Analog communication

V. Chandra Sekar
© Oxford University Press 2013

Introduction

Communication Basics
 Communication deals with the principle of transferring
information from one place to another.
 It involves transmission and reception, and processing of
information between these two locations.
 The source could be in continuous form as in the case of
analog communication and as discrete signals as in the case of
digital communication.
 Short distance transmission of information is called baseband
transmission.

Communication Basics
 For long distance transmission, information has to be
impressed upon an high frequency component to be
able to reach the reception end of communication.
 The high frequency component is termed as a carrier
and the entire process is called modulation.

Need For Modulation
 To translate the frequency of a low-pass signal to a higher
band so that the spectrum of the transmitted bandpass signal
matches the bandpass characteristics of the channel.
 For efficient transmission, it has been found that the antenna
dimension has to be of the same order of magnitude as the
wavelength of the signal being transmitted.
 Since C= f for a typical low-frequency signal of 2 kHz, the
wavelength works out to be 150 km. Even assuming the
height of the Antenna half the wavelength, the height works
out to be 75 km, which is impracticable.

Need For Modulation
 To enable transmission of a signal from several
message sources simultaneously through a single
channel employing frequency division multiplexing.
 To improve noise and interference immunity in
transmission over a noise channel by expanding the
bandwidth of the transmitted signal.

Frequency Translation
 The modulation process shifts the modulating
frequency to a higher frequency, which in turn
depends on the carrier frequency, thus producing
upper and lower sidebands.
 Hence, signals are upconverted from low frequencies
to high frequencies and downconverted from high
frequencies to low frequencies in the receiver.
 The process of converting a frequency or a band of
frequencies to another location in the frequency
spectrum is called frequency translation.

Types Of Modulation
Depending on whether the amplitude, frequency, or phase of
the carrier is varied in accordance with the modulation signal,
we classify the modulation as
 Amplitude modulation
 Frequency modulation
 Phase modulation.
The method of converting information into pulse form and
then transmitting it over a long distance is called pulse
modulation.

Transmitter
 The message as it arrives may not be suitable for direct
transmission. It may be voice signal, music, picture, or data. The
signals, which are not of electrical nature, have to be converted
into electrical signals. This is the role of transmitter. Typical block
diagram is illustrated below.

Receiver
 A receiver is meant to receive the electromagnetic signal which
carries the information. It is tuned to receive the required
information at a predetermined frequency. The output of the
receiver is usually fed into a transducer which converts the
information into understandable signal.

Multiplexing
 When it is required to transmit more signals on the same channel,
baseband transmission fails, as in the case of audio signals being
broadcast from different stations on the same channel.
 To encounter this problem either frequency division multiplexing
or time division multiplexing is employed.
 This method of transmitting several channels simultaneously is
known as frequency division multiplexing (FDM).
 In Time Division Multiplexing (TDM) several signals are
transmitted over a time interval. Each signal is allotted a time slot
and it gets repeated cyclically. The only difference compared to
FDM is that the signals are to be sampled before sending.

Signals – An Introduction

Signals:
 Any function that carries information.
 Shows how a parameter varies with another
parameter.
 Will be dealing with signals with time or frequency as
an independent variable
Signals

Signals are classified as:
 Continuous and discrete.
 Causal and Non causal.
 Even and Odd.
 Deterministic and Random
 Real and complex
 Energy and power type
Signals

Discrete Signals

Continuous Signals

Causal Signals

Even & Odd Signals

( )sin
sin ( ) ,
t
c t
t
π
π
=
Special Signals

Sgn(t) = 1, t > 0
= -1, t < 0
Signum Signals

Impulse or Delta signal
( ) 1
( ) ( ) ( )
t
and v t t dt v t
δ
δ
∞
−∞
∞
−∞
=
=
∫
∫

Classification Of Systems
 Discrete time and Continuous Time systems.
 Time Invariant and Time varying systems
 Causal and Non Causal system
 Instantaneous and Dynamic systems
 Stable and Unstable systems

Fourier Series & Transform
1. Fourier series:
- Any periodic of function of time x(t) having a fundamental
period ‘T’ and frequency 1/T can be represented as an infinite
series of sinusoidal waveforms of fundamental and its
harmonic frequencies.
2. If a function is x(t), its Fourier series is given by:
0
1 1
( 0 cos(2 ) sin(2 )n n
n n
x t a a fnt b fntπ π
∞ ∞
= =
=+ +∑ ∑

Where:
2
0
2
2
2
1
( )
2 2
( ) cos
2
( ) sin(2 )
T
T
T
n
T
n
a x t dt
T
nt
a x t dt
T T
b x t nt dt
T
π
π
−
−
=
=
= =
∫
∫

Fourier Transform
 To represent aperiodic function Fourier transform is used
 Unlike Fourier series, this representation will be continuous in
frequency domain
 It is given by:
 Also x(t) can be obtained from X(f) as:
x(t) =
2
( ) ( ) j ft
X f x t e dtπ
∞
−
−∞
= ∫
∫
∞
∞−
dfefX ftj π2
)(

Laplace Transform
1. It converts time domain signal into frequency domain a plane called ‘s’ plane having
as the real part and ω as the imaginary part.
2. Laplace transform is given by the expression:
3. The inverse Laplace transform is given by:
σ
( )
( ) ( )
. . ( ) ( )
st
jw t
x x t e dt
i e X x t e dtσ
ω
ω
∞
−
−∞
∞
− +
−∞
=
=
∫
∫
1
( ) ( )
2
st
x t X s e ds
jπ
∞
∞
= ∫

Z Transform
 Z transform is a polar representation compared to rectangular
representation in Laplace transform
 It is for discrete time function
 Z transform of a function x(t) is given by:
Inverse Z transform is given by:
 In Z transforms a term ROC is defined as “region of convergence”
where the Z transform of a function has finite value.
[ ] [ ] n
X z x n z−
= ∑
1
[ ] [ ]x n x z
Z
=

Amplitude Modulation

 Amplitude of the carrier is changed in proportion to the
instantaneous amplitude of a message signal
 Carrier frequency must be relatively higher than the
message frequency
 Modulation index ‘m’ is the ratio of Em/Ec
 Percentage of modulation = m x 100%
Amplitude Modulation

AM Envelope

Frequency Spectrum Of AM Wave

Power Spectrum Of AM

 Suppressed Carrier Systems
 Double side band (DSB) system
 Single side band system(SSB)
 SSB with pilot carrier
 Independent side band (ISB) system
 Vestigial side band (VSB) system
Other AM Systems

AM Waveforms For AM, DSB & SSB

Single Sideband
Advantages:
 Lesser power consumption.
 Conservation of bandwidth.
 Noise reduction.
 Less fading.
Disadvantages:
 Requires complex receiver.
 At the receiver, coherent carrier has to be generated.
 In case of pilot carrier, at the receiver end it has to be boosted
properly.

 Square law Modulators
 Switching Modulators
 Transistor Modulators
Low level
Medium level
High level
AM Modulators

Balanced Modulators
1. Balanced ring Modulator
2. Balanced bridge Modulator
3. Transistor balanced Modulator
4. FET balanced Modulator
SSB Generation
1. The filter method
2. The phase shift method
3. The Third method
Types Of Modulators

AM Demodulators
1. Rectifier detector
2. Envelope detector
Detector Distortions
1. Diagonal peak clipping
2. Negative peak clipping
SSB Reception
1. Coherent detection
2. Reception with pilot carrier
Demodulators, Distortions &
Reception

AM Transmitters
Low Level AM DSBFC Transmitter

High Level DSBFC Transmitter

SSB Transmitter
SSB suppressed carrier Transmitter: BPF is used to remove the other
sideband

Phase Shift Method

SSB Transmitter With Pilot
Carrier

AM Receiver
Super Heterodyne Receiver

SSB Pilot Receiver

Communication Receiver

 Selectivity
 Sensitivity
 Dynamic range
 Fidelity
 Bandwidth
 Noise temperature and equivalent noise
temperature
Receiver Parameters

Costas Loop

Angle Modulation

 Angle modulation includes both frequency and phase
modulations.
 In Frequency Modulation(FM), the frequency of the
carrier is changed with respect to amplitude of the
message signal
 In phase modulation(PM), the phase of the carrier is
changed with respect to amplitude of the message signal
 Unlike AM, both FM and PM are nonlinear, hence much
more difficult to implement and analyze.
Introduction

1. Modulation index for FM wave is given by:
Where ∆f is the frequency deviation and fm
is the modulating frequency
2. The expression for an FM wave is:
3. Modulation index for PM wave is given by:
where, is the phase deviation given by:
4. The expression for an PM wave is:
m
f
f
β
∆
=
( ) cos[2 sin{2 ( )}]FM c mf t A f t f tπ β π= +
p mm k E=
pK p
m
k
E
θ∆
=
( ) cos[2 cos{2 ( )}]PM c mf t A f t f tπ θ π= + ∆
Modulation Index & Deviation

Frequency & Phase Modulator
Phase modulator can be used to generate FM wave and FM modulator can be used to
generate PM wave as shown:

FM & PM Waves

 FM with β <<1 is called narrowband FM
 Expression for narrow band FM:
f(t) = Vc {cos ωct - cos (ωc – ωm) t + cos (ωc + ωm) t}
 Phasor diagram of narrowband FM:
Narrowband FM

 FM with β > 10 is called wideband FM
 Expression for wideband FM:
 f(t) = Jo(β) cos ωc t – J1(β){ cos(ωc – ωm) t – cos(ωc – ωm) t}+ J2 (β) { cos
(ωc - 2ωm) t + cos (ωc + 2ωm) t} – J3 (β) { cos (ωc - 3ωm) t – cos (ωc - 3ωm t) }
+ -------
 The function Jn(β) is called the Bessel function.
 The spectrum is composed of a carrier with an amplitude Jo (β) and a set
of side bands spaced symmetrically on either side of the carrier at
frequency separation of ωm, 2ωm, 3ωm --- and so on.
 Unlike AM, FM has an infinite number of side bands along with carrier.
These side bands are separated from the carrier by fm, 2fm, 3fm ---------.
Wideband FM

Bessel Function As A Function Of β

Bessel Function Values

 Carson’s formula for bandwidth of FM system
Band width = 2(∆f + fm) HZ
 For low modulation index, in case of narrow band FM since 2∆f << fm,
equation reduces to Band width = 2fm and for wide band FM where ∆f >> fm,
equation reduces to Band width = 2∆f.
 Average power in sinusoidal wideband FM:
PT = Vc
2 Jo
2 (β) /R + 2Vc
2 /R { J1
2 (β) + J2
2 (β) + J3
2 (β) + ---------- }
= Vc
2 /R [ J0
2 (β) + 2 { J1
2 (β) + J2
2 (β) + J3
2 (β) + -------------- }]
= Pc [ Jo
2 (β) + 2 { J1
2 (β) + J2
2 (β) + J3
2 (β) + ------------------- }]
where Pc is the unmodulated power Vc
2 /R.
Bandwidth Requirements For Angle
Modulated Waves

The expression for sinusoidal FM is:
Kp em(t) = Kp Em sin ωm t = ∆ Φ sin ωm t
where ∆ Φ = Kp Em, ∆ Φ is defined as “Peak phase
deviation” and is directly proportional to the peak
modulating signal.
Sinusoidal Phase Modulation

Phasor Representation

 FM generation
 Varactor diode modulators
 Reactance modulators
 Modulators using linear integrated circuits
 Indirect methods for narrow band and wideband
 PM generation:
 Varactor diode in direct PM modulators
 Direct method with transistor
FM & PM Generation

 Slope detector
 Balance slope detector
 Foster Seeley discriminator
 Ratio detector
 Demodulator using PLL
 Quadrature detector
 Zero crossing detector
FM Detectors

 Crosby Direct FM Transmitter:
FM Transmitter

Indirect FM Transmitter

Super heterodyne Receiver
FM Receivers

Double Superheterodyne Receiver

Phased Lock Loop
 It is a feedback system that generates a signal that has a
fixed relation to the phase of a reference signal .
 A phase locked loop circuit responds to both the
frequency and phase of the input signals, by changing the
frequency of the voltage controlled oscillator until it
matches to the reference input in both frequency and
phase. Hence it is a negative feedback system except that
the feedback error signal is a phase rather than a current
or voltage signal as usually the case in conventional
feedback system.

PLL Block Diagram (Analog)

PLL Block Diagram (Digital)

 Data and Tape Synchronization
 Modems
 FSK Modulation
 FM Demodulation
 Frequency Synthesizer
 Tone Decoding
 Frequency Multiplication and Division
PLL Applications

 Is a powerful technique to generate RF signals.
 A direct digital synthesizer operates by storing the
points of a waveform in digital format, and then
recalling them to generate the waveform.
 The rate at which the synthesizer completes one
waveform then determines the frequency.
Direct Digital Synthesis

Direct Digital Synthesis
Block Diagram :

Pulse Modulation

Pulse Modulation
 In analog pulse modulation, the carrier is a periodic
pulse train
 The amplitude, position and width of the carrier pulse
train are varied in a continuous manner in accordance
with the corresponding sample value of message
signal.
 Thus in Pulse modulation, information is transmitted
basically in analog form, but the transmission takes
place at discrete times.

 In the case of digital pulse modulation the message signal
is represented in a form that is discrete in both time and
amplitude
 The data is transmitted as a sequence of coded pulse.
 This type of modulation is also called pulse code
modulation (PCM).
 PCM is the most widely used form in the field of
Telecommunication.
 Digital Data transmission provides a higher level of noise
immunity, more flexibility in the band width
 Power tradeoff possibility of providing more security to
data and ease of implementation using large scale
integrated circuits.

 Pulse width modulation (PWM)
 Pulse position modulation (PPM)
 Pulse amplitude modulation (PAM)
 Pulse code modulation (PCM)
Predominant Methods Of Pulse
Modulation

Pulse Width Modulation

Pulse Amplitude Modulation

Pulse Modulation Technique

 PCM offers a method of over coming some of the disadvantages
of other type of pulse modulation.
 In PCM the instantaneous amplitude of the sample is
represented by a binary code resulting in a series of ones and
zeros or mark and space.
 All pulses have the same height and same shape
 Since only ones and zeros are sent. The receiver has only to
detect the presence or absence of a pulse.
 A distorted pulse does not degrade the signal as long as the
pulse can still be recognized. Hence PCM is less sensitive to noise
than wither PAM or PWM
Pulse Code Modulation (PCM)

PCM Transmitter & Receiver

 When more than one application or connection share the
capacity of one link it is called multiplexing.
 This results in better utilization of resources.
 A typical example is, many conversations over telephone
line, trunk line, wireless channel, etc.
 A few examples of multiplexing are:
 TDM- Time division multiplexing
 FDM- Frequency division multiplexing
 WDM- Wavelength division multiplexing
 CDMA- Code division multiple access
Multiplexing

FDM Transmitter

FDM Receiver

Synchronous TDM Transmitter

Synchronous TDM Receiver

Analog Carrier System Using FDM

Digital Carrier System Using TDM

Analog communication

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