COMMUNICATION ENGINEERING
PRINCIPLES
Text Book Used
1. Electronic Communication
Systems (Kennedy and
Davis)
2. Electronic
Communications (Roddy
Coolan)
1
Course Outline
Modulation Techniques:
Amplitude Modulation and Demodulation, SSB, DSB,
Vestgial Sideband Modulation Frequency Division
Multiplexing,
Angle Modulation,
Frequency Modulation and Demodulation.
Phase Lock Loop, Limiting of FM Waves, FM Radio,
Stereophonic FM Broadcasting
2
Course Outline
Television: Modulation Techniques, TV Standards, B/W
Transmission and Reception, Color Transmission and Reception,
Digital Transmission: Pulse Code Modulation, Sampling
quantization, Coding, Differential Pulse code Modulation.
Delta Modulation, Digital Multiplexing for telephony,
Digital Modulation Techniques: ASK, FSK, PSK, QPSK,
Noise in Analog Receiver Models, Noise in AM Reception, Noise
in FM Reception, FM Threshold Effect, Pre-emphasis and Deemphasis in FM.
3
INTRODUCTION
1.1 Human Communication
1. Communication is the process of exchanging
information.
2.
Main barriers are language and distance.
3.
Contemporary society’s emphasis is now the
accumulation, packaging, and exchange of information.
4
INTRODUCTION
1.2 Methods of communication:
1. Face to face
2. Signals
3. Written word (letters)
4. Electronic communications/innovations:
 Telegraph
 Telephone
 Radio
 Television
 Internet (computer)
5
1.3: COMMUNICATION SYSTEMS

Basic components:
Transmitter
Channel or medium
Receiver

Noise degrades or interferes with transmitted
information.
1.4: COMMUNICATION SYSTEMS
Figure 1-2: A general model of all communication systems.
1-2: COMMUNICATION SYSTEMS
Transmitter
The transmitter is a collection of electronic components
and circuits that converts the electrical signal into a
signal suitable for transmission over a given medium.
Transmitters are made up of oscillators, amplifiers,
tuned circuits and filters, modulators, frequency mixers,
frequency synthesizers, and other circuits.
1-2: COMMUNICATION SYSTEMS
Communication Channel
The communication channel is the medium by which
the electronic signal is sent from one place to another.
Types of media include
 Electrical conductors
 Optical media
 Free space
 System-specific media (e.g., water is the medium for
sonar).
1-2: COMMUNICATION SYSTEMS
Receivers
A receiver is a collection of electronic components and
circuits that accepts the transmitted message from the
channel and converts it back into a form understandable
by humans.
Receivers contain amplifiers, oscillators, mixers, tuned
circuits and filters, and a demodulator or detector that
recovers the original intelligence signal from the
modulated carrier.
1-2: COMMUNICATION SYSTEMS
Transceivers
A transceiver is an electronic unit that incorporates
circuits that both send and receive signals.
Examples are:
Telephones
• Fax machines
• Handheld CB radios
• Cell phones
• Computer modems
•
1-2: COMMUNICATION SYSTEMS
Attenuation
Signal attenuation, or degradation, exists in all media
of wireless transmission. It is proportional to the square
of the distance between the transmitter and receiver.
1-2: COMMUNICATION SYSTEMS
Noise
Noise is random, undesirable electronic energy that
enters the communication system via the
communicating medium and interferes with the
transmitted message.
1-3: TYPES OF ELECTRONIC
COMMUNICATION

Electronic communications are classified
according to whether they are
1.
2.
One-way (simplex) or two-way (full duplex or half
duplex) transmissions
Analog or digital signals.
1-3: TYPES OF ELECTRONIC
COMMUNICATION
Simplex
The simplest method of electronic communication is
referred to as simplex.
This type of communication is one-way. Examples are:
Radio
 TV broadcasting
 Beeper (personal receiver)

1-3: TYPES OF ELECTRONIC
COMMUNICATION
Full Duplex
Most electronic communication is two-way and is
referred to as duplex.
When people can talk and listen simultaneously, it is
called full duplex. The telephone is an example of this
type of communication.
1-3: TYPES OF ELECTRONIC
COMMUNICATION
Half Duplex
The form of two-way communication in which only one
party transmits at a time is known as half duplex.
Examples are:
Police, military, etc. radio transmissions
 Citizen band (CB)
 Family radio
 Amateur radio

1-3: TYPES OF ELECTRONIC
COMMUNICATION
Analog Signals
An analog signal is a smoothly and continuously
varying voltage or current. Examples are:
Sine wave
 Voice
 Video (TV)

1-3: TYPES OF ELECTRONIC
COMMUNICATION
Figure 1-5: Analog signals (a) Sine wave “tone.” (b) Voice. (c) Video (TV) signal.
1-3: TYPES OF ELECTRONIC
COMMUNICATION
Digital Signals
Digital signals change in steps or in discrete increments.
Most digital signals use binary or two-state codes.
Examples are:
Telegraph (Morse code)
 Continuous wave (CW) code
 Serial binary code (used in computers)

1-3: TYPES OF ELECTRONIC
COMMUNICATION
Figure 1-6: Digital signals (a) Telegraph (Morse code). (b) Continuous-wave (CW)
code. (c) Serial binary code.
1-3: TYPES OF ELECTRONIC
COMMUNICATION
Digital Signals
Many transmissions are of signals that originate in
digital form but must be converted to analog form to
match the transmission medium.
Digital data over the telephone network.
 Analog signals.

They are first digitized with an analog-to-digital
(A/D) converter.
 The data can then be transmitted and
processed by computers and other digital
circuits.

AMPLITUDE MODULATION
(AM)
CHAPTER 2
CHAPTER 2:
PRINCIPLES OF MODULATION
OBJECTIVES
 To describe the principles of Modulation and AM
 To define and analyze the modulation index
 To analyze the spectral analysis and bandwidth
calculation
 To analyze the power distribution of AM
MODULATION
 Changing a signal to convey information
 From Music:
Volume
Pitch
Timing
26
MODULATION
 Changing a signal to convey information
 Ways to modulate a sinusoidal wave
Volume: Amplitude Modulation (AM)
Pitch:Frequency Modulation (FM)
Timing: Phase Modulation (PM)
 In our case, modulate signal to encode a 0 or a 1.
(multi-valued signals sometimes)
27
AMPLITUDE MODULATION
 AM: change the strength of the signal.
 Example: High voltage for a 1, low voltage for a 0
0 0 1 1 0 0
1 1 0 0 0 1 1 1 0 0 0 1 1 0 0 0
1 1 1 0
28
1
0
1
0
1
FREQUENCY MODULATION
 FM: change the frequency
0
1
1
0
1
1
0
0
0
1
29
PHASE MODULATION
 PM: Change the phase of the signal
1
0
1
0
30
LECTURE OVERVIEW
2.1 Principles of amplitude modulation (AM)
2.2 Modulation index
2.3 Spectral analysis and bandwidth calculation
2.4 Power analysis of AM
2.1 PRINCIPLES OF AM
Definitions:
The process of changing the amplitude of a relatively
high frequency carrier signal in proportion with the
instantaneous value of modulating signal (information)
A process of translating information signal from low band
frequency to high band
frequency.
CONT’D…
Information signal cannot travel far. It needs carrier
signal of higher frequency for long distance
destination.
Inexpensive, low quality form of modulation
CONT’D…
Amplitude of the carrier signal varies with
the information signal.
The modulated signal consist of carrier
signal, upper sideband and lower sideband
signals
The modulated AM signal needs to go
through demodulation process to get back
the information signal.
THE AM ENVELOPE
AM double-sideband full carrier (AM DSBFC) is
the most commonly used and the oldest and
simplest form of AM modulation.
Sometimes called conventional AM or simply
AM.
The outline of the positive and negative peaks
of the carrier frequency re-create the exact
shape of the modulating signal known as
envelope.
Note that the repetition rate of the envelope is
equal to the frequency of the modulating
signal.
THE GENERATION OF AM ENVELOPE
AM FREQUENCY SPECTRUM AND
BANDWIDTH
 An AM modulator is a non-linear device.
 Nonlinear mixing results in a complex output
envelope consists of the carrier frequency
and the sum (fc + fm) and difference (fc – fm)
frequencies (called cross-products).
 The cross-products are displaced from the
carrier frequency by fm on both sides of it.
 AM modulated wave contains no frequency
component of fm.
AM FREQUENCY SPECTRUM AND
BANDWIDTH
Note the modulation index is
Communication Research Group
and
respectively………..equ 1
We can show that the frequencies present in the AM
wave are the carrier frequency and the first pair of
sideband frequency
4/3/2024
 Let the carrier voltage and modulating voltage be
…………………………………..equ2
38
AM FREQUENCY SPECTRUM AND
BANDWIDTH

 A=
1
)
Communication Research Group
obtain an expression for the AMPLITUDE of the AM
voltage
4/3/2024
 Note that by combining equation 1 and 2 we can
39
AM FREQUENCY SPECTRUM AND
BANDWIDTH
4/3/2024
 The instantaneous voltage of the resulting
Vc
Carrier frequency
signal (volts)
+
"
cos #
# $
Lower side frequency
signal (volts)
"
)
, ℎ!
cos #
# $
Communication Research Group
amplitude modulated wave is

= 1
Recall from trigonometry identity that
1 cos
cos
2
Upper side frequency
signal (volts)
40
FREQUENCY SPECTRUM OF AN AM
DSBFC WAVE
BANDWIDTH (BW)
 The BW of an AM DSBFC wave is equal to the
difference between the highest upper side
frequency and lowest lower side frequency:
 BW = [fc + fm(max)] – [fc – fm(max)]
= 2fm(max)
 For efficiency transmission the carrier and
sidebands must be high enough to be propagated
thru earth’s atmosphere.
REPRESENTATION OF AM
 Mathematically, the modulation index is
m = modulation index
m
Em = peak change in the amplitude output
waveform (sum of voltages from upper and
lower side frequencies)
Ec = peak amplitude of the unmodulated
carrier
E
E
m
c
 And the percentage of modulation index is
E
% m 
E
m
c
x 100 %
DETERMINING MODULATION INDEX
FROM VMAX AND VMIN
CONT’D…
 If the modulating signal is a pure, single-freq sine wave
and the process is symmetrical then the modulation
index can be derived as follows:
1
E  (V  V )
2
1
E  (V  V )
2
 Therefore,
m
max
min
c
max
min
1
(Vmax  Vmin )
(V  Vmin )
m 2
 max
1
(Vmax  Vm in )
(Vmax  Vmin )
2
CONT’D…
 Since the peak change
of modulated output
wave Em is the sum of the usf and lsf voltages
hence,
E E E
m
 Then
usf
lsf
1
(V
 Vmin )
Em 2 max
Eusf  Elsf 

2
2
1
 (V max  Vmin )
4
where E  E
usf
lsf
Eusf = peak amplitude
of the upperside
frequency (volts)
Elsf = peak amplitude
of the lower side
frequency (volts)
CONT’D…
 From the modulated wave displayed in the previous
slide, the maximum and minimum values of the
envelope occurs at
+Vmax = Ec + Eusb + Elsb
+Vmin = Ec – Eusb – Elsb
-Vmax = -Ec - Eusb - Elsb
-Vmin = -Ec + Eusb + Elsb
MODULATION INDEX FOR TRAPEZOIDAL
PATTERNS
 Modulation index, m can be calculated using
the equation:
m = Emax – Emin/ Emax + Emin
= Em / Ec
= (A - B) / (A + B)
CONT’D…
% MODULATION OF AM DSBFC
ENVELOPE
CONT’D…
 For proper AM operation, Ec > Em means that 0≤ m
≤ 1.
 If Ec < Em means that m > 1 leads to severe
distortion of the modulate wave.
 If Vc = Vm the percentage of modulation index goes
to 100%, means the maximum information signal is
transmitted. In this case, Vmax = 2Vc and Vmin = 0.
AM POWER DISTRIBUTION
 In any electrical circuit, the power dissipated
is equal to the voltage squared (rms) divided
by the resistance.
 Mathematically power in unmodulated carrier
is
2
2
Pc 
(Vc / 2 )
V
 c
R
2R
Pc = carrier power (watts)
Vc = peak carrier voltage (volts)
R = load resistance i.e antenna (ohms)
CONT’D
 The upper and lower sideband powers will
be
2
2
(mV c / 2)
m Vc
Pus b  Plsb 

2R
8R
 Rearranging in terms of Pc,
2

m  Vc  m 2
Pus b  Plsb 

P
c


4  2R 
4
2
2
CONT’D…
 The total power in an AM wave is
Pt  Pc  Pusb  Plsb
 Substituting the sidebands powers in terms of PC yields
m2
m2
Pt  Pc 
Pc 
Pc
4
4
m2
m2
 Pc in
 modulated

]is the same as
Pc  Pc [1wave
 Since carrier power
2
2
unmodulated wave, obviously power of the carrier is
unaffected by modulation process.
POWER SPECTRUM FOR AM DSBFC WAVE WITH
A SINGLE-FREQUENCY MODULATING SIGNAL
CONT’D…
With 100% modulation the maximum power
in both sidebands equals to one-half the
carrier power.
One of the most significant disadvantage of
AM DSBFC is with m = 1, the efficiency of
transmission is only 33.3% of the total
transmitted signal. The less wasted in the
carrier which brings no information signal.
The advantage of DSBFC is the use of
relatively simple, inexpensive demodulator
circuits in the receiver.
TRANSMITTER EFFICIENCY
Transmitter efficiency ‫תּ‬,
= average power from sideband/total power
absorbed.
= m²/ ( 2+m² )
MODULATION BY A COMPLEX INFORMATION
SIGNAL
 Previous examples are all using a single frequency
modulation signal. In practice, however, modulating signal is
very often a complex waveform made up from many sine
waves with different amplitudes and frequencies.
 Example: if a modulating signal contains three
frequencies(fm1, fm2, fm3), the modulated signal will contain the
carrier and three sets of side frequencies, spaced
symmetrically about the carrier:
mV
mV
mV
cos[2 ( f  f )t ] 
cos[2 ( f  f )t ] 
cos[2 ( f  f )t ]
2
2
2
mV
mV
mV

cos[2 ( f  f )t ] 
cos[2 ( f  f )t ] 
cos[2 ( f  f )t ]
2
2
2
v (t )  V sin (2f t ) 
am
c
c
c
c
c
c
c
m1
c
c
c
m2
m1
c
c
c
m3
c
m3
m2
CONT’D..FREQUENCY SPECTRUM FOR
COMPLEX INFORMATION SIGNAL
Fc-fm3
Fc-fm2
Fc-fm1
fc
Fc+fm1 Fc+fm2 Fc+fm3
CONT’D..MODULATION INDEX FOR COMPLEX
INFORMATION SIGNAL
 When several frequencies simultaneously
amplitude modulate a carrier, the combined
coefficient of modulation is defined as:
m  m  m  m  ...  m
t
2
2
2
2
1
2
3
n
mt=total modulation index/coefficient of modulation
m1, m2, m3, mn= modulation index/coefficient of
modulation for input 1, 2 ,3 , n
CONT’D..POWER CALCULATION FOR
COMPLEX INFORMATION SIGNAL
 The combined coefficient of modulation can be
used to determine the total sideband power and
transmitted power, using:
Pc m t2

4
Pusbt  Plsbt
2
t
Pc m
P sbt 
2

m t2 

Pt  Pc  1 
2 

LOW LEVEL AM TRANSMITTER
HIGH LEVEL AM TRANSMITTER
HOME WORKS 1
 With the aid of diagram explain the concept of low
level and high level modulation in an AM
transmitter.
 Describe any two methods of generating AM
 Solve any two standard numeric problem on AM
64
HOME WORK 2
 Suppose that Vmax value read from the graticule on
an oscilloscope screen is 4.6 divisions and Vmin is
0.7 divisions. Calculate the modulation index and
percentage of modulation.
HOME WORK 3

a)
b)
c)
d)
e)
For the AM waveform shown in Figure 3
below, determine
Peak amplitude of the upper and lower
side frequencies.
Peak amplitude of the unmodulated
carrier.
Peak change in the amplitude of the
envelope.
Modulation index.
Percent modulation.
AM ENVELOPE FOR HOME WORK 3
HOME WORK 4

a)
b)
c)
d)
e)
One input to a conventional AM modulator is a
500-kHz carrier with an amplitude of 20 Vp. The
second input is a 10-kHz modulating signal that
is of sufficient amplitude to cause a change in the
output wave of ±7.5 Vp. Determine
Upper and lower side frequencies.
Modulation index and percentage modulation.
Peak amplitude of the modulated carrier and the
upper and lower side frequency voltages.
Maximum and minimum amplitudes of the
envelope.
Expression for the modulated wave.
HOME WORK 5

a)
b)
c)
d)
For an AM DSCFC wave with a peak
unmodulated carrier voltage Vc = 10 Vp, a load
resistor of RL = 10  and m = 1, determine
Powers of the carrier and the upper and lower
sidebands.
Total sideband power.
Total power of the modulated wave.
Draw the power spectrum.
HOME WORK 6

For an AM DSBFC transmitter with an unmodulated carrier
power, Pc= 100W that is modulated simultaneously by
three modulating signals, with coefficients of modulation
m1=0.2, m2= 0.4, m3=0.3, determine:
a)
Total coefficient of modulation
Upper and lower sideband power
Total transmitted power
b)
c)
CHAPTER 2:
END OF AMPLITUDE MODULATION