1) Amplitude modulation (AM) is described which involves modifying a carrier wave by a modulating signal. Block diagrams of basic AM transmitters and receivers are shown. 2) The document discusses the modulation index and calculations for AM signals including upper and lower sidebands. It also covers power calculations and transmission efficiency. 3) Key aspects of AM including double sideband suppressed carrier (DSB-SC), single sideband (SSB) and vestigial sideband (VSB) are introduced.

1. EEE 3513
COMMUNICATION PRINCIPLES
Amplitude Modulation (AM)

2. 2

3. 3
BLOCK DIAGRAM OF A TRANSMITTER
Modulating
Signal
Audio
Amplifier
Modulator
RF
Amplifier
Carrier
Signal
Transmitting
Antenna

4. 4
BLOCK DIAGRAM OF A RECEIVER
RF
Amplifier
Mixer
Local
Oscillator
Intermediate
Frequency
Amplifier
Demodulator
Audio
Amplifier
Destination
Receiving Antenna

5. 5
ANALOG
MODULATION

6. BASEBAND TRANSMISSION
Transmission without frequency shifting
Baseband signal is the information either in a digital or
analogue form.
Transmission of original information whether analogue or
digital, directly into transmission medium is called
baseband transmission.

7. 7
Microphone
Voice
Audio
Amplifier
Audio
Amplifier
Speaker
Voice
Wire
EXAMPLE: INTERCOM

8. 8
Baseband signal is not suitable for long distance
communication. Not suitable for radio/microwave and
satellite communication.
Hardware limitations, requires very long antenna.

9. 9
Baseband signal is an audio signal of low frequency. For
example voice, range of frequency is 0.3 kHz to 3.4 kHz.
The length of the antenna required to transmit any signal at
least 1/10 of its wavelength (λ). Therefore, L = 100km
(impossible!)

10. 10
Interference with other waves
Simultaneous transmission of audio signals will
cause interference with each other. This is due to
audio signals having the same frequency range
and receiver stations cannot distinguish the
signals.

11. 11
MODULATION
 Modulation – defined as the process of modifying a
carrier wave (radio wave) systematically by the
modulating signal.
 This process makes the signal suitable for
transmission.
 Resultant signal – modulated signal.

12. 12
2 TYPES OF MODULATION
Analogue Modulation.
Amplitude.
Angle (FM , Phase)
Digital Modulation.
ASK – Amplitude Shift Keying.
FSK – Frequency Shift Keying.
PSK – Phase Shift Keying.
)]
(
[
cos
)
( t
t
E
t
v c
c
c
c 
 


13. 13
PURPOSE OF MODULATION
By using high frequency carrier signal, the information
(e.g. voice) can travel and propagate through the air at
greater distances and shorter transmission time.
Also, high frequency signal is less prone to noise and
interference. Certain types of modulation have the
useful property of suppressing both noise and
interference.
For example, FM use limiter to reduce noise and
keep the signal’s amplitude constant.

14. 14
Amplitude Modulation (AM)
Objectives:
Recognize AM signal in the time domain, frequency
domain and trigonometric equation form.
Calculate the percentage of modulation index.
Calculate the upper sidebands, lower sidebands and
bandwidth of an AM signal by given the carrier and
modulating signal frequencies.

15. 15
Calculate the power related in AM signal.
Define the terms of DSBSC, SSB and
VSB.
Understand the modulator and
demodulator operations.

16. 16
 Modulation
 The alteration carrier Signal in accordance with modulating
signal.
 Carrier Signal
 Sinusoidal wave,
 Modulating Signal/Base band
 Information signal,
 Modulated Wave
 Higher frequency signal which is being modulated
t
E
t
v c
c
c 
cos
)
( 
t
E
t
v m
m
m 
cos
)
( 
)
(t
vAM )
(t
vFM
)
(t
vPM

17. 17
Full AM - DSB-FC
• AM modulation is a fundamental modulation process in
communication system.
• Carrier frequency signal >> than modulating frequency signal.
=> fc >> fm.
• Modulator is used to generate AM signal, amDSB-FC(t). It is shown
in block diagram below.
  t
t
v
E
t
v c
m
c
AM 
cos
)
(
)
( 

vm(t)
vc(t)
AM Modulator
Modulating
signal
Carrier signal
AM modulated signal

18. 18
Let
and
therefore, amDSBFC signal can be expressed:
 
 
  t
t
E
E
t
v
t
t
v
E
t
v
c
m
m
c
AM
c
m
c
AM



cos
cos
)
(
cos
)
(




t
E
t
v m
m
m 
cos
)
( 
t
E
t
v c
c
c 
cos
)
( 

19. 19
AMPLITUDE MODULATION
Vc
- Vc
Vm
- Vm
Vam
- Vam
 
 
  t
t
E
E
t
v
t
t
v
E
t
v
c
m
m
c
AM
c
m
c
AM



cos
cos
)
(
cos
)
(





20. 20
Modulation Index, m
min
max
min
max
min
max
min
max














V
V
V
V
V
V
V
V
m
p
p
p
p
p
p
p
p
V
V
V
V
m







min
max
min
max
where m
c E
E
V 


max m
c E
E
V 


min
   
   
c
m
m
c
m
c
m
c
m
c
E
E
E
E
E
E
E
E
E
E
m








or
and
Therefore
m
E
m
E
m
E
m
E
 
t
m
E
Envelope m
c 
cos
1


 
t
m
E
Envelope m
c 
cos
1



max

V
min

V
min

V
max

V
c
E

c
E


21. 21
amDSBFC can be deduced to:
Modulation index :
From trigonometry identities:
Therefore:
c
m
E
E
m 
  t
t
m
E
t
v c
m
c
AM 
 cos
cos
1
)
( 

   
B
A
B
A
B
A 


 cos
2
1
cos
2
1
)
cos(
)
cos(
   t
mE
t
mE
t
E
t
t
mE
t
E
t
v
m
c
c
m
c
c
c
c
m
c
c
c
c
AM















cos
2
cos
2
cos
cos
cos
cos
)
(
  t
t
E
E
t
v c
m
m
c
AM 
 cos
cos
)
( 


22. 22
• Signal frequency spectrum ; amDSBFC
   
 
t
t
mE
t
E
t
v m
c
m
c
c
c
c
AM 



 



 cos
cos
2
cos
)
(
Carrier signal Sidebands signal
)
(V
Amplitud
)
( 1

rads

c
 m
c 
 
m
c 
 
0
c
E
2
c
mE
2
c
mE
m

2
2
m
c E
mE

m
E
where
Jalur Sisi Bawah
LSB
Jalur Sisi Atas
USB
Carrier band
Modulating band

23. 23
 
1
0 
 m
c
m
E
E
m 
)
(
,
1 c
m E
E
m 

)
(
,
1 c
m E
E
m 

)
(
,
1 c
m E
E
m 

The modulation index is given by :
Modulation indices range :

24. 24
• The phase change for carrier signal when over-modulation
occurs and must be avoided.
• Modulation depth greater than 100% must be
avoided, ( m > 1 > 100%)
Phase change

25. 25
2.2.3 Power, AM
 In the modulation process signal has been converted to
electrical signal in terms of current or voltage.
 The expression of AM signal components can be represented as
follows:
Pemodulatan Amplitud
)
(V
Amplitud
)
( 1

rads

c
 m
c 
 
m
c 
 
0
c
E
2
c
mE
2
c
mE
2
2
m
c E
mE

Where:
   t
mE
t
mE
t
E
t
v m
c
c
m
c
c
c
c
AM 



 



 cos
2
cos
2
cos
)
(
Carrier signal LSB signal USB signal
c
V LSB
V USB
V

26. 26
Pemodulatan Amplitud
R
E
m
R
E
m
R
E
R
mE
R
mE
R
E
R
V
R
V
R
V
P
P
P
P
c
c
c
c
c
c
USB
LSB
c
USB
LSB
c
T
rms
rms
rms
8
8
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2






























R
E
P c
c
2
2

4
2
c
USB
LSB
P
m
P
P 

where
Therefore
2
4
4
2
2
2
c
c
c
USB
LSB
SB
P
m
P
m
P
m
P
P
P



















2
1
2
2
2
m
P
P
m
P
P
P
P
c
c
c
SB
c
T
and
Therefore the relationship between
the total power transmitted, PT and
the carrier signal power, Pc is as
follows:

27. 27











2
1
2
m
P
P
P c
AM
T
%
100


T
SB
P
P

Watt
Transmission efficiency, η for AM:
2
2
2
2
2
2
2
2
1
2
2
1
2
m
m
m
m
m
P
P
m
c
c























where PSB is the total sidebands signal power that contains information
If m = 1 (100% modulation), the average power, PSB = 50% Pc= Pc/2
It shows that the PSB is dependent on m.

28. 28
2
2
2 m
m



From:
The transmission efficiency with m = 1 is only 33.33% .
Therefore we can conclude that the transmitted power signal is mostly
carrier power signal contributing of 66.67% from the total AM signal.
Whereas signal contains information in the LSB and USB transmitted is
33.33% from the total AM signal.
In practice, information signal is complex or non periodic signal,
eg: music, voice, image and etc. Its consists of many frequencies and
harmonics components.
Its can be represented:
.....
2
;
2
......
cos
cos
)
(
2
2
1
1
2
2
1
1
m
m
m
m
m
m
m
m
m
f
f
where
t
E
t
E
t
v












29. 29
Therefore total modulated power:
 
 
,.....
,
,
and
...
2
1
1
...
2
1
1
3
3
2
2
1
1
2
/
1
2
3
2
2
2
1
2
2
3
2
2
2
1
c
m
c
m
c
m
eff
eff
c
c
AM
E
E
m
E
E
m
E
E
m
m
m
m
m
where
m
P
m
m
m
P
P



























30. 30
Each sideband is equal in bandwidth to that of the modulating signal and is a
mirror image of the other.
Amplitude modulation is inefficient in terms of power usage and much of it is
wasted. (66.67%)
At least two-thirds of the power is concentrated in the carrier signal, which
carries no useful information
The remaining power is split between two identical sidebands, though only
one of these is needed since they contain identical information.
)
( 1

rads

c
 m
c 
 
m
c 
 
0
c
E
2
c
mE
2
c
mE
2
2
m
c E
mE

where
Amplitude
(V)
Summary