EC8394 Analog and Digital Communication

Unit - 1
Analog Communication
Presentation Prepared by
S. Alwyn Rajiv AP/ECE
Kamaraj College of Engineering and Technology
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What is Modulation?
 The process of shifting the baseband signal to
passband range is called Modulation.
 The process of shifting the passband signal to
baseband frequency range is called Demodulation.
Modulation is the process by which some characteristics of high
frequency carrier signal is varied in accordance with the instantaneous
value of the another signal called modulating or message signal (base
band signal)
 Need for Modulation 3

 For easy transmission

 Narrow banding
 Reduction in height of the antenna ( h = λ/2 )
 Multiplexing
 Modulation for frequency assignment or frequency
translation
 Modulation to reduce noise and interference
Types of Modulation 4
Modulation

Analog Modulation Digital Modulation Pulse Modulation

Amplitude Angle Amplitude Shift Pulse Position

Modulation Modulation Keying Modulation

Frequency Frequency Shift Pulse Amplitude

Modulation Keying Modulation

Phase Shift Pulse Width

Phase
Modulation Keying Modulation

Quadrature
Pulse Code
Amplitude
Modulation
Modulation
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Types of Modulation
The message signal contain some useful information. The carrier
signals(usually high frequency signal or Radio frequency signal) used to
carry modulating signals(Message Signal).
Let us sinusoidal carrier wave in analog communication is given by

Vc=A= Amplitude of the carrier

signal
Wc=Angular frequency = 2πfc
Ф= Phase angle
 Types of Modulation 7

Depends upon the Amplitude, frequency and phase variation it can be

classified into 3 types
1. Amplitude Modulation
2. Frequency Modulation
Angle Modulation
3. Phase Modulation
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Principle of Amplitude Modulation
❑ Amplitude Modulation(AM) is the process of changing the
amplitude of the carrier signal in proportion with the
instantaneous value of the modulating signal(Message
Signal).
❑ Frequency and phase of the carrier signal are not altered
during this process.
❑ The amplitude Modulation system is a nonlinear system
two input, one input.
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Modulating Signal
Vm (t) = Em sin (2πfmt)

Carrier Signal
Vc (t) = Ec sin (2πfct)
Where, Where,
Vm (t) → Time varying voltage of modulating signal (V) Vc (t) → Time varying voltage of carrier signal (V)
Em → Peak amplitude of modulating signal (V) Ec → Peak amplitude of carrier signal (V)
fm → Frequency of modulating signal (Hz) fc → Frequency of carrier signal (Hz)
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Peak amplitude of modulated signal

According to the definition the

amplitude of the carrier signal is
changed after modulation
EAM = Ec + Vm (t)
Where
EAM → Peak amplitude of modulated signal (V)
Ec → Peak amplitude of carrier signal (V)
Vm (t) → Time varying voltage of modulating signal (V)
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EAM = Ec + Em sin (2πfmt)
= Ec { 1 + Em/ Ec sin (2πfmt)}
= Ec { 1 + m sin (2πfmt) }
EAM = Ec { 1 + m sin (2πfmt) }
Modulation Index: m = Em/ Ec
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AM ENVELOPE
❑ Amplitude modulation are several
types but the commonly used
technique is Double Side Band
Full Carrier (DSB-FC)
❑ The amplitude modulation is also
called as Conventional
amplitude modulation or simply
amplitude modulation.
❑ The shape of the modulated wave
is called the AM envelope.
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The instantaneous amplitude of modulated signal or AM
Envelope
VAM (t) = EAM sin (2πfct)
VAM (t) = Ec { 1 + m sin (2πfmt) } sin (2πfct)
= Ec sin (2πfct) + m Ec sin (2πfmt) sin (2πfct)
Sin A Sin B = ½ { cos (A-B) - cos (A+B) }
VAM (t) = Ec sin (2πfct) + m Ec/2 { - cos 2π (fc + fm) t + cos 2π (fc - fm) t

VAM (t) = Ec sin (2πfct) - m Ec/2 cos 2π (fc + fm) t + m Ec/2 cos 2π (fc - fm) t
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AM Frequency Spectrum and Bandwidth
VAM (t) = Ec sin (2πfct) - m Ec/2 cos 2π (fc + fm) t + m Ec/2 cos 2π (fc - fm) t

 From this equation have 3 terms

 1st term is Ec sin (2πfct) → Carrier signal (V)
 2nd term is - m Ec/2 cos 2π (fc + fm) t → Upper side frequency signal (V)
 3rd term is + m Ec/2 cos 2π (fc - fm) t → Lower side frequency signal (V)
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Bandwidth is the difference between upper frequency and lower frequency.

Bandwidth = fc+ fm (max) - (fc- fm (max))
= fc+ fm (max) - fc + fm (max)
B = 2 fm (max) Hz
Where, fc → Carrier frequency
fm (max) → Maximum modulating frequency
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Phasor Representation of AM with Carrier
 It is easy way to represent of AM wave
m Ec/2
 VC carrier wave phasor, taken as a reference. (USB)

 The two side band frequency of wc+wm and wm

wc-wm are represented by two phasor VAM (t)

rotating in two opposite direction. The Carrier

angular frequency of wm
 That is the phasor for the carrier and LSB wm m Ec/2
(LSB)
and USB combine sometimes or some time
subtract.
 m Ec/2 19
(USB)
wm
VAM (t)
Carrier

wm m Ec/2
(LSB)

❑The positive amplitude envelop occurs if carrier LSB and USB all are have positive
value
Vmax=Vc+VLSB+VUSB
❑The minimum amplitude envelop occurs if carrier LSB and USB are in out of phase

Vmin=Vc-VLSB-VUSB
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MODULATION INDEX
 It is also called as co-efficient of modulation.
 The amount of amplitude change occurred in AM envelope is called
modulation index.
 It can be expressed as,
m= Em/Ec ; 0 < m < 1
Where,
m→ Modulation index (unit less)
Em → Peak amplitude change in modulation signal (V)
Ec → Peak amplitude change in carrier signal (V)
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Modulation Index in Terms of Voltage
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Modulation Index in Terms of Voltage
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PERCENT MODULATION
 The percent change on the amplitude of output wave is called as
percent modulation.
 It can be expressed as,
M= m x 100 %
M= Em/Ec x 100 %
Where, M→ Percent Modulation (%)
Em → Peak amplitude change in modulation signal (V)
Ec → Peak amplitude change in carrier signal (V)
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Degrees of Modulation
Case (i)
Ec = Em
m= Em/Ec = Em/Em = 1
m=1
This condition is called as critical modulation.

Case (ii)
Em < Ec
m= Em/Ec < 1 m<1

This condition is called as under modulation.

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Degrees of Modulation

Case (iii)
Em > Ec
m= Em/Ec >1
m>1
This condition is called as over modulation.
 AM VOLTAGE DISTRIBUTION 27

VAM (t) = Ec sin (2πfct) - m Ec/2 cos 2π (fc + fm) t + m Ec/2 cos 2π (fc - fm) t
Where,
Ec sin (2πfct) → Carrier signal (V)
- m Ec/2 cos 2π (fc + fm) t → Upper side frequency signal (V)
+ m Ec/2 cos 2π (fc - fm) t → Lower side frequency signal (V)
This expression represents AM voltage distribution.
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AM POWER DISTRIBUTION:
The total power can be expressed as,
Pt = Pc + PUSB + PLSB
Where,
Pt → Total power (W)
Pc → Carrier power (W)
PUSB → Upper Side Band Power (W)
PLSB → Lower Side Band Power (W)
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Carrier Power
Pc = {Vrms2 / R}
Vrms = Amplitude / √2
Pc = {Vrms2 / R}
= [Ec / √2 ]2 / R
Pc = Ec2 / 2R
This expression is called as carrier power.
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Sideband Power
The amplitude of upper and lower side band is equal. So, power
dissipated in upper and lower band is equal.
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Total Power
Pt = Pc + {m2Pc/4} + { m2Pc/4}
= Pc [1+ (2m2/4)]
Pt= Pc [1+ (m2/2)]
This equation is represented as total power dissipated by Amplitude
modulation
 Total side band power 32

PSBT = PUSB + PLSB

= {Ec2 m2 / 8R} + {Ec2 m2 / 8R}
PSBT = Ec2 m2 / 4R
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Current Distribution
EFFICIENCY 34