AC PPT.ppt

Block diagram of communication system

Types of communication systems
• Analog communication:
AM,FM,PM etc.
• Digital communication:
ASK,FSK,PSK,QPSK etc.
• Microwave communication:
Communication through radio/microwave/frequencies.
• Optical communication:
Communication through light.

Base band vs Pass band Transmission
• Base band Signal:- Information bearing Signal or
Message Signal.
• The term Base band refers to the band of
frequencies representing the original signal
obtained from the source (or Base).
– Voice (0-4kHz)
– TV (0-6 MHz)
• A signal may be sent in its base band format when
a dedicated wired channel is available.
• Otherwise, it must be converted to pass band.

Need for Modulation
• Size of the antenna
• For efficient radiation, the size of the antenna should be λ/10
or more (preferably around λ/4 ), where λ is the wavelength
of the signal to be radiated.
• Easy to Multiplex
• Several message signals can be transmitted on a given
channel, by assigning to each message signal an appropriate
slot in the channel.
• Channel Selectivity
• Each station can be assigned a suitable carrier so that the
corresponding program material can be received by tuning to
the station desired.

Need for Modulation
• Improved Signal to Noise Ratio
• Will be dealt in future lectures
• Less Fading of transmitted signal
• As the energy of a signal is proportional to its frequency,
fading by the atmospheric particle is less

What is Modulation?
• So for better transmission, we need to send a high
frequency signal.
• But message signal is of low frequency.
• If we alter the frequency of message signal, the
information will be lost.
• We can send a high frequency signal which reflects
the characteristics of message signal.
• This high frequency signal is called CARRIER
SIGNAL

What is Modulation?
• The message signal is called MODULATING SIGNAL
or BASEBAND SIGNAL.
• The word modulation means the systematic
alteration of one waveform, called the carrier,
according to the characteristic of another
waveform, the modulating signal or the message.
• We use c(t ) and m(t ), to denote the carrier and
the message waveforms respectively.

What is Modulation?
• The resultant signal after modulation is
called Modulated signal .
• For study purpose, the commonly used
carrier and message signal is Sinusoidal
wave.
• Transmitter Side - Modulation
• Receiver Side - Demodulation

Definition for Modulation
• Modulation is defined as the process by
which some characteristic of a carrier wave
is varied in accordance with the message
signal.

Types of Modulation
• Modulation - Characteristics of Carrier Wave is
varied in accordance with the characteristics of
message signal.
• Consider a Carrier wave:
c(t) = Ac Cos ( θ )
Instantaneous
Value
Maximum
Amplitude
Angle
( 2πfct + φ )
Frequency
Phase

Types of Modulation
MODULATION
Angle
Modulation
Amplitude
Modulation (AM)
Phase
Modulation (PM)
Frequency
Modulation (FM)
AM DSB FC
AM DSB SC
SSB
VSB
NBFM
WBFM
NBPM
WBPM

Amplitude & Angle Modulation -
Definition
• The amplitude modulation, in which the
amplitude of a sinusoidal carrier is varied in
accordance with the message signal.
• The angle modulation, in which the
instantaneous frequency or phase of the
sinusoidal carrier is varied in accordance with
the message signal.

AM, FM & PM
• AM – The amplitude of the carrier signal is varied in
accordance with the instantaneous amplitude of the
message signal.
• FM – The frequency of the carrier signal is varied in
message signal.
• PM – The phase of the carrier signal is varied in
message signal.

AM & FM Waveforms
AM
FM
MESSAGE
CARRIER

FM & PM Waveforms
FM
PM
MESSAGE
CARRIER

Types of Amplitude Modulation
AMPLITUDE
MODULATION
Non Linear
AM
Linear
AM
AM DSB FC
AM DSB SC
SSB
VSB

AM DSB FC (or simply AM)
• Introduction
• Signal & Spectrum representation of AM
• Power Relation
• Modulators
– Switching Modulator
– Square Law Modulator
• Demodulators
– Square Law Demodulator
– Envelope Detector

AM DSB FC or simply AM
• Consider a Carrier Signal:
• Message signal m(t) and Carrier signal c(t) are
independent.
• AM is defined as the process in which the amplitude of
the Carrier Signal, c(t) is varied about a mean value,
linearly with the Base band Signal, m(t).
• where Ka = 1/Ac, is the Amplitude Sensitivity Factor or
Modulation Sensitivity measured in volt-1

Non Linearity in AM DSB FC
Does Full-Amplitude Modulation Satisfy the Linearity Property ?
 Amplitude modulation, as defined in Eq. (2.2), fails the
linearity test (i.e. Super Position Theorem)in a strict sense.
1) Suppose that m(t) = m1(t) + m2(t). Let s1(t) and s2(t) denote the AM
waves produced by these two components acting separately.
2) Let the operator H denote the amplitude modulation process,
therefore we have:
 
1 1
( ) 1 ( ) cos( )
c a c
s t A k m t t

   
2 2
( ) 1 ( ) cos( )
c a c
s t A k m t t

 
and
   
1 2 1 2
1 2
( ) ( ) 1 ( ) ( ) cos( )
( ) ( )
c a c
H m t m t A k m t m t t
s t s t

   
 
 
 
The superposition principle is violated!

Modulation Index of AM
• Ka = 1 / Ac
• Ka*m(t) = (1/Ac) * Am Cos (2πfmt)
= (1/Ac) * Am (1)
= Am / Ac
= ka*Am
• This “ Ka * Am ” is called as Modulation Index.
• It is denoted using m or μ

Modulation Index of AM
Two cases arise, depending on the magnitude of
kam(t), when comparing with unity:
1) Under modulation, which is governed by the condition
( ) 1 for all
a
k m t t
 1 + kam(t) > 0
2) Over modulation, which is governed by the weaker
condition
( ) 1 for some
a
k m t t

Percentage of modulation  kam(t) 100%

Modulation Index
Important conclusion:
1. The envelope of the AM wave has a waveform that
bears a one-to-one correspondence with that of the
message signal if the percentage modulation is less
than or equal to 100%.
2. If percentage modulation > 100%, the modulated wave
is said to suffer from envelope distortion.

Signal Representation of AM
An un modulated RF
carrier wave
A carrier wave
amplitude
modulated (AM) with
a simple audio tone

Signal Representation of AM
1st
Condition
Envelope
Distortion

Spectrum Representation of AM
• To draw the spectrum of any wave, we need to
find out the Fourier Transform of that signal.
• Cos (x) = (1/2)*(e jx + e -jx)
• F [m(t) Cos (x)] = M(f – x)/2 + M(f + x)/2
   
B
A
B
A
B
A 


 cos
2
1
cos
2
1
)
cos(
)
cos(
   t
A
A
K
t
A
A
K
t
A
t
t
A
A
K
t
A
t
s
m
c
m
c
a
m
c
m
c
a
c
c
m
c
m
c
a
c
c















cos
2
cos
2
cos
cos
cos
cos
)
(

40 Chapter 2: Signal Analysis and Mixing 2009
Frequency shifting (Modulation)
 
   
0
0
j t
f t e F

 
  
Therefore multiplying a time function by causes its
spectral density to be translated in frequency by ω0.
0
j t
e 
Example
 
     
0 0 0
1
cos
2
f t t F F
    
 
    
 
 
F 
Some properties of the Fourier transform


 
0
1
2
F  
  
0
1
2
F  


 
 
   
)
(
)
(
2
)
(
)
(
2
)
2
cos(
)
(
)
2
cos(
)
(
)
(
c
c
c
a
c
c
c
c
a
c
c
c
AM
AM
f
f
M
f
f
M
A
k
f
f
f
f
A
t
f
t
m
k
A
t
f
A
F
t
s
F
f
S
















• From Eqn. (2.5), we can draw the spectrum
as:

Spectral Overlap (2nd Condition)
Spectral overlap phenomenon in amplitude modulation. The
phenomenon arises when the carrier frequency c is less than the
highest frequency component m of the modulating signal.
2nd
Condition
Spectral
Overlap

Time domain & Frequency domain

Carrier Power Vs Sideband Power

AM Modulators
1. Switching Modulator
– Utilizing the Switching characteristic or time varying
characteristic of a diode.
2. Square Law Modulator
– Utilizing the non linear characteristic of any square
law device ( like Diode, Transistor etc.)

Switching Modulator
• Assume that
• Let
• The diode will turn on if and will turn off if
• The output across the load resistor is
• Since g(t) is a periodic rectangular function, the Fourier series is
)
(t
m
Ac 
0
)
( 
t
c 0
)
( 
t
c
)
2
cos(
)
( t
f
A
t
c c
c 

)
(
)]
2
cos(
)
(
[
)
(
)
(
0
)
(
0
0
)
(
)
(
)
(
0
t
g
t
f
A
t
m
t
g
t
v
t
c
t
c
t
v
t
v
c
c
i
i











AM Demodulators
1. Square Law Demodulator
2. Envelope Detector

Envelope Detector
• The operations of the circuit requires
careful selection of t=RC
• If RC is too large, discharging will be
slow and the circuit cannot follow a
decreasing envelope.
• When RC is too small the ripples will
be high.
• The ripples are finally removed by
LPF.
• The DC value is blocked by a
capacitor.

Features of AM
• AM system is very cheap to build and
maintain.
• AM is wasteful of power - max efficiency
is 33%
• AM is wasteful of bandwidth - twice the
message bandwidth is required

Demerits of AM DSB FC
An unmodulated RF
carrier requires narrow
bandwidth
Modulation results in
creation of a carrier and 2
Sidebands. This
requires more power.
Moreover carrier contains
no information.

Why DSB SC?
 The carrier contains no information.
 So we can think of avoiding or suppressing carrier.

Linear Modulation
In its most general form, linear modulation is defined
by:
where sI(t) is the in-phase component and sQ(t) the
quadrature component of the modulated wave s(t).
In linear modulation, both sI(t) and sQ(t) are low-pass
signals that are linearly related to the message signal
m(t).

In-Phase and Quadrature
Components of Linear Modulation
Depending on sI(t) and sQ(t), three types of linear
modulation are defined:
1) DSB SC modulation, where only the upper and lower
sidebands are transmitted.
2) SSB modulation, where only the lower or the upper
sideband is transmitted.
3) VSB modulation, where only a vestige of one of the
sidebands and a modified version of the other sideband are
transmitted.

In-Phase and Quadrature
Components of Linear Modulation
There are 2 important points to be noted from this table:
1). The in-phase component sI(t) is solely dependent on the message m(t).
2). The quadrature component sQ(t) is a filtered version of m(t). Spectral
modification of the modulated wave s(t) is solely due to sQ(t) .

Linear Modulation Schemes
1. AM DSB SC (AM Double Side Band Suppressed Carrier)
2. SSB (Single Side Band)
3. VSB (Vestigial Side Band)

AM DSB SC
• Derivation
• Signal & Spectra
• Modulators
– Product Modulator
• Balanced Modulator
• Ring Modulator (Double Balanced Modulator)
• Demodulator
– Coherent Detector
– Costas Receiver

Introduction
DSB-SC modulation is generated by using a
product modulator that simply multiplies the
message signal m(t) by the carrier wave
Accos(2fct). Specifically, we write:
s(t) = Acm(t) cos(2fct) (2.8)

The modulated signal s(t) undergoes a phase reversal
whenever the message signal m(t) crosses zero.
This is called double side-band suppressed carrier (DSB-SC)
modulation.
).
2
cos(
)
(
)
( t
f
t
m
A
t
s c
c
DSB 

Introduction
1. Transmission bandwidth is same as standard AM.
2. Transmitted power is less than that used by standard AM.

Signal Representation
Double-sideband-suppressed carrier modulation. (a) Message signal.
(b) DSB-SC modulated wave, resulting from multiplication of the
message signal by the sinusoidal carrier wave.

Spectrum Representation
• The envelope of a DSB-SC signal is different from
the message signal; unlike the case of an AM wave
that has a percentage modulation < 100 %.
• From Eq. (2.8), the Fourier transform of s(t) is
obtained as:

Spectrum of AM DSB SC
When m(t) is limited to the interval -W < f < W, as
in Figure 2.6a, the spectrum S(f) of the DSB-SC
wave s(t) is as illustrated in Figure 2.6b.

Spectrum of AM DSB SC
Because it doesn’t have
components of the
carrier, we call this
kind of modulation
suppressed carrier

Time domain Vs Frequency domain
Time-domain
(on the left)
and frequency-
domain (on the
right)
characteristics
of DSB-SC
modulation
produced by a
sinusoidal
modulating
wave. (a)
Modulating
wave. (b)
Carrier wave. (c)
DSB-SC
modulated
wave. Note that
 = 2.

Modulators
Product Modulator
• Balanced Modulator
• Ring Modulator (Double Balanced Modulator)

Ring Modulator
• Therefore, we have
• Since c(t) is a periodic function, the Fourier series can be
expressed as:
• The desired DSB-SC AM signal is obtained by passing
through a bandpass filter with center frequency and
bandwidth 2W.
)
(
)
(
)
( t
c
t
m
t
vo 
)
(
0 t
v
c
f

Demodulators
1. Coherent Detector
• AM DSB SC Modulator + Filter
• Also called Synchronous or Homodyne
Detector.
• Quadrature Null Effect – Phase Error.
2. Costas Receiver
• Employs two Coherent detectors.
• Avoids Quadrature Null Effect.

Coherent Detector – Quadrature
Null Effect
• Assume the Local Oscillator signal have same
frequency of that of the Carrier, but a different
phase.
• Let the Phase difference is Ø.
• The LO signal is:

Coherent Detector – Quadrature
Null Effect

Costas Receiver
• I-channel:
• After down conversion,
• At the output of the lowpass filter, with |H(0)| = 1,
• Q-channel:
   
 
 






cos
2
cos
)
(
2
cos
cos
)
(
)
(






t
t
m
A
t
t
t
m
A
t
v
c
c
c
c
c
I
)
(
cos
2
)
( t
m
A
t
m c
I 
 
 
 


 sin
2
sin
)
(
2
)
( 

 t
t
m
A
t
v c
c
Q
)
(
sin
2
)
( t
m
A
t
m c
Q 
 

Costas Receiver
• Feedback path:
• At the output of the multiplier,
• At the output of the lowpass filter,
• The purpose of hf (t) is to smooth out fast time variations
of me(t).
• The output of the VCO is described by



2
sin
)
(
8
cos
sin
)
(
4
)
(
2
2
2
2
t
m
A
t
m
A
t
m
c
c
e







 t
t
t d
t
h
m
t
m f
e
ef )
(
)
(
)
(
 
( ) cos ( )
VCO c
x t t t
 
 

Costas Receiver
Where c is the VCO’s reference frequency and is
the residual phase angle due to the tracking error.
The constant kv is the frequency sensitivity of the VCO in rad/s/volt.
The instantaneous frequency in radians/sec of the VCO’s output is
given by:
Clearly, if (t) were small, then the instantaneous frequency would be
close to c and the output of the I-path would also be proportional to
m(t).
,
)
(
)
(
0


t
ef
v d
m
k
t t
t

  ),
(
)
(
t
m
k
dt
t
t
d
ef
v
c
c







Why SSB?
 The carrier contains no audio information.
The sidebands contains duplicated information

AM SSB SC (SSB)
• Hilbert Transform
• Derivation
• Signal and Spectrum
• Modulators
– Frequency Discriminator
– Phase Discriminator (Hartley Modulator)
• Demodulators
– Coherent Detector
– Envelope Detector

Introduction to SSB
• Two main parameters to be considered while
designing a Communication System are :
• In AM DSB FC, both are very high.
• In AM DSB SC Transmission Power is less than AM
DSB FC, but Transmission Bandwidth is same as
that of AM DSB FC.

Introduction to SSB
• In AM SSB SC or SSB, only one Sideband will be
Transmitted (Both the Sidebands contain the same
information).
• The Transmission Power as well as the Transmission
Bandwidth can be reduced.
• Transmission Bandwidth will be reduced to half of
that of AM DSB FC & AM DSB SC.
• To accomplish these merits, the Equipment Design in
more Complex.

SSB Derivation
• A single sideband AM signal can be
represented mathematically as:
USSB AM
LSSB AM

Spectrum of SSB
Suppose we want to transmit the upper sideband, then using an
ideal bandpass filter with center frequency yields the desired
result, namely,
2
m
c
W
f 

SSB Modulators
• Frequency Discriminator
–Generating SSB signal from DSB SC signal by
using BPF
• Phase Discriminator
–Generating SSB signal by using Hilbert
Transform

Frequency Discriminator
GDSBSC()
C
+2B

C
2B C
C
C
+2B
C
2B
USB
LSB
LSB
USB
M()
+2B

2B C
C
GUSB()
C+2B

C
C
C2B
USB
USB
GLSB()

C2B C
C C+2B
LSB
LSB
HUSB()
C+2B

C2B C
C C+2B
C2B
HLSB()
C+2B

C2B C
C C+2B
C2B
BW = 2B (B Hz)
Center Freq = c+B
BW = 2B (B Hz)
Center Freq = c– B

SSB Demodulators
• Coherent Detector
• Envelope Detector

Coherent Detector
• Same Coherent Detector used for AM DSB SC.

Demerit of SSB
• Selective Filtering using filters with sharp cutoff
characteristics. Sharp cutoff filters are difficult to design.
• The audio signal spectrum has no dc component, therefore ,
the spectrum of the modulated audio signal has a null around
the carrier frequency.
• This means a less than perfect filter can do a reasonably good
job of filtering the DSB to produce SSB signals.

VSB
• Derivation
• Signal and Spectra
• Modulators
– Frequency Discriminator
• Demodulators

Introduction to VSB
To produce SSB signal from DSB signal ideal filters should be used.
In VSB system one sideband and a vestige of other sideband are
transmitted together.
The resulting signal has a bandwidth > the bandwidth of the
modulating (baseband) signal but < the DSB signal bandwidth.

Generation of VSB
• Generation of VSB AM
– generate a DSB-SC AM signal
– pass the DSB-SC AM signal through a sideband filter with
frequency response H(f)

Basics of Signal to Noise Ratio

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