oo

Sulaimany
Polytechnic
University
Technical College
of Engineering
Communication
Engineering Dept
Second stage
(2018-2019)

Analog Modulation
Prepared by :Kaso Baxtiar Abdelkarim
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Analog Modulation

Contents
Introduction into modulation :...................................................................................................................2
Modulation techniques...............................................................................................................................2
Need for modulation...................................................................................................................................3
Types of modulation...................................................................................................................................6
Amplitude Modulation...............................................................................................................................8
The main disadvantage of using AM system.........................................................................................9
The main advantage of AM system........................................................................................................9
Application of Amplitude modulation..................................................................................................10
expression of AM..................................................................................................................................10
Modulation by single sine wave...........................................................................................................10
Frequency spectrum..............................................................................................................................12
Band width of Am wave........................................................................................................................12
Modulation index..................................................................................................................................13
Power of modulated signal...................................................................................................................19
Modulation by complex information...................................................................................................22
Generation of AM................................................................................................26
AM Transmission...................................................................................................................................30
DSB-FC...................................................................................................................................................30
DSB-SC...................................................................................................................................................31
SSB-SC....................................................................................................................................................32
SSB-RC...................................................................................................................................................34
ISB.........................................................................................................................................................35
VSB........................................................................................................................................................35
Angle Modulation.....................................................................................................................................37
Modulating index..................................................................................................................................39
Deviation ration....................................................................................................................................41
present modulation..............................................................................................................................42
frequency spectrum of FM wave..........................................................................................................42
Bandwidth requirements......................................................................................................................45
Transmission of FM (Wideband and narrowband transmission)..........................................................46
Main advantage of FM.........................................................................................................................49
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Analog Modulation
Main Disadvantage of FM....................................................................................................................50

Introduction into modulation :

Modulation techniques
In general term modulation is proses of producing signals that the
base band or message signal which is could be (video ,voice, radio)
modifies another signal that called carrier signal ,
So carrier usually is high frequency continues AC wave used to carry
intelligence (base band signal ) from the transmitter to receiver ,and
there is different kind of carrier accordance to intelligence to be
transmitted ,as we know the transmitting of high frequency signal(the
base band that normally have high frequency without carrier ) is cheap
and reliable ,while the low frequency signal is can not travel greater
distance and their transmitting is required much greater power than
low frequency need so the advantage of using high frequency carrier is
that it has much greater power that the base band has and supporting
it to get to its destination ,and also high frequency carrier is mean
availability of larger bandwidth of base band signal to be transmitted,
so somehow the advantage of using high frequency carrier is become
the advantage modulation technique too .
Modulation techniques are classified accordance to changing the
parameter of the carrier signal such as ( amplitude ,frequency ,phase )
during the prosses
•Amplitude modulation: in amplitude modulation the amplitude of the
carrier signal is varied according to the amplitude variation of the base
band signal to be transmit ,the frequency and the phase of the carrier
remains constant.
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Analog Modulation
•Frequency modulation: in frequency modulation the frequency of the
carrier is varied according to the base band signal ,amplitude and
phase of carrier remains constant.
•Phase modulation: in phase modulation the phase of signal is varied
according to base band signal .

Need for modulation

There is a few reason that avoid base band signal to be direct transition
so we use the modulation technic for the base band signal, Hence the
advantage of using modulation techniques are as given below:
•Reduces the height of the antenna
•Avoiding missing signal
•Increases the range of communication
•Allows multiplexing of signals
•Allows adjustments in the band width
•Improves the quality of reception

1.Reduces the height of the antenna

The height of antenna required for transmission and reception the
radio waves in radio transmission is a function of wavelength of the
frequency used ,the minimum height of antenna is given as λ/4
The wavelength λ is given as

c
λ=
f
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Analog Modulation
Where c is velocity of light
f is frequency of signal
from the above equation it can be easily noticed at low frequency the
wavelength will be very high so on the antenna height ,for example
consider the base band signal with (f=15KHz) then
λ 3∗108
height of antenna= = =5000 meters
f ∗4 15∗103∗4

This 5000 meters of vertical antennas is unthinkable und unpracticable,

In other hand if we consider a modulated signal with 1MHz frequency
in the broadcast band the height of antenna is given as
λ 3∗108
height of antenna= = =75 meters
f ∗4 10∗106∗4

but this height of antenna is practical and can be installed.

2.Avoid mixing of signals
All sound that humans can hear is between range 20Hz to 20KHz ,
In order to sperate the viruses signals , it is necessary to translate them
all to different portions of the channel ,each must be given its own
bandwidth commonly known as channel bandwidth ,this can be
achieved by taking different carrier frequency for different band passes
as shown in Fig 1.1 Once the signal has been transmitted ,a tuned
circuit at the receiver end with selection of the channel bandwidth is
tuned for therefor modulating band passes by different frequency
carrier avoid mixing of signals .
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Analog Modulation

Figure 1 modulation avoid mixing of signal

3.Increasing range of communication
in low frequency base band signals can not be transmitted from the
receiver to transmitter properly because the radiation and
broadcasting signals in low frequency are poor and get highly
attendance and therefor its quite impossible to directly transmit the
base band signal so we use modulation technique to increase the
frequency of signal to be radiated and thus increase the distance over
which signals can be transmitted properly
4.Allows multiplexing of signals
Modulation is main reason that let multiplexing to be happened ,
Multiplexing means transmitting of one or two signals at the same time
through a same channel the common example of the multiplexing of
signals is television signals that transmitting through same channel or
radio station broadcasting the signals in MW (medium wave ) and SW
(small wave ) bands simultaneously .
The different signals from different station can be separated in the
receiver since the carrier frequencies of those signals are different,
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Analog Modulation
This is commonly known as tuning the receiver to the desired station
,by tuning process the desired signal is selected and at the same time
other signals will be rejected .
5.Allow adjustment in the bandwidth
Bandwidth of the modulated signal may be smaller or larger that the
band pass signal ,signals to the noise ration of the receiver which is
function of the signal bandwidth can thus be improved by proper
control of bandwidth at the modulating stage .

6.Improves quality of reception

One of most useful aim of modulation technique is to reduce noise
effect ton great extent and reducing in noise improving the reception.

Types of modulation
Generally the modulation technique will be classify into two groups
depending on the type of the carrier signal , in continues wave
modulation the carrier signal is the continues sinusoidal wave and in
case that the amplitude of carrier signals is varied accordance to the
base band signal the modulation type will be AM (amplitude
modulation ) and while the angle of the carrier is varied we have angle
modulation and the angle modulation subdivided into two parts FM
(frequency modulation) and PM (phase modulation ) ,in which the
insaneness frequency and phase of the carrier respectively are varied
in accordance with baseband signal .
And the other part which is pulse modulation define that the carrier
signal is periodic sequence of rectangular pulse ,and the pulse
modulation subdivided into two analog and digital type ,
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Analog Modulation
analog pulse modulation the amplitude or duration or position of pulse
is varied accordance to the base band signal to have pulse amplitude
modulation(PAM) and pulse duration modulation,(PDM),pulse position
modulation(PPM) receptively .

Figure 2 represent type of modulation

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Analog Modulation
Amplitude Modulation
amplitude modulation is defined as proses ,in which the amplitude of
high frequency carrier is varied accordance to the instantaneous value
of the modulating signal and the carrier frequency remains constant .
and modulation technique can be represent in both time domain and
frequency domain,
Representation of amplitude modulation process

TD FD

Figure 3 representation of amplitude modulation

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Analog Modulation
The main disadvantage of using AM system

 Because of amplitude modulation wavelength, AM signals can

propagate longer distances.
 For amplitude modulation, we use simple and low cost circuit; we
don’t need any special equipment and complex circuits that are
used in frequency modulation.
 The Amplitude modulation receiver will be wider when compared
to the FM receiver. Because, atmospheric propagation is good for
amplitude modulated signals.
 Bandwidths limit is also big advantage for Amplitude modulation,
which doesn’t have in frequency modulation.
 Transmitter and receiver are simple in Amplitude modulation.
When we take a demodulation unit of AM receiver, it consists of
RC filter and a diode which will demodulate the message signal or
modulating signal from modulated AM signal, which is unlike in
Frequency modulation.
 Zero crossing in Amplitude modulation is equidistant.

The main advantage of AM system

 Adding of noise for amplitude modulated signal will be more when

compared to frequency modulated signals. Data loss is also more
in amplitude modulation due to noise addition. Demodulators
cannot reproduce the exact message signal or modulating signal
due to noise.
 More power is required during modulation because Amplitude
modulated signal frequency should be double than modulating
signal or message signal frequency. Due to this reason more power
is required for amplitude modulation.
 Sidebands are also transmitted during the transmission of carrier
signal. More chances of getting different signal interfaces and
adding of noise is more when compared to frequency modulation.
Noise addition and signal interferences are less for frequency
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Analog Modulation
modulation. That is why Amplitude modulation is not used for
broadcasting songs or music.

Application of Amplitude modulation

 Used to carry message signals in early telephone lines.

 Used to transmit Morse code using radio and other communication
systems.
 Used in Navy and Aviation for communications as AM signals can
travel longer distances.
 Widely used in amateur radio.

expression of AM
practically there is two kind of amplitude modulation depending on the
type of the modulating signal .
Amplitude modulation

Modulation by single sine wave Modulation by complex information

Modulation by single sine wave

this kind of modulation consists of a modulating signal (base band )
which is single sine wave with low frequency compared to the carrier
and this type is non usual type of modulation because its rarely used in
practical ,so in following sections we discus the behaving of this type
and its main characteristic .
And also the modulation process
the instantons values of modulating signal (intelligence)and carrier
signals can be expressed as given below .
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Analog Modulation

Instantons values of modulating signal

e m=E m sin ωm t

Where
em is instantons amplitude
Em is Maximum amplitude
ω m=2 π f m=angular frequency
fm is frequency of modulating signal
Instantons values of carrier signal
e c =Ec sin ωc t

Where
ec is instantons amplitude
Ec is Maximum amplitude
ω c =2 π f c=angular frequency
fc is frequency of carrier signal
Instantons values of modulated signal
From above mathematical equation of modulating signal and carrier
signal we can express the modulates signal I mathematical equation.
E AM =E c + em

¿ Ec + Em sin ω m t ∵ e m=E m sin ωm t

So
e AM =E AM sin θ

¿ E AM sin ωc t
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Analog Modulation
e AM =(E c + Em sin ωm t) sin ωc t

Frequency spectrum
The modulated signals consists of three components ,carrier
signal(carrier frequency ) with side bands or side frequency
Which placed directly in above(USB) and below(LSB) of carrier
frequency as we know it occur in frequency domain ,the said bands
produced from the modulating signal frequency when it goes throw
the process .
f USB=f C + f m

f LSB =f C −f m

f USB stands for (upper side band) , and f LSB is stand for(lower side band).

Figure 4 modulated signal representation in frequency domain

Band width of Am wave
Is defined as frequency range between lowest frequency in
lower side band and highest frequency in upper side band
BW =f USB−f LSB

BW =f C + f m−(f C −f m)

BW =2 f m
 13
Analog Modulation

So the band width required for amplitude modulation is equal to the

twice of the frequency of the modulating signal ,because the USB is
below of carrier frequency by amount of highest frequency of
modulating signal and the LSB is above the carrier by the same amount
hence the result will be as above .
Modulation index
1-calculating modulating index depending on amplitude of the carrier
and modulating signal
And its also called modulation factor or modulation coefficient Gives
the relation between the relative amplitude of modulating signal and
amplitude of unmodulated carrier ,the modulation index is number
lying between 0 and 1,and very often it expressed in percentage and
called percentage modulation if pick to pick of modulating signal is
equal to the unmodulated carrier modulation is said to be 100% ,and if
its less than 100% the modulation is called normal modulation ,in 100%
modulation (full modulation) during the positive value the pick to pick
of the unmodulated carrier is doubled ,where as during the negative
values pick to pick of carrier signal will be equal to zero ,
Em
m=
Ec
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Analog Modulation

Figure 5 normal Amplitude modulation

2-calculating modulating index depending upon the amplitude of the
modulated signal

Em
We know that m= with this relation we can calculate modulating
Ec
index from the amplitude of modulated wave form as shown in fig 2.5

E max −Emin
Em =
2
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Analog Modulation

And Ec =E max −Em

By substitution the value of Em in Ec we get

E max−E min
¿ Emax −( )
2
2 E max−E max + Emin
¿
2
Emax + Emin
Ec =
2

And substitution value of Ec ∈m we get

Em (E ¿ ¿ max−Emax )/2
m= = ¿
Ec (E ¿ ¿ max+ Emax )/2 ¿

Emax −Emax
m=
Emax + Emax

And also percentage modulation expressed as :

Em
percentge of modulation= ×100 %
Ec

Em
Which Ec is known as modulation index (m) .

And know we can derive the equation of instantons amplitude of

modulated signal in frequency domain .
e AM =(E c + Em sin ωm t) sin ωc t
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Analog Modulation
Em
We know that m=
Ec

Em =m E c

Substitution value of Em in main equation we get

e AM =( E c + m Ec sin ωm t) sin ω c t

¿ Ec (1+m sin ωm t) sin ωc t

¿ Ec sin ωc t+m Ec sin ωm t sin ω c t

1
∵ sin a × sin b= [ cos ( a−b )−cos ( a+ b ) ] we get
2

m Ec m Ec
e AM =Ec sin ωc t+ cos ( ω c −ω m ) t− cos ( ωc −ω m ) t
2 2

Carrier LSB USB

And the Fig 4 shows spectrum of AM signal for a single modulating

component .
And the equation show that for each modulating frequency component
gives rise to two side bands frequency component , and the amplitude
of these side bands depend upon the modulating index that the
modulating frequency provide ,and we can notice that the amplitude of
the carrier signal is twice of the side bands amplitude .
Types of modulation : basically there is three types of modulation depending in
modulation index or modulation factor .
Types of modulation
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Analog Modulation
The most widely seen modulation level is for a signal that has 100%
modulation. Under these circumstances the signal level falls to zero and
rises to twice the value with no modulation. In this case the voltage
rises to a maximum of twice the normal level – this means that the
power will be four times that of the quiescent value, i.e. 22 the value of
the no modulation level.

Figure 6 full amplitude modulation

if less than 100% modulation is applied, then the carrier will not fall to
zero, no will it rise to twice the level, but the deviation will be less than
this from the quiescent level. The diagram below shows a level of 50%
modulation, but the principle holds good for any value between 0 and
100% modulation.
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Analog Modulation

Figure 7 normal amplitude modulation

If the level of modulation is raised up above a modulation index of 1, i.e.
more than 100% modulation this causes what is termed over-
modulation. The carrier experiences 180° phase reversals where the
carrier level would try to go below the zero point. These phase reversals
give rise to additional sidebands resulting from the phase reversals
(phase modulation). These sideband caused by the phase reversal
extend out, in theory to infinity. This can cause serious interference to
other users if not filtered. Figure 8 over amplitude modulation

Figure 9 over
Broadcast amplitude
stations modulation
using amplitude modulation take measures to
ensure that the carries of their transmissions never become over
modulated. The transmitters incorporate limiters to prevent more than
100% modulation. They also normally incorporate automatic audio gain
controls to keep the audio levels such that near 100% modulation levels
are achieved for most of the time. In this way the signal sounds clearer
and stronger when demodulated , and the reason look after that the
over modulation is undesirable is that during negative peaks of
modulating signal the peak to peak amplitude of unmodulated carrier
 19
Analog Modulation
should fall below zero since it cannot fall below zero the intelligence
may get suppressed during part of negative cycle ,thus after the
demodulation the part of negative cycle will be clipped and this
introduce distortion which is highly undesirable .

Power of modulated signal

From the above equation we get that each modulated signal consists
of three component the modulated carrier and the side bands ,and if
we notice that the amplitude of the unmodulated carrier and
modulated carrier are the same and however the modulated signal
has two side band component as well so its obviously that the
modulated signal contain a power greater than the carrier have before
the modulation took place , and since the amplitude of side bands
depend upon the modulation index its anticipated that the power of
modulated signal depend upon the modulation index .
Therefor the total power of the modulated signal is equal to sum of the
individual power of the carrier PC and side bands P LSB , PUSB .
PTOTAL=PC + P LSB + PUSB

E C 2 E LSB2 EUSB2
¿ + +
R R R

Where all three voltages represent r.m.s values ,and the R is stand for
the antenna impedance which power of signal is dissipated .
Carrier power
The carrier power is as given as ,
EC 2
PC =
R

( E ¿¿C /√ 2)2
the average power= ¿
R
 20
Analog Modulation

EC 2
PC =
2R

Power in side bands

Similarly the average power for two side bands can be given as
E SB 2
P LSB=PUSB=
R

m Ec 2 m Ec
¿(
2
)×
1 ∵E SB =
2
√2 R

m2 E c 2
P LSB=PUSB=
8R

EC 2
We know that PC =
2R

m2
P LSB=PUSB= PC
4

Total power
EC 2 m 2 E c 2 m2 Ec 2
The average total power PTOTAL=
2R
+
8R
+
8R

E C2 m2 m 2
¿ (1+ + )
2R 4 4

E C2 m2
¿ (1+ )
2R 2

m2
PTOTAL=PC (1+ )
2
 21
Analog Modulation
From the above equation its obviously that maximum total power of
modulated signal is equal to the 1.5 PC and that’s occur in case of full
modulation mean while modulation index is equal to 1.
And also we can derive the expression of modulation index in terms of
the carrier power and total power

m 2 P TOTAL =(1+ m )
2

We know that , PTOTAL=PC (1+ 2

)
PC 2

PTOTAL
√
m= 2(
PC
−1)

Modulation by complex information

In general the wave form of modulating signal will be complex, ,like in
case of speech ,will have large number of component frequency
together with there harmonic ,so its necessary to consider a briefly of
effect of more than one frequency and amplitude components in the
modulating signal
 22
Analog Modulation
Figure 10 modulating by complex information
We assume that the modulating signal contains two frequencies (
fm1∧fm 2),the modulated frequency contain a carrier and two sets of
side frequencies placed symmetrically about the carrier .
Let us consider there are two message signals .
e m 1=E m 1 sin ω m 1 t and e m 2=E m 2 sin ω m 2 t

Total modulating signal will be:

e mtotal =E m1 sin ω m 1 t + Em 2 sin ω m 2 t

and the amplitude of modulated signal will be :

E AM =E C + ¿+ Em 2 sin ω m 2 t ¿

And the total modulated wave will be:

e AM =¿+ Em 2 sin ω m 2 t ¿ ¿ sin ω c t

¿ EC ( 1+m 1 sin ω m 1+ m 2 sin ω m 2) sin ω ct

Since we know that

1
sin a × sin b= [ cos ( a−b )−cos ( a+ b ) ] we get
2
m1 E C m1 EC
e AM =EC sin ω c t + cos ( ω c + ωm 1 ) t− cos ( ωc +ω m 1 ) t
2 2
+ m 2 EC m 2 EC
cos ( ω c + ωm 2 ) t− cos ( ωc +ω m 2) t
2 2

so we get that with two modulating signal we get four additional

frequencies two upper sidebands frequencies ( f c + f m 1)and ( f c + f m 2)
and two lower sidebands ( f c −f m 1)and ( f c −f m2) assuming f m 1< f m 2
we can draw the frequency spectrum of modulated signal
 23
Analog Modulation

Figure 11 frequency domain of modulated complex information

Total power in AM wave
The total power of AM wave can be determine by sum of power of the
individual sidebands and carrier ,
So
PTotal =Pc + PUSB 1+ PUSB 2 + P LSB 1+ P LSB 2

2 2 2 2 2 2 2 2 2
( E¿¿ c √ 2) m1 E c m1 E c m2 Ec m2 Ec
¿ + + + + ¿
R 8R 8R 8R 8R

Where m1 is modulating index for first signal

m2 is modulating index for second signal
R is characteristic resistance of antenna
Ec 2 m12 m22
∵ PTotal =
R ( 1+
2
+
2 )
m 12 m22
(
PTotal =Pc 1+
2
+
2 )

Total modulating index (mt )

 24
Analog Modulation
When several frequencies simultaneously modulate amplitude of the
carrier the combined modulating index is the square root of the
quadratic sum of the individual modulating indexes as follow
m total=√ m 12+ m22 +m 32+ m n2

The combination coefficient of modulation can be used to determine

the total sidebands and transmitted power as shown below .

P c m t2
Pusbt =Pisbt =
4

Pc m t2
And Psbt =
2

mt 2
Pt =Pc (1+ )
2

Where
Pusbt = total upper sideband power
Pisbt = total lower sideband power
Psbt = total sideband power
Pt = total transmitted power

Band width
The band width of AM with several modulating signal is given by

BW =2 f max
 25
Analog Modulation
Where f max is the maximum modulating frequency .

Transmission efficiency
Is defined as ration of transmitted power which care the intelligence
(power of USB,LSB) to the total power transmitted
P LSB + PUSB
Transmission efficiency ŋ=
P TOTAL

m2 m2 m2
[ PC + PC ]
4 4 2
ŋ= 2
=
m m2
[1+ ] PC 1+
2 2

m2
ŋ=
2+m 2

and we can express percentage Transmission efficiency as given below

m2
% ŋ= ×100 %
2+ m 2

Generation of AM
 26
Analog Modulation

Basically there is two kind of AM generation depending upon the

amplifier stage that AM wave take .
Low level modulation :In this technique ,the generation of AM waves
takes the place in the initial stage of the amplification ,in low power
level the generated AM signal is then amplified using numbers of
amplifiers stages ,or we can say a low level amplitude modulation
system, the modulation is done at low power level. At low power levels,
a very small power is associated with the carrier signal and the
modulation signal. Because of this the output power of modulation is
low. Therefore the power amplifiers are required to boost the
amplitude modulated signals up to the desired output level.

Figure 12 low level modulation

in figure it is clear that modulation is done at low power level. After this
the amplitude modulated signal is applied to a wide band power
amplifier . A wide band power amplifier is used just to preserve the
sidebands of the modulated signal. Amplitude modulated systems ,
employing modulation at low power levels are also called low level
amplitude modulation transmitters.
Square-law diode modulation and switching modulation are examples of
low-level modulation.
 27
Analog Modulation

High level modulation : In a high-level amplitude -modulation system,

the modulation is done at high power level. Therefore, to produce
amplitude modulation at these high power levels, the base band signal
and the carrier signal must be at high power levels. In block diagram of
figure the modulating signal and carrier signal are first power amplified
and then applied to AM  high level modulator. For modulating signal
the wide band power amplifier is required just to preserve all the
frequency components present in modulating signal.

Figure 13 high level modulation

On the other hand for carrier signal , the narrow band power amplifier
is required because it is a fixed frequency signal. The collector
modulation method is the example of high level modulation. Before we
discuss low level and high level modulation methods in detail, we shall
establish the fact that a non- linear resistance of non linear device can
be made to produce amplitude modulation when two different
frequencies are passed together through it.
 28
Analog Modulation

Figure 14 comparison between low and high level modulation

No. Parameters Low level modulation High level modulation

1 Point at which Modulation takes place Modulation takes place

modulation takes place at initial stages of at final stages of
amplification amplification
2 Power Level Modulation circuity has Modulation circuity has
to handle low power to handle high power
3 Complexity Modulation circuity is Modulation circuity is
simple as handle low quite complex as it has
power to handle high power
4 Prime factors in design Simplicity is prime Prime requirement is
requirement high efficiency and low
distortion
5 Audio power Low audio power is High power is required
required to produce to produce modulation
modulation
6 Design requirement to Each amplifier stage This is not the case
amplifier stages following modulation with high level
must handle sidebands modulation because in
power as well as the modulation takes
carrier , all this place in output stage
subsequent amplifiers
 29
Analog Modulation
must have sufficient
band width for
sideband frequencies
7 Amplifier used Transistor and Op- Vacuum tubes and
amps power transistor
8 Efficiency Lower than high level Very high
modulation
9 Amplifier used Linear amplifier such as High efficient class C
class A amplifier is amplifier are used
used because all stages
must be capable of
handling amplitude
variations caused by
the modulation
10 Applications Typically used in TV High power broadcast
transmitters AM transmitters

AM Transmission
the AM transmission are classified accordance to generation method

Forms of AM

double side band Single side band independenet vestigial sid

(DSB) (SSB) side band (ISB) band (VSB

double sideband Single sideband

full carrier full carrier
(DSB-FC) (SSB-FC)

Single sideband
suppresed
double sideband carrier (SSB-SC)
suppresed
carreir (DSB-SC) Single sideband
Figure 15 transmission form ofreduced
AM carrier
(SSB-RC)
 30
Analog Modulation
DSB-FC
is kind of AM form which both sidebands and the carrier are
transmitted to the receiver, because the baseband modulated by
single sine wave ,the resulting signal consists of three frequencies ,the
original carrier and two sidebands f m ± f c .

DSB-SC
we know that that the carrier signal do not convey any information
,the information carried by the sidebands but the two sidebands are
image for each others and they are carrying the same information,
considering this fact we can say that to recollecting the original signal
and the end of the receiver its not necessary to transmit all the three
frequencies ,to make AM more efficient its possible to simply suppress
the carrier and since the carrier dose not have any information there is
no reason to be transmitted ,after the carrier is removed the remaining
signal will contain upper and lower sidebands such signal referred to
double sideband suppressed carrier signal (DSB-SC),with this technique
no power will be wasted on the carrier while all saved power will be
added to sidebands to for stronger signal over long distance .
 31
Analog Modulation

Figure 16 double sideband with suppressed carrier modulation

Fig 16 show the DSBSC wave AM signal ,here the carrier modulated by
single sine wave and the carrier have been suppressed before the
transmission ,and the resulting is algebraic sum of lower and upper side
band .

Figure 17 DSB-SC frequency domain

As in figure shown the doted line represent suppressed carrier
SSB-SC
since both of sidebands carrying same information its obviously that
DSB is redundant because the same information have been transmitted
twice ,once of each sideband ,then its absolutely no reason to transmit
both sidebands in order to convey the information ,so we can simply
suppress one of the sidebands and the output signal will be single
sideband .
Figure 18 single side band with suppressed carrier
 32
Analog Modulation

Advantage of SSB over DSB and AM

1.The spectrum space occupied by the SSB signal is f m , which is only
half that of AM and DSB signals. In other words, we can say that SSB
required half the bandwidth required of AM and DSB signals. This
reduction in frequency spectrum or bandwidth allows more signals to
transmit in the same frequency range without interfering each other.
2. Due to suppression of carrier and one sideband power is saved and
saved power can be used to produce a stronger signal that will carry
farther a will be reliably received at greater distances.
3. When band width is less, the receiver circuits can be made with a
narrower bandwidth, filtering out most of the noise. We know that, the
 33
Analog Modulation
SSB signal has less bandwidth than an AM or a DSB signal, hence,
logically there will be less noise on it.
4. Fading is less in SSB system. Fading means that a signal alternately
increases and decreases in strength as it is picked up by the receiver. It
occurs because the carrier and sideband may reach the receiver shifted
in time and phase with respect to each other. The carrier and sideband
signals have different frequencies, which are affected by the
ionosphere in different ways. The ionosphere bends the carrier and
sideband signals at slightly different angles, resulting fading.
Disadvantages of SSB
1. It will be seen later, the generation and reception of SSB signal is a
complex
2. Since carrier is absent, the SSB transmitter and receiver need to have
an excellent frequency stability. A little change in frequency hampers
the quality of transmitted and received signals. Thus SSB is not used for
the transmission of good quality of signal such as music signal. It is
usually used for process transmission of speech signal.
Applications of SSB In practice
SSB is used to save power in applications where such a power saving is
required, i.e. in mobile systems. Single-side band modulation is also
used in applications in which bandwidth requirements are low. Point-
to-point communications land, air, and maritime mobile
communications, television, telemetry, military communications, radio
navigation, and amateur radio are the greatest users of SSB in one form
or another.
 34
Analog Modulation
SSB-RC
this technique the carrier has to be suppressed only after the
sidebands have been produced and after that the carrier has to be
regenerated at the receiver to achieve demodulation for which
presence of carrier is also essential in the same manner and its during
modulation this carrier has to be reinserted as the same frequency and
phase that the carrier has in the transmitter ,and instead of completely
suppress the carrier it may be transmitted along with the (SSB) or (DSB)
so that the proper synchronization can be achieved between the two
type carrier and the carrier is used for this is at very low level of power
and this also called carrier power ,and this technique also requires
excellent frequency stability on the part of both transmitter and
receiver because any frequency shift anywhere along the chain of
event through which the information must pass, will caused an equal
frequency shift received to the signal .
So the good quality music is difficult to transmit via pilot carrier and
the speech will also be impaired unless long term of stabilities ,pilot
carrier can be described by is technique that attenuated carrier signal
that added to transmission after the unwanted sidebands have been
removed ,the carrier is normally reinserted at a level of 16 Db or 26 dB
And below this values it would have had if it had not been suppressed
In the first place and it provide reference signal to help a demodulation
in the receiver , and the receiver can then use automatic frequency
control (AFC) .
Such system is widely used in transmarine point to point radio
telephone and in marine mobile communication especially at the
distress frequencies ,for high density traffic and in short or long-haul
and in deferent modulation technique are used as they are known as
 35
Analog Modulation
Frequency or time division multiplex.
ISB
this technique is essential consist of two (SSB) channels added to the
form two sidebands around the reduced carrier however each
sidebands are completely independent from the other ,it can
simultaneously convey a totally different transmission for example the
upper sideband can be used to hold the telephony while the lower
sideband carrier the telegraphy ,and also its not advisable to mix the
telephone and telegraphy channels in one sideband .and this technique
is commonly used at low or medium density point to point
communication
VSB
the SSB is not appropriate way of modulation when the message signal
contain a significant component in extremely low frequencies ,because
in such cases the upper and lower sidebands meet at the carrier
frequency and its difficult to isolate one of sideband ,to overcome this
difficulty the modulation technique know as vestigial sideband
modulation (VSB) is used in this technique one sideband is passed
almost completely whereas just a trace ,or vestige of the other
sideband is retained ,this compromise between SSB modulation and
DSBSC modulation , the television signal contain significant component
at the extremely low frequencies hence vestigial sideband modulation
used in television transmission .
 36
Analog Modulation

Figure 19 vestigial sideband representation in time domain

A tv signal consists of the picture (Video) signal and the audio signal
both having different carrier frequencies ,the audio carrier is frequency
modulated whereas the picture carrier is amplitude modulated .
heaSo the VSB is used when the transmission of low frequency are
important but the bandwidth required for double -sideband
transmission is unavailable or uneconomical .
Figure 20 vestigial sideband representation in time domain

Angle Modulation
We have seen that in amplitude modulation, the amplitude of the
carrier is varied according to the amplitude of the information
(modulating) signal. In angle modulation, the frequency or phase of the
carrier is varied according to the amplitude of the information
(modulating) signal. When information signal is used to vary the
 37
Analog Modulation
frequency of carrier signal, we have frequency modulation. On the
other hand, when information signal is used to vary the angle of the
carrier signal, we have phase modulation . As both modulations are
related to time they have some very similar properties. However, there
are some important differences also.
Now we are going to mention these two modulation techniques with
their similarities and differences.
The sinusoidal carrier wave has basically three characteristics, viz.
amplitude, frequency, and phase. In amplitude modulation, the
amplitude of the sinusoidal carrier is slowly varied in accordance with
the baseband signal required to be modulation transmitted .
Instead of amplitude, either frequency or phase of the sinusoidal carrier
can be changed according to the message, keeping the amplitude
constant. This is another method of modulation, termed "angle
modulation". In angle modulation the phase angle (0) of a sinusoidal
wave is varied with respect to time. An angle-modulated wave is
expressed mathematically as,
st =v c cos [ω c t +θt ]

where
st =angle modulated wave
v c=peak carrier amplitude
ω c =2 πfc =carrier radian frequency
θt = instantons phase deviation in radians
The angle modulation is expressed mathematically as
θt =F [m(t )]

Where m(t) is modulating signal and

 38
Analog Modulation
θt is the instantons phase deviation
Whenever, the frequency of a carrier is varied, the phase is also varied,
and vice-versa. Therefore, frequency modulation and phase modulation
both occur whenever either form of angle modulation is performed.
The difference between frequency and phase modulation lies in which
property of the carrier (the frequency or the phase) is directly varied by
the modulating signal and which property is indirectly varied. The FM is
generated when the frequency of the carrier is varied directly in
accordance with the modulating signal, and PM is generated when the
phase of the carrier is varied directly in accordance with the modulating
signal , the change in frequency and phase of the carrier The change is
frequency (∆ f ) is referred to as frequency deviation and change in
phase (∆ θ ) is referred to as phase deviation. Frequency deviation is the
relative displacement of the carrier frequency in hertz, and phase
deviation is the relative angular displacement in radians of the carrier in
respect to a reference phase.
angle modulation

frequency modulation phase modulation

 39
Analog Modulation

Figure 20 Analog modulation

Modulating index
Modulating in FM is generally expressed in term of modulating index
and the modulating index is the ration of frequency deviation to the
modulating frequency
frequency deviation
m f =modulating index=
modulating frequency

∆f
mf =
fm

The table below show that how modulating index is to the number of
effective sidebands :
Modulation 0.5 1 2 3 4
index
Numbers of 2 3 4 6 7
effective
 40
Analog Modulation
sidebands on
either side of
the carrier

figure 21 and 21 show effect of modulation index on significant numbers of sidebands

When the modulation index is less than 0.5, i.e. when the frequency
deviation is less than half the modulating frequency, the second and
higher-order sideband components are relatively small, and the
frequency band required to accommodate the essential part of the
 41
Analog Modulation
signal is the same as in amplitude modulation On the other hand, when
the modulation index exceeds unity, there are important higher-order
sideband components contained in the wave. As modulation index
reduces, number of effective sidebands bandwidth requirement less.
This results in the reduction in
Deviation ration
In FM broadcasting, the modulating, signal causes the carrier frequency
to shift above and below its center frequency. The Federal
Communications Commission (FCC) has laid down rules regarding the
carrier shift. The maximum frequency deviation should not exceed + 75
kHz from the center frequency. In addition, the FCC placed a 25 kHz
"guard band" on either side of the carrier's center frequency. Thus 75
kHz either side of the center frequency equals 150 kHz, plus a 25 kHz
guard band either side, allows a total of 200 kHz for one FM channel. In
commercial FM broadcasting, the maximum modulating frequency is
limited to 15 kHz. It is only 5 kHz in commercial AM broadcasting. Since
a limit is placed on the amount of deviation and maximum modulating
frequency, there is always a corresponding value of the modulation
index for the maximum modulating frequency. This is known as
"deviation ratio". For example, in commercial broadcasting the
maximum deviation of 75 kHz divided by the maximum modulating
frequency of 15 kHz results in a corresponding deviation ratio of 5. Thus

Maximum frequency devation

Deviation ration=
Maximim modulating frequency

present modulation
Percent Modulation The term "percent modulation" as it is used in
reference to FM. It is the ratio of the actual frequency deviation
produced by the modulating signal to the maximum allowable
 42
Analog Modulation
frequency deviation. The band between 88 MHz and 108 MHz has been
allocated by FCC for commercial FM broadcast. FCC sets a maximum
frequency deviation of 75 kHz for FM broadcast stations in this band.
sound accompanying the picture in TV broadcast is transmitted using
FM technique. For this sound transmission, FCC allows a maximum
frequency deviation of Thus, 100% modulation corresponds to 75 kHz
for the commercial FM The 25 kHz. broadcast and 25 kHz for TV sound
broadcast.
∆ f actual
M= ×100
∆ f maximum

frequency spectrum of FM wave

A For an AM wave, it is easy to tell what frequencies are present in the
modulated e. Unfortunately, the situation is quite complicated,
mathematically for FM. Since the equation is the sine of a sine, the only
solution involves the use of Bessel functions

Using Bessel function, it may be shown that the equation for FM

e= A sin ⁡[ωc +mf sin ω m t ]

It can be expand to obtain e= A { j 0 (m¿¿ f ) sin [ ωc t ] }¿

 43
Analog Modulation
+ j 1( m¿¿ f )¿[sin ( ω c + ωm ) t−sin ( ω c −ω m ) t ]

+ j 2 (m¿¿ f )¿[sin ( ω c + 2ω m ) t−sin ( ω c −2 ωm ) t ]

+ j 3 (m¿¿ f ) ¿[sin ( ω c + 3 ωm ) t−sin ( ω c −3 ωm ) t ]
+ j 4 (m¿¿ f ) ¿[sin ( ω c + 4 ω m ) t−sin ( ω c −4 ωm ) t ]
it is seen that the wave consists a carrier and apparently infinity
number of pairs of the sidebands each produced by j coffinite.
The amplitude of the different frequency components for the case of
sinusoidal adulation depends upon the modulation index mf , and can be
either calculated with the aid of a table of Bessel's functions or
obtained from the graph ( Fig 22 ) showing the relation of Bessel
functions with modulation index .

Figure 22 Bessel function

Observation
The following observations are made with Bessel functions
1.In AM, only three frequencies, viz the carrier and two side
frequencies are present in the output. However, FM has an infinite
number of sidebands, as well as the carrier in the output. The
 44
Analog Modulation
sidebands are separated from the carrier by etc. and the frequency
separation between successive sidebands is the modulating frequency
fm .

2. The sideband distribution is symmetrical about the carrier frequency,

i.e. the sidebands at equal distances from the carrier frequency have
equal amplitudes.
3. Some of the Bessel coefficients may have negative values, indicating
a 180 degree phase change for that particular pair of sideband .
4. How many sideband components have significant amplitudes is
dependent on the modulation index .
5. As modulation index increases, the value of the particular J
coefficient increases. Then, assuming that the frequency deviation is
constant, the relative amplitude of distant sidebands will increase as
the modulation frequency is lowered .
6. In AM, the sideband power and the total transmitted power
increases with the depth of modulation. However, in FM the total
transmitted power always remains constant. The increased depth of
modulation requires more bandwidth for transmission .
7. For certain values of modulation indices, called eigen values, the
Bessel coefficient Jo(m r) for the carrier term has zero value and the
entire FM wave consists of sideband components of various orders. The
carrier disappears for the modulating index mf ,equal to
2.4,5.5,8.611.8,and so on .
Bandwidth requirements
To find the bandwidth occupied by FM wave, the useful rule is the
modulated wave contains sideband components of importance on
either side of the either wave over a frequency interval approximating
the sum of 'the frequency deviation and the modulating frequency. The
 45
Analog Modulation
total bandwidth in which most of the energy of the wave is contained is
then twice this value.
BW FM =2 [mf +1]fm

frequency devation
¿ 2[ +1] fm
modulating frequency
∆f
¿ 2[ +1]fm
fm
BW FM =2 [∆ f + fm]

And also the modulating frequency changes usually by a factor

1:100,and the bandwidth occupied by the spectrum alters very little ,so
these explain why frequency modulating is sometimes referred to as a
constant=bandwidth system.
And also its important to determine the maximum bandwidth of the
FM signal depending upon the maximum values of modulating
frequency and frequency deviation present in the complex modulating
wave .
So
BW =2[mf +1]fm

BW =2[δ + fm max]

The above equation using for determining maximum bandwidth

required to transmit the FM wave is known as Carson rule .
Also the bandwidth of PM can be determined by using below equation .
The bandwidth of PM =2[ M p +1]f m

BW PM ¿ 2[k p Em +1] f m ∵ m p=k p E m

 46
Analog Modulation
Transmission of FM (Wideband and narrowband transmission)
by convention, wideband FM is defined as that in which the modulation
index usually is more than unity. The maximum permissible frequency
deviation in such B system is 75 kHz with modulating frequency range
from 30 Hz to 15 kHz. With a large frequency deviation noise is better
suppressed in wideband FM system .However this system requires
quite a large bandwidth. These considerations have resulted in
wideband systems which are being used for entertainment
broadcasting There are certain other communication services where
quality of sound is not so much important as it is in F.M. broadcasting
These services are F M. mobile communication services, such as police
wireless ambulances, short-range ship-to-shore communication, etc. In
these services, speech is transmitted where intelligibility of speech is
more important than its quality. The higher audio frequencies are
attenuated, similar to telephone systems, but the resulting speech
quality is still quite adequate Maximum frequency deviations of 5 to 10
kHz are allowed and the channel bandwidth up to 30 kHz is used. The
modulation index in narrowband FM is near unity. The maximum
modulating frequency used is typically 3 kHz and maximum frequency
deviation is normally 5 kHz. Narrowband systems with even lower
frequency deviations are in use. Similar to other FM transmission
systems, narrowband FM system uses pre-emphasis and de-emphasis
Let us compare wide and narrowband FM based on following
parameters
1.Modulation index: For wideband it is more than unity while for
narrowband it is less than unity but very close to unity
2. Maximum deviation :The maximum permissible frequency deviation
for wideband is 75 kHz while for narrowband it is 5 kHz
 47
Analog Modulation
3. Maximum modulation index: For wideband, maximum modulation
index is between 5 to 2500 while for narrow band it is just greater than
unity
4. Range of modulating frequency: For wideband it is 30 Hz to 15 kHz
while for narrowband it is 30 Hz to 3 kHz
5. Bandwidth: The bandwidth of wideband is much higher than the
narrowband For wideband it is about 15 times greater than for
narrowband
6. Noise: The noise gets better suppressed in wideband than the
narrowband
7. Applications: The wideband is suited for entertainment broadcasting
due to its high quality transmission. The narrow band is used in services
where quality of sound is not so much important but speech is more
important. Such communication services are police wireless,
ambulances, short range ship to shore communication etc.
FM PM
1 The equation for FM The equation for PM
wave is wave is
v= A sin [ w c t+ m f sin wm t ] to v= A sin [ w c t + mp sin w m t ] .
instantaneous amplitude
of the modulating.
2 he frequency deviation The phase shift of the
is linearly proportional carrier is linearly
signal. proportional to
instantaneous amplitude
of the modulating signal.
3 Frequency modulation is Phase modulation is
direct method of indirect method of
producing FM signal. producing FM.
4 The modulation index of The modulation index is
an FM signal is the ratio proportional to the
 48
Analog Modulation
of the frequency maximum amplitude of
deviation to the the modulating signal.
modulating frequency.
5 To have better quality of The amount of frequency
transmission and shift produced by a
reception of higher audio phase modulator
frequencies pre- increases with the
emphasis and de- modulating frequency.
emphasis circuits are Hence an audio equalizer
used. Is required to
compensate this.
6 Amplitude of the FM Amplitude of the PM
wave is constant. wave is constant.

7 Noise is better Noise immunity is

suppressed in FM system inferior to that of FM .
as compared to PM
system .
8 FM is mainly used for FM PM is used in mobile
broadcasting used for communication system .
entertainment purpose .
Figure 23 comparing between FM and PM

Main advantage of FM
1.The amplitude of FM wave is constant, and hence transmitted power
is constant, independent of modulation depth; whereas in A M,
modulation depth controls the transmitted power. This means that low
level modulation can be employed in FM transmitters. Since the power
is constant, the FM calculated signal, after low level modulation, can be
amplified by more efficient class-C.
 49
Analog Modulation
2. In AM, 67% of transmitted power is in the carrier, for maximum
modulation depth, which is totally wasted as carrier does not carry any
intelligence. In FM, all the transmitted power is useful.
3. FM receivers use amplitude limiter circuits to eliminate the
amplitude actions caused by noise. Due to this, FM reception is more
immune to noise than AM reception.
4. By increasing the frequency deviation in it is possible to improve
signal-to-noise ratio AM does not have this feature since in AM,
modulation depth cannot be increased beyond 100% .
5. For commercial FM transmitting stations a guard band of frequencies
is allocated, this reduces adjacent channel interference, as compared to
AM .
6. FM broadcasting is in the appear VHF and UHF frequency ranges. In
these frequency ranges, there is inherently less noise than in the MW
an SW ranges allotted to AM broadcast .
7.Since FM operates in VHF and UHF range, the propagation is line of
sight propagation by space wave. The radius of operation of an FM
transmitter is limited to radio horizon allows several independent FM
transmitters to work on the same frequency with very little interference
than would be possible with AM .

Main Disadvantage of FM

1. For transmission of FM signal, a much larger bandwidth is required

than that quired to transmit AM signal .
2. FM transmitter and receiver are quite complicated, especially from
the point of view of servicing and fault finding/repairing of FM
receivers.
 50
Analog Modulation
3. Since FM reception is limited to radio horizon up to line of sight, the
service area of FM transmitter is much less than that for AM Although
this is advantageous for reduction of adjacent char is a disadvantage for
FM mobile communication over a wide area
Comparison of PM and FM
No. FM AM
1 The equation for FM wave Is The equation for AM wave Is
is v= A sin [ w c t+ m f sin wm t ]. v=Ec [1+ msin ω m t]sin ω c t .
2 The modulation index can have The value of modulation index is
value either less than one or always between zero and one.
more than one.
3 Since in FM, amplitude of the Transmitted power is
carrier is constant, the dependent upon modulation
transmitted power is constant, m2
independent of the modulation index pT = pC [1+ ]
2
index.
4 The modulation index n an AM signal, only two
determines the number of sidebands are produced, for any
significant pairs of sidebands in value of modulation index.
an FM.
5 The amplitudes of the carrier The amplitudes of the
and side bands vary with the sidebands is dependent on the
modulation index and can b modulation index, and is always
calculated with Bessel less than the amplitude of
functions. carrier.
6 The carrier or sideband The sideband amplitude is
amplitudes are zero at some never zero for any value of
modulation indices. modulation index greater than
zero.
7 The bandwidth of an FM signal The bandwidth of an AM signal
is proportional to the is twice the highest modulating
modulation index. frequency.
8 For FM, the of modulation is the For AM, modulation is the ratio
ratio of the actual frequency of amplitude of modulating
deviation and the maximum voltage to the amplitude of the
permissible frequency deviation carrier multiplied by 100.
multiplied by 100.
9 The main advantage of FM over The AM system is more
AM is its noise immunity, as susceptible to noise and more
limiter stage in FM receiver clips affected by noise than FM.
off noise signals.
10 The capture effect in FM allows When two AM signals occupy
the strongest signal on a the same frequency, both
 51
Analog Modulation
frequency to dominate without signals will generally be heard
interference from the other regardless of their relative
signal. signal strength.
11 In FM, greater transmitter The efficiency of AM is less than
efficiency can be realized using that of FM due to use of class-B
class-C amplifiers, as amplitude amplifier The bandwidth
of FM signal is constant. required to transmit AM signal
is much less than that of FM
typically 10-kHz in AM
broadcasting.
12 The bandwidth of FM signal is he bandwidth required to
much under than the transmit AM signal is much less
bandwidth of AM. The than that of FM typically 10-kHz
bandwidth of a typical FM in AM broadcasting.
channel is 200 KHz.
13 The circuits to produce and The demodulation of AM signal
demodulate FM re usually more is very easy practically by use of
complex and expensive than a diode which is very simple in
AM circuits. operation and cheap cost wise.
 52
Analog Modulation

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Macdonald and Janes
Leon, W.C.(2007).Digital and analog communication systems .Upper saddle River , New
jersey :Pearson Prentice Hall
Manoj, K . M., Manisha.,& Manisha.(2007).Analog communication systems. Karol Bagh,
New Delhi :Tech india publication series.
Chitode,J.S.(2009).Signals and systems. Shaniwar Peth, Pune, India: Technical publication
pure.
Uday,A.B.,&Atul,P.G.(2009).Analog communication. Shaniwar Peth, Pune, India:
Technical publication pure.
Anil,A.M(2008).Electronic and communication simplified. Darya Ganj, New Delhi: Khanna
Publishers