Spectrum plot of an DSB-SC signal

DSB-SC is generated by a mixer. This consists of a message signal multiplied by a carrier

signal. The mathematical representation of this process is shown below, where the product-to-sum

trigonometric identity is used.

DEMODULATION

Demodulation is done by multiplying the DSB-SC signal with the carrier signal just like the

modulation process. This resultant signal is then passed through a low pass filter to produce a

scaled version of original message signal. DSB-SC can be demodulated by a simple envelope

detector, like AM, if the modulation index is less than unity. Full depth modulation requires

carrier re-insertion.

=

The equation above shows that by multiplying the modulated signal by the carrier signal, the

result is a scaled version of the original message signal plus a second term. Since this second

term is much higher in frequency than the original message. Once this signal passes through

a low pass filter, the higher frequency component is removed, leaving just the original

message.

DISTORTION AND ATTENUATION

For demodulation, the demodulation oscillator's frequency and phase must be exactly the

same as modulation oscillator's, otherwise, distortion and/or attenuation will occur.

To see this effect, take the following conditions:

Message signal to be transmitted:

Modulation (carrier) signal:

Demodulation signal (with small frequency and phase deviations from the modulation

signal):

VESTIGIAL SIDEBAND (VSB)

1. If portion or one of the (upper or lower) sideband of AM signal is removed, we will get VSB

signal.

a) AM :

( ) 1 cos cos

cos cos cos2

AM c m c

cc c c m c m

s t V m t t

mVV t t t

VSB Spectrum

(a) Message

(b) Modulated signal

(c) Frequency translated signal before LPF

VSB: ( ) cos cos2

cVSB c c c m

mVs t V t t

f 0 -fc fc fc + fm

fm -fm 0 f

(a) (b)

f 2fc 0 -2fc 2fc + fm fm

-fm

(c)

Average transmitted power, P P P Pt c m c 1

2

In general, P P kP Pt c m c where 0 5 1. k

Example: suitable for signal with significant low-frequency contents

Generation of VSB

Generation of VSB signal

a) VSB is derived by filtering AM (with VSB filter) in such a fashion that one sideband is

passed almost completely while just a trace, or vestige, of the other sideband is included.

VSB filter should have the following characteristics

i) odd symmetry about carrier frequency

ii) a relative response of ½ at fc

SSB have good bandwidth efficiency, but practical SSB modulation systems have poor

low frequency response. This is unpleasant when message (modulating) signal bandwidth is wide

or where one cannot disregard the low frequency component. Whereas, DSB-SC works well for

baseband signals with significant low frequency content but has a wider bandwidth and higher

power than SSB.

VSB gets over this problem while retaining the advantages of SSB(compromise solution

between SSB &DSB-SC). VSB relaxes the stringent sharp cutoff requirements of SSB by

retaining a part (vestige) of the unwanted sideband in the transmitted signal.

Offers a compromise between SSB and DSB-SC

VSB has lower power less bandwidth than full AM and higher power and slightly greater

bandwidth than SSB.

VSB is standard for transmission of TV and similar signals(TV has low frequency

component)

Bandwidth saving can be significant if modulating signals are of large bandwidth as in

TV and wideband data signals.

For example with TV the bandwidth of the modulating signal can extend up to 5.5MHz;

with full AM the bandwidth required is 11MHz this is excessive from transmission

bandwidth occupancy and cost points of view.

VSB modulated wave is obtained by passing DSBSC through a sideband shaping filter as shown

in fig below.

The exact design of this filter depends on the spectrum of the VSB waves.

The relation b/n filter transfer function H(f) and the spectrum of VSB waves is given by

S(f) = Ac /2 [M (f - fc) + M(f + fc)]H(f) -------------------------(1)

Where

M(f) is the spectrum of Message Signal.

Now, we have to determine the Specification for the Filter transfer function H(f).

It can be obtained by passing s(t) to a coherent detector and determining the necessary condition

for Undistorted version of the message signal m(t). Thus, s(t) is multiplied by a Locally

generated sinusoidal wave cos2πfct, which is synchronous with the carrier wave Accos2πfct in

both frequency and phase, as in fig below,

Then, v(t) = s(t). cos2πfct------ (2)

In frequency domain Eqn (2) becomes, V(f) = ½ [S( f - fc ) + S( f + fc )] -----(3)

Substitution of Eqn (1) in Eqn (3) gives

V(f) = ½[Ac /2 [M (f - fc - fc) + M(f - fc + fc)]H(f – fc ) + ½[Ac /2 [M (f + fc - fc) + M(f + fc +

fc)]H(f + fc )

V(f) = ½[Ac /2 [M (f -2 fc) + M(f )]H(f – fc ) + ½[Ac /2 [M (f ) + M(f +2fc)]H(f + fc )

V(f) = Ac /4 M(f)[H (f - fc) + H(f + fc)] + Ac /4 [M(f -2 fc) H (f - fc) + M(f + 2fc) H(f + fc)] –4

For a distortion less reproduction of the original signal m(t), Vo(f) to be a scaled version of M(f).

Therefore, the transfer function H(f) must satisfy the condition

H (f - fc) + H(f + fc) = 2H(fc)------(5)

Where H(fc) is a constant. Since m(t) is a band limited signal, we need to satisfy eqn (6) in the

interval - w≤f≤w. The requirement of eqn (6) is satisfied by using a filter whose transfer function

The Response is normalized so that H(f) at fc is 0.5. Inside this interval fc-fv≤f≤fc+fv response

exhibits odd symmetry. i.e., Sum of the values of H(f) at any two frequencies equally displaced

above and below is Unity

Time domain description: Time domain representation of VSB modulated wave, Procedure is

similar to SSB Modulated waves. Let s(t) denote a VSB modulated wave and assuming that s(t)

containing Upper sideband along with the Vestige of the Lower sideband. VSB modulated wave

s(t) is the output from Sideband shaping filter, whose input is DSBSC wave

The DSBSC Modulated wave is SDSBSC(t) = Ac m(t) cos2πfct -------(1)

It is a band pass signal and has in-phase component only. Its low pass complex envelope is

given by DSBSC (t) = Acm(t) --------------(2)

The VSB modulated wave is a band pass signal. Let the low pass signal (t) denote the complex

envelope of VSB wave s(t), then s(t) = e (t) exp(j2πfct)] -------(3)

Amplitude Modulators

There are two types of amplitude modulators. They are low-level and high-level modulators.

● Low-level modulators generate AM with small signals and must be amplified before

transmission.

● High-level modulators produce AM at high power levels, usually in the final amplifier stage of

a transmitter.

AM TRANSMITTERS

The block diagram is a simple AM transmitter. The microphone converts the audio

frequency input to electrical energy. The driver and modulator amplify the audio signal to the

level required to modulate the carrier fully. The signal is then applied to power amplifier. Power

amplifier combines the RF carrier and the modulating signal to produce the AM signal for

transmission. In SSB communications, carrier is suppressed (eliminated) and the sideband

frequencies produced by the carrier are reduced to a minimum. This means no carrier is present

in the transmitted signal. It is removed after the signal is modulated and reinserted at the receiver

during demodulation. Since there is no carrier, all the energy is concentrated in the side- band(s)

LOW LEVEL TRANSMITTER

If modulation takes place at early stage of transmitter i.e., in between modulator and

transmitter antenna one or more amplifier are connected because carrier power is low .A small

audio stage is used to modulate a low power stage; the output of this stage is then amplified

using a linear RF amplifier. This type of modulation is called low level modulation.

Low Level Modulation System

Pre amplifier is a class A linear voltage amplifier. It is used to amplify modulating signal

to a usable level with minimum distortion

Driver amplifiers are used to further amplify carrier and modulating signals to adequate

level to drive the modulator.

Modulated waves powers are quite low. To increase power level, amplification is

required.

Class A or Class B power amplifier stages are required to amplify carrier as well as

modulating signals. It provides sufficient bandwidth to accommodate frequencies,

otherwise sideband would cut off.

Matching network matches output impedance of power amplifier to transmission line and

antenna to radiate maximum signal power.

Advantages

Advantage of using a linear RF amplifier is that the smaller early stages can be modulated, which

requires a small audio amplifier to drive modulator.

Disadvantages

Disadvantage is amplifier chain is less efficient, because it has to be linear to preserve

modulation. Hence Class C amplifiers cannot be employed.

An approach which gets holdup the advantages of low-level modulation with the efficiency of a

Class C power amplifier chain is to arrange a feedback system to compensate for the substantial

distortion of the AM envelope. A simple detector at the transmitter output (which can be little

more than a loosely coupled diode) recovers the audio signal, and this is used as negative

feedback to the audio modulator stage. The overall chain then acts as a linear amplifier as far as

the actual modulation is concerned, though the RF amplifier itself still retains Class C efficiency.

Applications

Remote control and walkie talkies wireless intercom and pagers AM radiotelephones.

HIGH LEVEL TRANSMITTER

If carrier and modulating signals are amplified to desired level before modulation (to produce

100%) such a system is said to be a high level modulation. With high level modulation, the

modulation takes place at the final amplifier stage where the carrier signal is at its maximum

High Level Modulation System

When a large modulated power is to be transmitted, low level modulation system is quite

unsuitable because of low efficiency.

The crystal oscillator generates carrier signal; driver amplifier amplifies carrier signal to

desired level. Buffer amplifier isolates oscillator and driver amplifier to avoid loading

effect.

In this system both carrier and message signals are further amplified by power amplifiers

before modulation takes place to raise the power to desired level

Modulation which is usually carried out by using a collector modulated class ‘C’

amplifier, it provides three operations, such as modulator, final power amplifier and

frequency converter. Frequency converter simply translates the low frequency message

(AF) signal to R.F signal, which is fed into an antenna, it can radiate effectively into free

space.

In this transmitter power level of message signal and carrier signals are raised to the

desired level before modulation thus modulated signal is directly connected to antenna

through antenna matching network. The function of antenna matching network is

providing proper impedance matching between antenna and modulated amplifier to

radiate maximum power.

Power conversion efficiency is higher .This system require less critical adjustments.

Advantages

One advantage of using class C amplifiers in a broadcast AM transmitter is that only the

final stage needs to be modulated, and that all the earlier stages can be driven at a

constant level. These class C stages will be able to generate the drive for the final stage

for a smaller DC power input. However, in many designs in order to obtain better quality

AM the penultimate RF stages will need to be subject to modulation as well as the final

stage.

more power for modulation

A large audio amplifier will be needed for the modulation stage, at least equal to the

power of the transmitter output itself. Traditionally the modulation is applied using an

audio transformer, and this can be bulky. Direct coupling from the audio amplifier is also

possible (known as a cascode arrangement), though this usually requires quite a high DC

supply voltage (say 30 V or more), which is not suitable for mobile units.

DIFFERNCE BETWEEN LOW LEVEL AND HIGH LEVEL MODULATION

LOW LEVEL MODULATION HIGH LEVEL MODULATION

Modulated signals are amplified after

modulation takes to raise the power level

Carrier signals and modulating signals are

amplified before modulation takes to raise

power level, so no amplification after

modulation

Depth of modulation is less than 100% Depth of modulation maximum100%

Low gain and efficiency High gain and efficiency

Class B amplifier used to amplify

modulated signal

Class C amplifier used. So no need to

amplify after modulation

Base modulation is used. Collector or emitter modulation is used.

Used for wireless intercom ,remote control,

walkie talkie

Used for transmit radio and TV signals.

FREQUENCY MODULATION

Frequency modulation is defined as the modulation in which frequency of the carrier signal is

varied in accordance with amplitude of the modulating signal keeping the amplitude of

modulated carrier constant.

The shift in carrier frequency from its resting point compared to amplitude of modulating

voltage is called deviation ratio

Deviation ratio =(max)

(max)

dev

m

f

f where

fdev= frequency deviation of carrier

fm =frequency of modulating voltage

The general equation of an unmodulated wave or carrier is given by sinx A t

X= instantaneous value (voltage or current )

A= maximum amplitude

= angular velocity

= phase angle

By definition, amount by which carrier frequency is varied from its unmodulated wave

called deviation is made proportional to the instantaneous amplitude of modulating voltage. The

rate at which frequency deviation changes or takes place is equal to modulating frequency.

MATHEMATICAL REPRESENTATION OF FM

The instantaneous frequency of the resulting FM signal equals 1 cosf f kV tm mc

where fc =unmodulated carrier frequency

k=proportionality constant

cosV tm m

=instantaneous modulating voltage

Maximum deviation of a signal occurs when cosine term has its maximum value 1.

Under these condition instantaneous frequency becomes 1f f kVc m

The maximum deviation is given bymf

ckV

Instantaneous amplitude of FM signal will be given by sin , sinc mv A F A

where ,c mF is function of carrier and modulating frequencies

Consider the figure shown above to determine the value of θ which is traced by vector A

in time t rotating at a constant angular velocity. In this instance angular velocity is constant. It is

obtained by replacing f by angular velocity ω.

The instantaneous frequency of resulting FM signal now becomes 1 coskV tm mc

In order to find , must be integrated with respect to time.

1 cos

1 cos

sin

sin

c

c

m mc

m

m c mc

m

kV tm m

kV tm m

dt

dt

dt

kV tt

kV tt

Now replacing ω by angular velocity f. the above equation becomes

sin

sin

m c mc

m

c m

m

kV f tt

f

t t

since mf

ckV

The instantaneous value of FM is given by sin sinc c m

m

v V t t

The modulation index of FM is given by maximum frequency deviation δ

m = =f modulating frequency fm

Substituting in above equation we get sin sinc c f mv V t m t

If the modulating frequency decreases, maximum amplitude voltage remains constant,

modulation index increases.

Advantages of frequency modulation, FM

FM is used for a number of reasons and there are several advantages of frequency modulation. In

view of this it is widely used in a number of areas to which it is ideally suited. Some of the

advantages of frequency modulation are noted below:

Resilience to noise: One particular advantage of frequency modulation is its resilience

to signal level variations. The modulation is carried only as variations in frequency. This

means that any signal level variations will not affect the audio output, provided that the

signal does not fall to a level where the receiver cannot cope. As a result this makes FM

ideal for mobile radio communication applications including more general two-way radio

communication or portable applications where signal levels are likely to vary

considerably. The other advantage of FM is its resilience to noise and interference. It is

for this reason that FM is used for high quality broadcast transmissions.

Easy to apply modulation at a low power stage of the transmitter: Another advantage

of frequency modulation is associated with the transmitters. It is possible to apply the

modulation to a low power stage of the transmitter, and it is not necessary to use a linear

form of amplification to increase the power level of the signal to its final value.

It is possible to use efficient RF amplifiers with frequency modulated signals: It is

possible to use non-linear RF amplifiers to amplify FM signals in a transmitter and these

are more efficient than the linear ones required for signals with any amplitude variations

(e.g. AM and SSB). This means that for a given power output, less battery power is

required and this makes the use of FM more viable for portable two-way radio

applications.

FM modulation index

In terms of a definition: the FM modulation index is equal to the ratio of the frequency deviation

to the modulating frequency.

Thus the formula for the modulation index for FM is simple given by that shown below:

FM deviation ratio

FM deviation ratio can be defined as: the ratio of the maximum carrier frequency deviation to the

highest audio modulating frequency.

Where

D = Deviation ratio.

To give an example of how the deviation ratio may be calculated and used, take the example of

an FM broadcast transmitter. For these the maximum deviation is ±75 kHz and the maximum

modulation frequency is 15 kHz. This means that the deviation ratio is 75 / 15 = 5.

PHASE MODULATION

Phase modulation is defined as the modulation in which phase of the carrier signal is

varied in accordance with amplitude of the modulating signal keeping amplitude and frequency

of modulated carrier constant.

Let message signal be represented by vm(t)=Vm cos ωmt , carrier signal is represented by

vc(t)=Vc sin (ωct+θ) and the equation can also be rewritten as vc(t)=Vc sin

Vm= Maximum amplitude of modulating signal

Vc= Maximum amplitude of carrier signal

ωm= angular frequency of modulating signal

ωct= angular frequency of carrier signal

= phase angle of carrier

Phase angle of carrier is varied in accordance with amplitude of modulating signal i.e.

.

. cos

p m

p m m

K v

K V t

where

pK =is phase deviation sensitivity

After phase modulation, instantaneous voltage is given by

sin

sin cos

sin cos

c c

c c p m m

c c p m

v V t

V t K V t

v V t m t

pm p mm K V Modulation index of phase modulation

SPECTRUM OF AN FM SIGNAL

Narrow-band Frequency Modulation

Sinusoidal Modulating Signal

For small values of ,

cos ( sin(2 fm t) 1

sin ( sin(2 fm t)) sin(2 fm t)

Thus the expression for FM signal can be expanded as

( ) cos(2 ) sin(2 ) sin(2 )x t V f t V f t f tc c c c m

using BABABA sinsincoscoscos

which may be written as follows

1( ) cos(2 ) cos[2 ( ) ] cos[2 ( ) ]2

x t V f t A f f t f f tc c c c m c m

using BABABA coscos2

1sinsin .

Wide-band Frequency Modulation

The general expression for FM signal can be analyzed to give the spectral components of wide-

band FM signal.

In order to compute the spectrum of an angle-modulated signal with a sinusoidal message signal,

let

( ) sin( )tf

fmfmt

2

The corresponding FM signal

( ) cos(2 sin(2 ))x t V f t f tc c m

and may alternatively be written as

Amplitude spectrum (single-sided plot)

fc

Vc 1

2f cm V

f fc +fm fc -fm

1

2f cm V

Bandwidth=2fm

sin2

( ) Re( )j t j f t

c mx t V e ec

Where Re(x) denotes the real part of x.

The parameter is known as the modulation index and is the maximum value of phase

deviation of the FM signal.

Modulation index & FM bandwidth

It will often be seen that the terms narrowband or wideband FM are used when describing the

form of FM being used.

Narrowband FM: Narrow band FM is defined as an FM transmission where the value

of Β is small enough that the terms in the Bessel expansion, i.e. sidebands are negligible.

For this to be the case the modulation index must be less than 0.5, although a figure of

0.2 is often used. Narrowband FM is often used for short distance communications using

vehicle mount radios or hand carried equipment. Here the narrow band means that the

audio or data bandwidth is small, but this is acceptable for this type of communication.

Wideband FM: Wideband FM is defined as the situation where the modulation index is

above 0.5. Under these circumstances the sidebands beyond the first two terms are not

insignificant. Broadcast FM stations use wideband FM, and using this mode they are able

to take advantage of the wide bandwidth available to transmit high quality audio as well

as other services like a stereo channel, and possibly other services as well on a single

carrier.

The bandwidth of the FM transmission is a means of categorising the basic attributes for the

signal, and as a result these terms are often seen in the technical literature associated with

frequency modulation, and products using FM. This is one area where the figure for modulation

index is used.

Narrowband and Wideband FM

Narrowband FM NBFM

From the graph/table of Bessel functions it may be seen that for small , ( 0.3)

there is only the carrier and 2 significant sidebands, i.e. BW = 2fm.

FM with 0.3 is referred to as narrowband FM (NBFM) (Note, the bandwidth is

the same as DSBAM).

Wideband FM WBFM

For > 0.3 there are more than 2 significant sidebands. As increases the number of

sidebands increases. This is referred to as wideband FM (WBFM).

SL.NO WBFM NBFM

1

Modulation index is greater than 1

minimum value=5

Modulation index is less than 1

2

Frequency deviation =75 KHZ

Modulating frequency range from 30

Hz to 15 KHz

Frequency deviation =5 KHZ

Modulating frequency=3KHz

4

Bandwidth 15 times NBFM.

Bandwidth = 2 FM

5

Noise is more suppressed

Less suppressing of noise.

6

Use: Entertainment and broadcasting

Use: Mobile communication

AM DETECTOR CIRCUITS

ENVELOPE DETECTOR :

The detector is demodulator obtains the original modulation signal from the IF signal. Classified

as coherent and Non coherent detectors.

a. Diode detector or envelope detector

Envelope Detector

The most commonly used AM detector is simple diode detector.

The AM signal at fixed IF is applied to the transformer primary.

The signal at secondary is half wave rectified by diode D. This diode is the detector

diode.

The resistance R is load resistance to rectifier and C is the filter capacitor.

In the positive half cycle of AM signal, diode conducts and current flows through R,

whereas in negative half cycle , the diode is reverse biased and no current flows.

Therefore only positive half of the AM wave appears across resistance R as shown in fig.

The capacitor across R provides low impedance at the carrier frequency and much higher

impedance sat the modulation frequency.

Therefore capacitor reconstructs the original modulation signal as shown in fig and high

frequency carrier is removed.

a) Negative peak clipping in diode detector

This is the distortion that occurs in the output of diode detector because of unequal ac and

dc load impedances of the diode. The modulation index is defined as Em/ Ec. Therefore it

can also be defined as Im/Ic with

Im=Em/Zm and Ic = Ec/Rc

Here Zm is audio diode load impedance and Rc is the dc diode resistance.

The audio load resistance of the diode is smaller than the dc resistance.

Hence the AF current Im is larger, in proportion to dc current.

This makes the modulation index in the demodulated wave relatively higher than that of

modulated wave applied at the detector input.

This introduces the distortion due to over modulation in the detected signal for

modulation index near 100% .

This is illustrated in fig. In the figure observe that the negative peak of the detected signal

takes place because of over modulation effect taking place in detector.

b. Diagonal clipping in diode detector:

.

Diagonal clipping

As modulation frequency is increased, the diode ac load impedance, Zm does not remain

purely resistive.

It does have reactive component also. At high modulation depths, the current changes so fast

that the time constant of the load does not follow the changes.

Hence the current decays slowly as shown in. the output voltage follows the discharge law of

RC circuit. This introduces distortion in the detected signal and it is called diagonal peak

clipping

Demodulation of signals

An envelope detector can be used to demodulate a previously modulated signal by

removing all high frequency components of the signal. The capacitor and resistor form a low-

pass filter to filter out the carrier frequency. Such a device is often used to demodulate AM radio

signals because the envelope of the modulated signal is equivalent to the baseband signal.

Precision detector

An envelope detector can also be constructed to use a precision rectifier feeding into

a low-pass filter.

Drawbacks

The envelope detector has several drawbacks:

The input to the detector must be band-pass filtered around the desired signal, or else the

detector will simultaneously demodulate several signals. The filtering can be done with a

tunable filter or, more practically, a super heterodyne receiver

It is more susceptible to noise than a product detector

If the signal is over modulated, distortion will occur

Most of these drawbacks are relatively minor and are usually acceptable tradeoffs for the

simplicity and low cost of using an envelope detector.

An envelope detector is sometimes referred to as an envelope follower in musical environments.

It is still used to detect the amplitude variations of an incoming signal to produce a control signal

that resembles those variations. However, in this case the input signal is made up of audible

frequencies.

Envelope detectors are often a component of other circuits, such as a compressor or an auto- or

envelope-followed filter. In these circuits, the envelope follower is part of what is known as the

"side chain", a circuit which describes some characteristic of the input, in this case its volume.

Both expanders and compressors use the envelope's output voltage to control the gain of an

amplifier.. The voltage-controlled filter of an analog synthesizer is a similar circuit.

Modern envelope followers can be implemented:

1. directly as electronic hardware,

2. indirectly using DSPs

3. completely virtually in software.

4.

AM Modulator:

Modulator circuits cause carrier amplitude to be varied in accordance with modulating

signals. Circuits produce AM, DSB, and SSB transmission methods.

The basic equation for an AM signal is

νAM = Vcsin 2πfct + (Vmsin 2πfmt)(sin 2πfct)

The first term is the sine wave carrier

The second term is the product of the sine wave carrier and modulating signals.

AM in the Frequency Domain

The product of the carrier and modulating signal can be generated by applying

both signals to a nonlinear component such as a diode.

A square-law function is one that varies in proportion to the square of the input

signals. A diode gives a good approximation of a square-law response.

Bipolar and field-effect transistors (FETs) can also be biased to give a square-law

response. Diodes and transistors whose function is not a pure square-law function

produce third-, fourth-, and higher-order harmonics, which are sometimes referred

to as intermodulation products.

Intermodulation products are easy to filter out.

Tuned circuits filter out the modulating signal and carrier harmonics, leaving only

carrier and sideband.

AM in the Time Domain

Amplitude modulation voltage is produced by a circuit that can multiply the

carrier by the modulating signal and then add the carrier.

If a circuit’s gain is a function of 1+ m sin 2πfmt, the expression for the AM signal

is νAM = A(νc) , Where A is the gain or attenuation factor.

There are two types of amplitude modulators. They are low-level and high-level

modulators.

Low-level modulators generate AM with small signals and must be amplified before

transmission.

High-level modulators produce AM at high power levels, usually in the final amplifier

stage of a transmitter.

SQUARE-LAW CIRCUIT FOR PRODUCING AM

LOW-LEVEL AM: DIODE MODULATOR

Diode modulation consists of a resistive mixing network, a diode rectifier, and an

LC tuned circuit.

The carrier is applied to one input resistor and the modulating signal to another

input resistor.

This resistive network causes the two signals to be linearly mixed (i.e.

algebraically added).

A diode passes half cycles when forward biased.

The coil and capacitor repeatedly exchange energy, causing an oscillation or

ringing at the resonant frequency.

Amplitude modulation with a diode Low-Level AM: 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.

Low-Level AM: PIN Diode Modulator

Variable attenuator circuits using PIN diodes produce AM at VHF, UHF, and

microwave frequencies.

PIN diodes are special type silicon junction diodes designed for use at frequencies

above 100 MHz.

When PIN diodes are forward-biased, they operate as variable resistors.

Attenuation caused by PIN diode circuits varies with the amplitude of the

modulating signal.

The LC tuned circuit oscillates (rings) to generate the missing half cycle

Low-Level AM: Differential Amplifier Differential amplifier modulators make excellent amplitude modulators because

they have a high gain, good linearity and can be 100 percent modulated.

The output voltage can be taken between two collectors, producing a balanced, or

differential, output.

The output can also be taken from the output of either collector to ground,

producing a single-ended output.

High-Level AM

In high-level modulation, the modulator varies the voltage and power in the final

RF amplifier stage of the transmitter.

The result is high efficiency in the RF amplifier and overall high-quality

performance.

High-Level AM: Collector Modulator

The collector modulator is a linear power amplifier that takes the low-level

modulating signals and amplifies them to a high-power level.

A modulating output signal is coupled through a modulation transformer to a class

C amplifier.

The secondary winding of the modulation transformer is connected in series with the collector

supply voltage of the class C amplifier

Amplitude Demodulators

Demodulators, or detectors, are circuits that accept modulated signals and recover the original

modulating information

DIODE DETECTOR

On positive alternations of the AM signal, the capacitor charges quickly to the

peak value of pulses passed by the diode.

When the pulse voltage drops to zero, the capacitor discharges into the resistor.

The time constant of the capacitor and resistor is long compared to the period of

the carrier.

The capacitor discharges only slightly when the diode is not conducting.

The resulting waveform across the capacitor is a close approximation to the

original modulating signal.

A DIODE DETECTOR AM DEMODULATOR

REACTANCE MODULATOR

The circuit is the three-terminal reactance that may be connected across the tank circuit

of the oscillator to be frequency-modulated.

The value of this reactance is proportional to the transconductance of the device, which

can be made to depend on the gate bias and its variations.

Theory of reactance modulators:

In order to determine z, a voltage v is applied to the terminals A-A between which the

impedance is to be measured, and the resulting current i is calculated.

The applied voltage is then divided by this current, giving the impedance seen when

looking into the terminals. In order for this impedance to be a pure reactance (it is

capacitive here), two requirements must be fulfilled.

The first is that the bias network current ib must be negligible compared to the drain

current. The impedance of the bias network must be large enough to be ignored.

The second requirement is that the drain-to-gate impedance (XC here) must be greater

than the gate-to-source impedance (R in this case), preferably by more than 5:1.

The following analysis may then be applied:

g b

c

Rv i R

R jX

The FET drain current is 1

1m v cm g

c m

g R jXi g v

R jX g R

If cX R in above equation will reduce to cjXz

R

The impedance seen at terminal A-A is

11

m v

c

c

m

vz

i

g Rv

R jX

jX

g R

The impedance is quite clearly a capacitive reactance

1

2

1

2

ceq

m

m

eq

XX

g R

fg RC

fC

From above equation, input impedance of device at A-A is a pure reactance is given by

eq mC g RC

The following should be noted from the above equation

1. This equivalent capacitance depends on the device trans conductance and can therefore be

varied with bias voltage.

2. Capacitance can be originally adjusted to any value, within reason, by varying R and C.

3. The expression gm RC has correct dimensions of capacitance; R, is measured in ohms, and

gm, measured in Siemens (s), cancel each other’s dimensions, leaving C as required.

4. It was stated earlier that the gate-to-drain impedance must be much larger than the gate-to-

source impedance. If XC/R had not been much greater than unity, z would have had a

resistive component as well.

1

1 1

2

cX nRC

CnR fnR

Substituting the values of C and cX in

eqC we get

2

2

eq m

m

m

C g RC

g R

fnR

g

fn

SUPERHETERODYNE RECEIVER

Heterodyne – to mix two frequencies together in a nonlinear device or to transmit one frequency

to another using nonlinear mixing.

Block diagram of super heterodyne receiver :

. RF section

Consists of a pre-selector and an amplifier

Pre-selector is a broad-tuned band pass filter with an adjustable center frequency

used to reject unwanted radio frequency and to reduce the noise bandwidth.

RF amplifier determines the sensitivity of the receiver and a predominant factor in

determining the noise figure for the receiver.

Mixer/converter section

Consists of a radio-frequency oscillator and a mixer.

Choice of oscillator depends on the stability and accuracy desired.

Mixer is a nonlinear device to convert radio frequency to intermediate frequencies

(i.e. heterodyning process).

The shape of the envelope, the bandwidth and the original information contained

in the envelope remains unchanged although the carrier and sideband frequencies

are translated from RF to IF.

IF section

Consists of a series of IF amplifiers and band pass filters to achieve most of the

receiver gain and selectivity.

The IF is always lower than the RF because it is easier and less expensive to

construct high-gain, stable amplifiers for low frequency signals.

IF amplifiers are also less likely to oscillate than their RF counterparts.

DETECTOR SECTION

To convert the IF signals back to the original source information (demodulation).

Can be as simple as a single diode or as complex as a PLL or balanced demodulator.

RECEIVER OPERATION.

Frequency conversion in the mixer stage is identical to the frequency conversion in the

modulator except that in the receiver, the frequencies are down-converted rather that up-

converted.

In the mixer, RF signals are combined with the local oscillator frequency

The local oscillator is designed such that its frequency of oscillation is always above or

below the desired RF carrier by an amount equal to the IF center frequency.

Therefore the difference of RF and oscillator frequency is always equal to the IF

frequency

The adjustment for the center frequency of the pre-selector and the local oscillator

frequency are gang-tune (the two adjustments are tied together so that single adjustment

will change the center frequency of the pre-selector and at the same time change the local

oscillator)

when local oscillator frequency is tuned above the RF – high side injection

when local oscillator frequency is tuned below the RF – low side i injection

Local oscillator tracking – the ability of the local oscillator in a receiver to oscillate either

above or below the selected radio frequency carrier by an amount equal to the intermediate

frequency throughout the entire radio frequency band.

With high side injection- local oscillator should track above the incoming RF carrier by a

fixed frequency equal to fRF + fIF

With low side injection- local oscillator should track below the incoming RF carrier by a

fixed frequency equal to fRF - fIF

Frequency conversion

Frequency conversion in the mixer stage is identical to the frequency conversion in the

modulator except that in the receiver, the frequencies are down-converted rather that up-

converted.

In the mixer, RF signals are combined with the local oscillator frequency

The local oscillator is designed such that its frequency of oscillation is always above or

below the desired RF carrier by an amount equal to the IF center frequency.

Therefore the difference of RF and oscillator frequency is always equal to the IF

frequency

The adjustment for the center frequency of the pre-selector and the local oscillator

frequency are gang-tune (the two adjustments are tied together so that single adjustment

will change the center frequency of the pre-selector and at the same time change the local

oscillator)

when local oscillator frequency is tuned above the RF – high side injection

when local oscillator frequency is tuned below the RF – low side injection.

Image frequency

o The higher the IF, the farther away the image frequency is from the desired radio

frequency. Therefore, for better image frequency rejection, a high IF is preferred.

o However, the higher the IF, it is more difficult to build a stable amplifier with

high gain. I.e. there is a trade-off when selecting the IF for a radio receiver (image

frequency rejection vs IF gain and stability)

Image frequency rejection ratio (IFRR) – a numerical measure of the ability of a pre-

selector to reject the image frequency

Mathematically expressed as,

where ρ= (fim/fRF) – (fRF/fim)

Q = quality factor of a pre-selector

Once an image frequency has down-converted to IF, it cannot be removed. In order to

reject the image frequency, it has to be blocked prior to the mixer stage. I.e. the

bandwidth of the pre-selector must be sufficiently narrow to prevent image frequency

from entering the receiver.

Pre Emphasis, & De Emphasis

In telecommunications emphasis is the intentional alteration of the amplitude-vs.-

frequency characteristics of the signal to reduce adverse effects of noise in a communication

system. The whole system of pre-emphasis and de-emphasis is called emphasis.

221 QIFRR

The high-frequency signal components are emphasized to produce a more equal modulation

index for the transmitted frequency spectrum, and therefore a better signal-to-noise ratio for the

entire frequency range.

Emphasis is commonly used in LP records and FM broadcasting

Pre-emphasis

At the transmitter the modulating signal is passing through a simple network which amplifies

the high frequency component more the low-frequency component

The simplest form of such circuit is a simple high pass filter

The pre-emphasis circuit increases the energy of the higher content of the higher-frequency

signals so that will tend to become stronger than the high-frequency noise component.

This improves the signal-to-noise ratio.

To return the frequency response to its normal level, a de-emphasis circuit is used at the

receiver.

The de-emphasis circuit provides a normal frequency response.

The combined effect of pre-emphasis and de-emphasis is to increase the high-frequency

components during the transmission so that they will be stronger and not masked by noise

Pre-emphasis is achieved with a pre-emphasis network which is essentially a calibrated filter.

The frequency response is decided by special time constants. The cutoff frequency can be

calculated from that value.

Pre-emphasis is commonly used in telecommunications, digital audio recording, record cutting,

in FM broadcasting transmissions, and in displaying the spectrograms of speech signals.

One example of this is the RIAA equalization curve on 33 rpm and 45 rpm vinyl records.

Another is the Dolby noise-reduction system as used with magnetic tape.

In high speed digital transmission, pre-emphasis is used to improve signal quality at the output of

a data transmission. In transmitting signals at high data rates, the transmission medium may

introduce distortions, so pre-emphasis is used to distort the transmitted signal to correct for this

distortion. When done properly this produces a received signal which more closely resembles the

original or desired signal, allowing the use of higher frequencies or producing fewer bit errors.

Pre-emphasis is employed in frequency modulation or phase modulation transmitters to equalize

the modulating signal drive power in terms of deviation ratio. The receiver demodulation process

includes a reciprocal network, called a de-emphasis network, to restore the original signal power

distribution.

De-emphasis

In telecommunication, de-emphasis is the complement of pre-emphasis, in the anti noise system

called emphasis. Emphasis is a system process designed to decrease, (within a band of

frequencies), the magnitude of some (usually higher) frequencies with respect to the magnitude

of other (usually lower) frequencies in order to improve the overall signal-to-noise ratio by

minimizing the adverse effects of such phenomena as attenuation differences or saturation of

recording media in subsequent parts of the system.

Special time constants dictate the frequency response curve, from which one can calculate

the cutoff frequency.

Pre-emphasis is commonly used in audio digital recording, record cutting and FM

radio transmission.

In serial data transmission, de-emphasis has a different meaning, which is to reduce the level of

all bits except the first one after a transition. That causes the high frequency content due to the

transition to be emphasized compared to the low frequency content which is de-emphasized. This

is a form of transmitter equalization; it compensates for losses over the channel which are larger

at higher frequencies. Well known serial data standards such as PCI

Express, SATA and SAS require transmitted signals to use de-emphasis.

FOSTER–SEELEY DISCRIMINATOR

The Foster Seeley detector or as it is sometimes described the Foster Seeley discriminator has

many similarities to the ratio detector. The circuit topology looks very similar, having a

transformer and a pair of diodes, but there is no third winding and instead a choke is used.

Like the ratio detector, the Foster-Seeley circuit operates using a phase difference between

signals. To obtain the different phased signals a connection is made to the primary side of the

transformer using a capacitor, and this is taken to the centre tap of the transformer. This gives a

signal that is 90 degrees out of phase.

When an un-modulated carrier is applied at the centre frequency, both diodes conduct, to

produce equal and opposite voltages across their respective load resistors. These voltages cancel

each one another out at the output so that no voltage is present. As the carrier moves off to one

side of the centre frequency the balance condition is destroyed, and one diode conducts more

than the other. This results in the voltage across one of the resistors being larger than the other,

and a resulting voltage at the output corresponding to the modulation on the incoming signal.

The choke is required in the circuit to ensure that no RF signals appear at the output. The

capacitors C1 and C2 provide a similar filtering function.

Both the ratio and Foster-Seeley detectors are expensive to manufacture. Wound components

like coils are not easy to produce to the required specification and therefore they are

comparatively costly. Accordingly these circuits are rarely used in modern equipment.

FOSTER-SEELEY DETECTOR ADVANTAGES & DISADVANTAGES

As with any circuit there are a number of advantages and disadvantages to be considered when

choosing between the various techniques available for FM demodulation.

ADVANTAGES DISADVANTAGES

Offers good level of performance

and reasonable linearity.

Simple to construct using discrete

components.

Does not easily lend itself to being

incorporated within an integrated

circuit.

High cost of transformer.

As a result of its advantages and disadvantages the Foster Seeley detector or discriminator is not

widely used these days. Its main use was within radios constructed using discrete components.

BALANCED SLOPE DETECTOR:

The circuit shows the balanced slope detector.

It consists of two identical circuits connected back to back.

The FM signal is applied to the tuned LC circuit. Two tuned LC circuits are connected in series.

The inductance of this secondary tuned LC circuit is coupled with the inductance of the primary (or input side)

LC circuit. Thus it forms a tuned transformer.

In fig , the upper tuned circuit is shown as Tc and lower tuned circuit is shown as Tc.

The input side LC circuit is tuned to fc, carrier frequency, T1 is tuned to fc+ δf, which represents highest

frequency. And lower LC circuit R2 is tuned to fc-δf, which represents the minimum frequency of FM signal .

The input FM signal is coupled to T1 and T2 180⁰ out of phase.

The secondary side tuned circuits (T1 and T2) are connected to diodes D1 and D2 with RC loads.

The total output Vout is equal to difference between Vo1 and Vo2, since they subtract.

The fig shows the characteristic of the balanced slope detector. It shows Vout with respect to input frequency.

Balanced Slope Detector

When the input frequency is equal to fc, both T1 and T2 produce the same voltage.

Hence Vo1 and Vo2 are identical and they subtract each other. Therefore Vout is Zero .

This is shown in the fig , when the input frequency I fc+ δf, the upper circuit T1 produces maximum voltage

since it is tuned to this frequency (i.e. fc+ δf) whereas lower circuit T2 is tuned to fc- δf. Which is quite away

from fc+ δf.

Hence T2 produces minimum voltage, hence the output V01 is maximum where V02 is minimum. Therefore

Vout- Vo1- Vo2 is maximum positive for fc+ δf.

When input frequency is fc- δf, the lower circuit T2 produces maximum signal since it is tuned to it.

But upper circuit T1 produces minimum signal. Hence rectified outputs Vo2 is maximum and Vo1 is

minimum, Therefore output Vout=Vo1-Vo2 is maximum negative for fc- δf. This is shown in fig.

For the other frequencies of input, the output Vout is produced according to the characteristic shown in fig. For

example if input frequency tries to increase above fc then Vo1 will be greater than Vo2 and output Vout will be

positive, it is desirable that the characteristic shown in fig. should be linear between fc- δf and fc+ δf , then

only proper detection will take place.

The linearity of the characteristic depends upon alignment of tuning circuits and coupling characteristics of the

tuned coils.

Characteristic of Balanced Slope Detector or S curve

Disadvantages:

1. Amplitude limiting cannot be provided

2. Linearity is not sufficient when compared to slope detector

3. Difficult to align because of three different frequency to which various tuned circuits are to be tuned

4. Tuned circuits is not purely band limited and hence low pass filter of envelope detector introduces

distortion

FM slope detection advantages & disadvantages FM slope detection is not widely used, and yet it has some limited applications. Knowing the

advantages and disadvantages enables the technique to be used where applicable.

ADVANTAGES DISADVANTAGES

Simple - can be used to provide FM

demodulation when only an AM

detector is present.

Enables FM to be detected without

any additional circuitry

Not linear as the output is dependent

upon the curve of a filter.

Not particularly effective as it relies

on centering the signal part of the

way down the filter curve where

signal strengths are less.

Both frequency and amplitude

variations are accepted and therefore

much higher levels of noise and

interference are experienced.

Comparison of AM and FM (Advantages of FM )

i) In AM system there are three frequency components,(the carrier ,LSB and USB

terms) and hence the bandwidth is finite. but FM system has infinite number of

sidebands in addition to a signal carrier. Each sideband is speared by a frequency, fm

hence its B.W is infinite.

ii) In FM, the sidebands at equal distance from fc has equal amplitude s ,ie sideband

distribution is symmetrical about the carrier frequency .The ‘J’ coefficient(Bessel

Coefficients) occasionally have negative values signifying a 1800 phase change for

the particular pair of side band.

iii) The amplitude of frequency modulated wave in FM is independent of modulation

index, whereas the amplitude of modulated wave in AM is dependent of modulation

index.

iv) In AM, increased modulation index increases the sideband power and there fore

increased the total transmitted power .In FM the total transmitted power always

remains constant but an increase in the modulation index increases the bandwidth of

system.

v) In FM system all transmitted power is useful whereas in AM most of the transmitted

power is used by the carrier .But the carrier does not contains any useful information

.Hence the power is wasted.

vi) Noise is very less in FM, hence there is an increase in the signal to noise ratio. There

are 2 reasons for this

There is less noise at frequencies where FM is used.

FM receivers use amplitude limiters to remove the amplitude variation caused by

noise, this feature does not exit in AM.

vii) Due to frequency allocations by CCIR (International Radio Consultative Committee)

there are guard bands between FM stations so that the there is less adjacent channel

interface than in AM.

viii) FM system operated in UHF and VHF range of frequencies s and at these frequencies

the space wave is used for propagations, so that the radius of reception is limited

slightly more than line of sight .It is thus possible to operate several independent

transmitters on the same frequency with considerably less interference than would be

possible with AM.

Comparison of FM & PM

Phase modulation is equivalent to frequency modulation with a modulation index

mp= m Thus holds only when its modulation is sinusoidal.

The spectrum of PM wave is similar to that of an FM wave.

Disadvantages in FM

1. A much wider channel is required in FM up to 10 times as large as that needed by AM

2. FM transmitter and receiver tend to be more complex.

3. Since reception is limited to line of sight, area of reception for FM is much smaller than AM.

Generation of FM

The FM systems have some definite advantages.

i) Firstly, the excessive power dissipation due to extreme peaks in the waveform need

not be bothered.

ii) Secondly, the non liner amplitude distortion has no effect on message transmission,

since the information resides in zero crossing of the wave and not in the amplitude

.How ever phase shift or delay distortion is intolerable.

iii) To avoid this problem a limiter circuit is used to clip the spurious amplitude variation

without disturbing the messages.

The frequency modulated signals can be generated in 2 ways:

i) Direct method of FM

ii) Indirect method of FM.

The prime requirement of FM generation sis a viable output frequency. The frequency is

directly propositional to the instantaneous amplitude of the modulating voltage.

The subsidiary requirement of FM generation is that the frequency deviation is independent

of modulating frequency. However if the system does not properly produce these charctictics,

corrections can be introduced during the modulation process.

Varactor diode modulator

Figure how the characteristics curve of a typical variable capacitance diode (varactor

diode) displaying the capacitance as function of reverse bias.

Transfer characteristics of Varactor Diode

Increasing the bias increase the width of PN junction and reduces the capacitance .It can be

mathematically written as

reversewhereVV

C 1

Bias voltage

Figure shows the basic circuit for FM generation .Here the varactor diode is connected across the

resonant circuit of an oscillator through a coupling capacitor of relatively large value .This

coupling capacitor isolated the varactor diode from eh oscillator as far as DC is connected and

provide an effective short circuit at the operation frequencies.

Basic varactor diode modulator circuit for FM Generation Operation

The D.C bias to the varactor diode is regulated in such a ways that the oscillator

frequency is not affected by varactor supply fluctuations. The modulating signal is fed in

series with this regulated supply and at any instant the effective bias to the varactor diode

equals the algebraic sum of the d.c bias volt ‘V’ and the instantaneous values of the

modulating signal.

As a result, the capacitance changes with amplitude of the modulating signal resulting in

frequency modulating of the oscillator output.

The rate of change of carrier frequency depends on the information signal. Since the

information signal directly controls the frequency of the oscillator the output is frequency

modulated .The chief advanced for this circuit is the use of two terminal devices but

makes its applications limited.

Applications

i) Automatic frequency control

ii) Remote tuning.

Disadvantage of direct method of FM generation

The direct modulators can’t employ crystal oscillators to obtain high frequency stability.

This problem becomes more accurate when the narrow band FM is multiplied by

appropriate frequency multiplying networks in order to achieve the desired wide band

FM

This is because crystal frequency cant be varied as required in FM therefore non crystal

oscillators are used which don’t have sufficient stability for use in commercial system

.More over the reactance modulator has to be stabilized which makes already complex

circuitry even more complex.

Indirect method of FM wave generation

In this method, first the modulating signal is integrated and then phase modulated with

ethers carrier signal, as a result of which some form for FM signal is obtained .Later

frequency multipliers are used to get the desired wideband FM.

To overcome the disadvantage of direct method of FM wave generations, in the indirect

method a stable crystal oscillator is used to generate PM from which narrow band FM is

obtained.

Then suitable frequency multiplying circuits are used to obtain the desired wide and FM.

This method is called the Armstrong method of FM wave generation.

ARMSTRONG METHOD

The block diagram of an Armstrong system is shown.

The system terminates at the output of the combining network; the remaining blocks are

included to show how wideband FM might be obtained.

The effect of mixing on an FM signal is to change the center frequency only, whereas the

effect of frequency multiplication is to multiply center frequency and deviation equally.

The carrier of the amplitude-modulated signal has been removed so that only the two

sidebands are added to the unmodulated voltage.

This has been accomplished by the balanced modulator, and the addition takes place in

the combining network.

The resultant of the two sideband voltages will always be in quadrature with the carrier

voltage.

As the modulation increases, so will the phase deviation, and hence phase modulation has

been obtained.

The resultant voltage coming from the combining network is phase-modulated, but there

is also a little amplitude modulation present.

The AM is no problem since it can be removed with an amplitude limiter.

The output of amplitude limiter is phase modulation. Since frequency modulation is the

requirement, modulating voltage will have to be equalized before it enters the balanced

modulator.

If a frequency-modulated signal fc ± δ is fed to a frequency doubler, the output signal will

contain twice each input frequency.

For the extreme frequencies here, this will be 2fc - 2δ and 2fc + 2δ.

The frequency deviation has quite clearly doubled to ±2δ, with the result that the

modulation index has also doubled.

In this fashion, both center frequency and deviation may be increased by the same factor

or, if frequency division should be used, reduced by same factor.

When a frequency-modulated wave is mixed, the resulting output contains difference

frequencies (among others).

The original signal might again be fc ±δ. When mixed with a frequency f0, it will yield fc –

f0 – δ and fc – f0 + δ as the two extreme frequencies in its output.

It is seen that the FM signal has been translated to a lower center frequency fc – f0, but

the maximum deviation has remained a ± δ.

It is possible to reduce (or increase, if desired) the center frequency of an FM signal

without affecting the maximum deviation.

Since modulating frequency has obviously remained constant in the two cases treated,

modulation index will be affected in the same manner as the deviation.

It will thus be multiplied together with center frequency or unaffected by mixing.

Also it is possible to raise the modulation index without affecting the center frequency by

multiplying both by 9 and mixing the result with a frequency eight times the original

frequency.

The difference will be equal to the initial frequency, but the modulation index will have

been multiplied nine fold.

AMPLITUDE MODULATION FREQUENCY MODULATION

Amplitude of carrier is varied in

accordance to instantaneous amplitude of

Frequency of carrier is varied in

accordance to instantaneous amplitude of

modulating signal modulating signal

It has carrier LSB and USB and so

bandwidth is finite

It has infinite side band and so bandwidth

is infinite

Transmitted power is used by carrier and

so efficiency is low.

All transmitted power is useful and so

efficiency is high.

Noise interference is more Comparatively less

Amplitude of AM wave depends on

modulation index and transmitted power

is varied accordingly

Amplitude of FM wave is independent on

modulation index and hence transmitted

power is constant.

Modulation should be less than 1 No limitation

Used in broadcasting medium frequency

and high frequency.

Used in broadcasting very high frequency

and ultra high frequency.

Adjacent channel interference is more. Adjacent channel interference is less.

Simple to generate Complicated process

Amplitude of modulated signal is varied

but frequency of carrier remains constant.

Amplitude of modulated signal remains

constant but frequency of carrier is

varied.

AM has poor fidelity due to narrow band

width.

FM has better fidelity due to large band

width.

Transmission equipment is simple Transmission equipment is complex

PHASE MODULATION FREQUENCY MODULATION

Phase of carrier is varied in accordance to

instantaneous amplitude of modulating

signal

Frequency of carrier is varied in

accordance to instantaneous amplitude of

modulating signal

It has two side bands and so bandwidth is

finite

It has infinite side band and so bandwidth

is infinite

Maximum phase duration depends on

amplitude of modulating voltage.

Maximum frequency duration depends on

amplitude of modulating voltage and

frequency.

Modulation index is low. Modulation index is high.

Modulation index remains constant. Modulation index is increased if

modulation frequency is decreased.