21EEB0A12_Deepak_Chaurasiyahugytfrd5er.pptx

Amplitude modulation
1
Roll Number : 21EEB0A12
B-TECH III YEAR
S e c t i o n - A
Created by- Deepak
Chaurasiya

Communication System
2
 Communication is simply the process of exchanging information.
 Transmitter converts information into a signal that is suitable for transmission
over a medium.

Modulation
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 Modulation is a process of mixing a signal with a sinusoid to produce a new
signal.
 This new signal, will have certain benefits over an un-modulated signal.
 Mixing of low frequency signal with high frequency carrier signal is called
modulation.
 Low frequency signals cannot be transmitted for longer distances.
 So we modify the carrier signal with respect to modulating (message) signal.
 Modulating Signal: Message signal
 Carrier Signal: Signal which carries message signal.
 Modulated Signal: The resultant signal after modulation.

Need for Modulation
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1. Reducing the height of antenna
 Usually the size of antenna is around 𝜆/4.
ƒ 3×103
 For voice signal ranging from 300 Hz to 3.4 KHz, 𝜆 = 𝑐
= 3×108
= 105 𝑚 =
100 km. So the height of antenna becomes 25 km which is not practically
possible.
 If we modulate the signal to higher frequency (1MHz), 𝜆 = 3×108
= 300 𝑚.
1×106
So antenna size becomes 300/4 = 75 m, which can be installed easily.

Need for Modulation
5
2. Multiplexing
 If 3 users are transmitting voice signals simultaneously, all the signals get
mixed together and a receiver can not separate them from each other.
 If we modulate the 3 voice signals on to 3 different carrier signals, they will
not interfere with each other.
 This process is called Frequency Division Multiplexing (FDM).
 Multiplexing is a process in which two or more signals can be transmitted
over the same communication channel simultaneously.

Need for Modulation
6
3. Increase the Range of Communication
 Low frequency signals can not travel long distance when they are transmitted
as they get heavily attenuated .
 The attenuation reduces with increase in frequency of the transmitted signal,
and they travel longer distance .
 The modulation process increases the frequency of the signal to be
transmitted. Therefore, it increases the range of communication..

Types of Modulation
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 Consider a carrier signal 𝑐 𝑡 = 𝐴𝑐cos(2𝜋ƒ𝑐𝑡 + ɸ𝑐).
 𝐴𝑐 is the amplitude of the carrier.
 ƒ𝑐 is the frequency of the carrier.
 ɸ𝑐 is the phase of the carrier.
 Modulation is the process of varying the characteristic of a carrier signal in
accordance with the modulating (message) signal.
 Amplitude Modulation
 Frequency Modulation
 Phase Modulation

Amplitude Modulation (AM)
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 Consider a carrier signal 𝑐 𝑡 = 𝐴𝑐cos(2𝜋ƒ𝑐𝑡).
 Amplitude Modulation is defined as a process in which the amplitude of
carrier wave 𝑐 𝑡 is varied linearly with message signal 𝑚 𝑡 .
Time-Domain Description ofAM
 The standard form ofAM wave is defined by
𝑠 𝑡 = 𝐴𝑐 1 + 𝑘𝑎𝑚(𝑡) cos(2𝜋ƒ𝑐𝑡)
 where 𝑘𝑎 is a constant called amplitude sensitivity of the modulator.
 The amplitude of time function multiplying cos(2𝜋ƒ𝑐𝑡) is called the envelope
of theAM wave.

Time-Domain Description ofAM
9
 The amplitude of time function multiplying cos(2𝜋ƒ𝑐𝑡) is called the envelope
ofAM wave, denoted by
𝑎 𝑡 = 𝐴𝑐 1 + 𝑘𝑎𝑚(𝑡)
 The maximum absolute value of 𝑘𝑎𝑚(𝑡) multiplied by 100 is referred to as the
percentage modulation.
 If 𝑘𝑎𝑚(𝑡) < 1, Under modulated
 If 𝑘𝑎𝑚(𝑡) = 1, Critically modulated
 If 𝑘𝑎𝑚(𝑡) > 1, Over modulated

Frequency-Domain Description ofAM
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 The standard form ofAM wave is defined by
𝑠 𝑡 = 𝐴𝑐 1 + 𝑘𝑎𝑚(𝑡) cos 2𝜋ƒ𝑐𝑡
𝑠 𝑡 = 𝐴𝑐 cos 2𝜋ƒ𝑐 𝑡 + 𝐴𝑐 𝑘𝑎 𝑚(𝑡) cos 2𝜋ƒ𝑐 𝑡
 To determine frequency description of this AM wave, take Fourier transform on
both sides.
=
𝐴𝑐
+
𝑘𝑎𝐴𝑐
2 2
𝑆 ƒ ð ƒ − ƒ + ð ƒ + ƒ 𝑀 ƒ − ƒ + 𝑀 ƒ + ƒ
𝑐 𝑐 𝑐
𝑐
 𝑀(ƒ) is the FT of 𝑚(𝑡) and 𝑚(𝑡) is band-limited to the interval −W ≤ ƒ ≤ W
 ƒ𝑐 > W

 The spectrum consists of two delta functions weighted by factor 𝐴𝑐/2 occurring
at ±ƒ𝑐 and two versions of the baseband spectrum translated in the frequency by
±ƒ𝑐 and scaled in amplitude by 𝑘𝑎𝐴𝑐/2
 For positive frequencies, the portion of spectrum lying above carrier frequency
is called upper sideband and the symmetric portion below ƒ𝑐 is called lower
sideband.
 The condition ƒ𝑐 > W ensures that the sidebands do not overlap. Otherwise the
modulated wave exhibits spectral overlap and therefore frequency distortion.
 For positive frequencies, the highest frequency component of AM wave is
ƒ𝑐 + W and lowest frequency component is ƒ𝑐 − W.
 22

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 The difference between these two frequencies defines transmission bandwidth
B ofAM wave, which is exactly twice the message bandwidth W.
 B=2W
 This spectrum of the AM wave is full i.e., the carrier, the upper sideband, and
the lower sideband are all completely represented.
 Hence this form of amplitude modulation is treated as standard.

Single-Tone modulation ofAM
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 Consider a modulating wave m 𝑡 = 𝐴𝑚cos(2𝜋ƒ𝑚𝑡).
 TheAM wave is described by
𝑠 𝑡 = 𝐴𝑐 1 + 𝑘𝑎𝐴𝑚cos(2𝜋ƒ𝑚𝑡) cos 2𝜋ƒ𝑐𝑡
𝑠 𝑡 = 𝐴𝑐 1 + 𝜇 cos(2𝜋ƒ𝑚𝑡) cos 2𝜋ƒ𝑐𝑡
 where 𝜇 = 𝑘𝑎𝐴𝑚 is called modulation factor or modulation index.
 To avoid envelope distortion due to over modulation, the modulation factor 𝜇
must be kept below unity.
 Let 𝐴𝑚𝑎𝑥 and 𝐴𝑚i𝑛 be the maximum and minimum values of the envelope of
the modulated wave.

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𝐴𝑚𝑎𝑥
=
𝐴𝑐(1 + 𝜇)
𝐴𝑚i𝑛 𝐴𝑐(1 − 𝜇)
 That is
𝜇 =
𝐴𝑚𝑎𝑥 − 𝐴𝑚i𝑛
𝑠 𝑡
𝐴𝑚𝑎𝑥 + 𝐴𝑚i𝑛
= 𝐴𝑐 1 + 𝜇 cos(2𝜋ƒ𝑚𝑡) cos 2𝜋ƒ𝑐𝑡
𝑠 𝑡 = 𝐴𝑐 cos 2𝜋ƒ𝑐𝑡 + 𝐴𝑐 𝜇 cos(2𝜋ƒ𝑚𝑡)cos 2𝜋ƒ𝑐𝑡
1
 Using the relation cos(A)cos(B) = 2
[cos(A+B)+cos(A-B)]
𝑠 𝑡 = 𝐴
1 1
𝑐 cos 2𝜋ƒ𝑐 𝑡 +
2
𝐴𝑐 𝜇 cos 2𝜋 ƒ𝑐 + ƒ𝑚 𝑡 +
2
𝐴𝑐 𝜇 cos 2𝜋 ƒ𝑐 − ƒ𝑚 𝑡

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𝑠 𝑡 = 𝐴
1 1
2
2
 FT of s(t) is
𝑆 ƒ =
𝐴𝑐
2
ð ƒ − ƒ𝑐 𝑐
+ ð ƒ + ƒ +
𝐴 𝜇
𝑐
4
ð ƒ − ƒ𝑐 − ƒ𝑚 + ð ƒ + ƒ𝑐 + ƒ𝑚
+
𝐴 𝜇
𝑐
4
ð ƒ − ƒ𝑐 + ƒ𝑚 + ð ƒ + ƒ𝑐 − ƒ𝑚
 Thus the spectrum of an AM wave, for special case of sinusoidal modulation,
consists of delta functions at ±ƒ𝑐, ƒ𝑐 ± ƒ𝑚 and −ƒ𝑐 ± ƒ𝑚.

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Power Calculation inAM
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𝑠 𝑡 = 𝐴
1 1
2
2
 Power = 𝑉𝑟𝑚𝑠𝐼𝑟𝑚𝑠 =
𝑉r𝑚𝑠
2
=
𝑉𝑝
2 1 𝑉𝑝
2 𝑉𝑝
2
2 𝑅
= 2𝑅
= 2
when R=1.
𝑐
 Carrier Power, 𝑃 =
Æ𝑐
𝑅
2
2
𝑈𝑆
𝐵
 Upper Sideband Power, 𝑃 =
𝐿𝑆
𝐵
 Lower Sideband Power, 𝑃 =
Æ𝑐𝜇/2 2
= Æ𝑐
2𝜇2
= 𝑃𝑐𝜇2
2 8 4
Æ𝑐𝜇/2 2
= Æ𝑐
2𝜇2
= 𝑃𝑐𝜇2
2 8 4
 Total power, 𝑃𝑡 = 𝑃𝑐 + 𝑃𝑈𝑆𝐵 𝐿𝑆
𝐵
𝑐
+ 𝑃 = 𝑃 + 𝑐
𝑃 𝜇
4
+ 𝑐
𝑃 𝜇
2 2
4 𝑐
= 𝑃 + 𝑐
𝑃 𝜇 2
2

Power Calculation inAM
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𝑡
𝑐
= Æ𝑐
2
 Total power, 𝑃 = 𝑃 1 + 𝜇2
1 + 𝜇2
2 2 2
 Transmission Efficiency, 5 =
𝑇o𝑡𝑎𝑙 𝑠i𝑑e𝑏𝑎𝑛𝑑 𝑝owe𝑟
𝑇o𝑡𝑎𝑙 𝑡𝑟𝑎𝑛𝑠𝑚i𝑡𝑡e𝑑 𝑝owe𝑟
=
𝑃 𝜇2
𝑐 2
𝑐
𝑃 1+
𝜇2
2
=
𝜇2
2+𝜇2
2 2
 Let 𝑃𝑡 = 300W, 𝜇 = 1 then 300 = 𝑃𝑐 1 + 1
= 3
𝑃𝑐
 i.e., 𝑃𝑐 = 200W, 𝑃𝑆𝐵 = 𝑃𝑡 − 𝑃𝑐 = 100W
 So 200W of the power is wasted to transmit carrier. 2/3rd of power is lost in
transmitting carrier and only 1/3rd of power is used to transmit sidebands.

Generation ofAM Waves
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Square Law Modulator
 It requires 3 features:
 a means of summing the carrier and modulating waves,
 a nonlinear element, and
 a band pass filter for extracting the desired modulation products.

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 Semiconductor diodes and transistors are the most common nonlinear devices
used for implementing square law modulators.
 The filtering requirement is usually satisfied by using a single or double tuned
filter.
1
 The nonlinear device can be modeled as, 𝑣2 𝑡 = 𝑎1𝑣1 𝑡 + 𝑎2𝑣2(𝑡)
 where 𝑎1 and 𝑎2 are constants.
 The input voltage 𝑣1 𝑡 consists of the
carrier wave plus the modulated wave
i.e., 𝑣1 𝑡 = 𝑚 𝑡 + 𝐴𝑐 cos 2𝜋ƒ𝑐𝑡

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 𝑣2 𝑡 = 𝑎1[𝑚 𝑡 + 𝐴𝑐 cos 2𝜋ƒ𝑐𝑡 ] + 𝑎2[𝑚 𝑡 + 𝐴𝑐 cos 2𝜋ƒ𝑐𝑡 ]2
2 𝑐
 𝑣2 𝑡 = 𝑎1𝑚 𝑡 + 𝑎1𝐴𝑐 cos 2𝜋ƒ𝑐𝑡 + 𝑎2𝑚2 𝑡 + 𝑎 𝐴2𝑐o𝑠2 2𝜋ƒ𝑐𝑡 + 2𝑎2𝑚(𝑡)𝐴𝑐 cos 2𝜋ƒ𝑐𝑡
 𝑣2 𝑡 = 𝑎 𝐴
1 𝑐
2𝑎2
𝑎1
2 2 2
𝑐 1 2 2 𝑐 𝑐
1 + 𝑚(𝑡) cos 2𝜋ƒ 𝑡 + 𝑎 𝑚 𝑡 + 𝑎 𝑚 𝑡 + 𝑎 𝐴 𝑐o𝑠 2𝜋ƒ 𝑡
 The first term is the desiredAM wave with amplitude sensitivity 𝑘𝑎 = 2𝑎2/𝑎1.
 The remaining 3 terms are unwanted and are removed by appropriate filtering.

Switching Modulator
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 It is assumed that carrier wave applied to diode is larger in amplitude.
 We assume that diode acts as an ideal switch, it is short circuited (zero
impedance) when it is forward biased and is open circuited (infinite impedance)
when it is reverse biased.

Switching Modulator
• This diode is assumed to be operating as a switch .
• The modulating signal x(t) and the sinusoidal carrier signal c(t) are connected in
series with each other. Therefore, the input voltage to the diode is given by :
• The amplitude of carrier is much larger than that of x(t) and c(t) decides the status of
the diode (ON or OFF ) .

Switching Modulator
• In other words , the load voltage v2(t) varies periodically between the
values v1(t) and zero at the rate equal to carrier frequency fc .
• We can express v2(t) mathematically as under :
• where, gp(t) is a periodic pulse train of duty cycle equal to one half
cycle period i.e. T0 /2 (where T0 = 1/fc) .

Switching Modulator
Let us express gp(t) with the help of Fourier series as
under :

Switching Modulator
• The odd harmonics in this expression are unwanted, and therefore, are assumed to
be eliminated .
• Hence,
• in this expression, the first and the fourth terms are unwanted terms whereas the
second and third terms together represents the AM wave .
• Clubbing the second and third terms together , we obtain:
• This is the required expression for the AM wave with m=[4/πEc] . The unwanted
terms can be eliminated using a band-pass filter (BPF) .

Detection ofAM Waves
 The process of detection or demodulation means recovering the
message signal from an incoming modulated wave.
 Detection is the inverse of modulation.
• Square Law Detector
 A square law detector is obtained by using a square law
modulator for the purpose of detection.
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1
 The nonlinear device can be modeled as, 𝑣2 𝑡 = 𝑎1𝑣1 𝑡 + 𝑎2𝑣2(𝑡)
 The input to the detector isAM wave given by
𝑣1 𝑡 = 𝐴𝑐 1 + 𝑘𝑎𝑚(𝑡) cos 2𝜋ƒ𝑐𝑡

Square Law Detector
32
 Substituting 𝑣1 𝑡 in 𝑣2 𝑡 , we get
2
𝑣2 𝑡 = 𝑎1𝐴𝑐 1 + 𝑘𝑎𝑚(𝑡) cos 2𝜋ƒ𝑐𝑡 + 𝑎2 𝐴𝑐 1 + 𝑘𝑎𝑚(𝑡) cos 2𝜋ƒ𝑐𝑡
𝑣2 𝑡 = 𝑎1𝐴𝑐 1 + 𝑘𝑎𝑚(𝑡) cos 2𝜋ƒ𝑐𝑡
+𝑎2𝐴2 1 + 2𝑘 𝑚 𝑡 + 𝑘2𝑚2(𝑡)
𝑐 𝑎 𝑎
1 + cos 4𝜋ƒ𝑐𝑡
2
 The desired signal, 𝑎 𝐴2𝑘 𝑚 𝑡 is due to the 𝑎 𝑣2(𝑡)
2 𝑐 𝑎 2 1
description square law detector.
term, hence the
 This component can be extracted by means of a low pass filter.
 This is not the only contribution within the baseband spectrum, because
2 𝑐 𝑎
𝑎 𝐴2𝑘2𝑚2(𝑡)/2 will give rise to a plurality of similar frequency components.

Square Law Detector
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 The ratio of wanted signal to distortion is equal to 𝑐
𝑐
𝑎
𝑎2Æ2k2𝑚2(𝑡)/2
𝑎2Æ2k𝑎𝑚 𝑡 2
k𝑎 𝑚 𝑡
= .
 To make this ratio large, we choose 𝑘𝑎𝑚 𝑡 small compared to unity.

Envelope Detector
34
 An envelope detector is a simple yet highly effective device that is well suited
for demodulation of a narrowband AM wave (carrier frequency is large
compared with message bandwidth), for which percentage modulation is less
than 100%.
 Ideally an envelope detector produces an output signal that follows the envelope
of the input signal exactly.
 Envelope detector consists of a diode and
a resistor capacitor filter.

Envelope Detector
35
 On the +ve half cycle of input signal, the diode is forward biased and capacitor
charges up rapidly to the peak value of input signal.
 When input signal falls below this value, the diode becomes reverse biased and
the capacitor discharges slowly through the load resistor Rl.
 The discharging process continues until the next +ve half cycle.
 When the input signal becomes greater than the voltage across the capacitor, the
diode conducts again and the process is repeated.
 We assume that the diode is ideal and the envelope detector is supplied by a
voltage source of internal impedance Rs.

Envelope Detector
36
 The charging time constant 𝑅𝑠𝐶 must be short compared with the carrier period,
1/ƒ𝑐 , that is
𝑠
𝑅 𝐶 ≪
1
ƒ𝑐
 Hence, capacitor charges rapidly and thereby follows the applied voltage up to
the positive peak when the diode is conducting.
 On the other hand, the discharging time constant 𝑅𝑙𝐶 must be long enough to
ensure that the capacitor discharges slowly through the load resistor 𝑅𝑙 between
positive peaks of carrier wave, but not so long that capacitor voltage will not
discharge at maximum rate of change of the modulating wave, that is
𝑙
1
≪ 𝑅 𝐶 ≪
1
ƒ𝑐 W

Envelope Detector
37
 where W is the message bandwidth.
 The result is that the capacitor voltage or the detector output is very nearly same
as the envelope ofAM wave.
 The detector output usually has a small ripple at carrier frequency, which is
removed by low pass filtering.

Applications ofAM
41
 In radio broadcasting, a central transmitter is used to radiate message signals for
reception at a large number of remote points.
 AM broadcasting is radio broadcasting using amplitude modulation (AM)
transmissions.
 One of the most important factors which promoted the use of AM in radio
broadcasting is the simple circuitry required at the receiver’s end.
 A simple diode circuit is enough at the receiver’s end to properly receive the
modulated signal and get the original message.

Applications ofAM
42
 Since, while broadcasting, there are a large number of receivers which are the
common masses of public, it is essential that circuitry involved be simple and
compact so that everyone can accommodate and use it properly.
 Amplitude modulation serves this purpose perfectly as explained above and
hence is used for broadcasting.

Double Sideband Suppressed Carrier (DSB-
SC)
43
 The spectrum of standard AM wave is full i.e., the carrier, the upper sideband,
and the lower sideband are all completely represented.
 Hence it is called as Double Sideband with Full Carrier.
 But 2/3rd of power is lost in transmitting carrier and only 1/3rd of power is used
to transmit sidebands. i.e., Transmission efficiency is only 33.33% when 𝜇=1.
 Transmission Efficiency, 5 =
𝑇o𝑡𝑎𝑙 𝑠i𝑑e𝑏𝑎𝑛𝑑 𝑝owe𝑟
𝑇o𝑡𝑎𝑙 𝑡𝑟𝑎𝑛𝑠𝑚i𝑡𝑡e𝑑 𝑝owe𝑟
=
𝜇2
2+𝜇
2
 This is the main drawback of standardAM wave.
 To overcome this drawback, we can suppress the carrier component from the
modulated wave resulting in Double Sideband Suppressed Carrier modulation.

Double Sideband Suppressed Carrier (DSB-
SC)
44
wave that is
 Thus by suppressing the carrier, we obtain a modulated
proportional to the product of carrier wave and message signal.
Time-Domain Description of DSB-SC
 DSB-SC wave can be expressed as
𝑠 𝑡 = 𝑐 𝑡 𝑚(𝑡)
𝑠 𝑡 = 𝐴𝑐 cos 2𝜋ƒ𝑐𝑡 𝑚(𝑡)
 This modulated wave undergoes a phase reversal whenever the message signal
crosses zero.
 Hence the envelope of DSB-SC modulated wave is different from the message
signal.

Time-Domain Description of DSB-SC
45

Frequency-Domain Description of DSB-SC
46
 By taking the Fourier transform on both sides of time-domain signal, 𝑠 𝑡
𝑠 𝑡 = 𝐴𝑐 cos 2𝜋ƒ𝑐𝑡 𝑚(𝑡)
=
𝐴𝑐
2
𝑆 ƒ 𝑀 ƒ − ƒ + 𝑀 ƒ + ƒ
𝑐 𝑐
 where 𝑆 ƒ is the FT of modulated wave, and 𝑀(ƒ) is the FT of message signal
 When message signal is limited to the interval −W ≪ ƒ ≪ W, the modulation
process simply translates the spectrum of baseband signal by ±ƒ𝑐.

Frequency-Domain Description of DSB-SC
47
 The transmission bandwidth required
by DSB-SC modulation is same as that
for standardAM, i.e., 2W.
 However, the carrier is suppressed in
DSB-SC as there are no delta functions
at ±ƒ𝑐.

Generation of DSB-SC Waves
 A DSB-SC wave consists simply the product of the message signal
and the carrier wave.
 A device for achieving this requirement is called a product modulator.
 We have two forms of product modulator namely balanced
modulator and ring modulator.
48

Balanced Modulator
49
 A balanced modulator consists of two standard amplitude modulators arranged
in a balanced configuration so as to suppress the carrier wave.
 We assume that the two modulators are identical,
except for the sign reversal of the modulating
wave applied to the input of one of them.

Balanced Modulator
50
 Thus the outputs of two modulators may be expressed as
𝑠1 𝑡
𝑠2 𝑡
 Subtracting 𝑠2 𝑡
= 𝐴𝑐 1 + 𝑘𝑎𝑚(𝑡) cos 2𝜋ƒ𝑐𝑡
= 𝐴𝑐 1 − 𝑘𝑎𝑚(𝑡) cos 2𝜋ƒ𝑐𝑡
from 𝑠1 𝑡 , we obtain
𝑠 𝑡 = 𝑠2 𝑡 − 𝑠1 𝑡 = 2𝑘𝑎𝐴𝑐 cos 2𝜋ƒ𝑐𝑡 𝑚(𝑡)
 Hence, except for the scaling factor 2𝑘𝑎, the balanced
modulator output is equal to the product of modulating
wave and carrier, as required.

DSB-SC De-Modulator
51
The process of extracting an original message signal from DSBSC
wave is known as detection or demodulation of DSBSC. The
following demodulators (detectors) are used for demodulating
DSBSC wave.
•Coherent Detector
•Costas Loop

Coherent Detector
52
Here, the same carrier signal (which is used for generating DSBSC
signal) is used to detect the message signal. Hence, this process of
detection is called as coherent or synchronous detection. Following is
the block diagram of the coherent detector.

Coherent Detector
53
In this process, the message signal can be extracted from DSBSC wave
by multiplying it with a carrier, having the same frequency and the phase
of the carrier used in DSBSC modulation. The resulting signal is then
passed through a Low Pass Filter. Output of this filter is the desired
message signal.
Let the DSBSC wave be
s(t)=Accos(2πfct)m(t)
The output of the local oscillator is
c(t)=Accos(2πfct+ϕ)
From the figure, we can write the output of product
modulator as:
v(t)=s(t)c(t)
v(t)=Ac^2/2cosϕm(t)+Ac^2/2cos(4πfct+ϕ)m(t)
In the above equation, the first term is the scaled version of the
message signal. It can be extracted by passing the above signal
through a low pass filter.

Coherent Detector
54
Therefore, the output of low pass filter is:
v0t=Ac^2/2cosϕm(t)
The demodulated signal amplitude will be maximum, when ϕ= 0 degree, That’s
why the local oscillator signal and the carrier signal should be in phase, i.e., there
should not be any phase difference between these two signals.
The demodulated signal amplitude will be zero, when ϕ= +-90
degrees. This effect is called as quadrature null effect.

Costas Receiver
55
Costas loop is used to make both the carrier signal (used for DSBSC
modulation) and the locally generated signal in phase. Following is the
block diagram of Costas loop.

Costas Receiver
56
Costas loop consists of two product modulators with common
input s(t), which is DSBSC wave. The other input for both
product modulators is taken from Voltage Controlled
Oscillator (VCO) with −90 degree phase shift to one of the
product modulator as shown in figure.
The output of this Low pass filter has -90 Degrees phase difference with
the output of the upper low pass filter.
The outputs of these two low pass filters are applied as inputs of the
phase discriminator. Based on the phase difference between these two
signals, the phase discriminator produces a DC control signal.
This signal is applied as an input of VCO to correct the phase error in VCO
output. Therefore, the carrier signal (used for DSBSC modulation) and the
locally generated signal (VCO output) are in phase.

Single Side Band (SSB) Modulation
57
The process of suppressing one of the sidebands, along with the
carrier and transmitting a single sideband is called as Single
Sideband Suppressed Carrier system, or simply SSB-
SC or SSB. It is plotted as shown in the following figure.

Generation of SSB
58
There are two methods used for SSB
Transmission.
1.Filter Method
2.Phase Shift Method

De-Modulation of SSB
59
The demodulation of single-sideband (SSB) signals requires special attention, because
simple mixing leads to superposition of the upper and lower sidebands at audio
frequencies. The following article gives an overview of the different methods for SSB
demodulation and their use in software defined radios. SSB modulators and
demodulators are sometimes also referred to as image reject mixers.
1. The Filter Method (Superhet)
The filter method is the traditional SSB reception method in analog superhet receivers.
Typically a first mixer translates the signal to an intermediate frequency (IF) first. At the IF a
sharp band-pass filter with fixed frequency response simply selects only one of the two
sidebands and suppresses the other one. Then the second mixer converts the remaining
sideband to audio frequencies. If required, the audio spectrum can be inverted by using
high-side local oscillator injection in one of the mixer stages.

61
2. The Phasing Method (Direct Conversion):
The phasing method uses complex IQ processing to resolve the
superposition of lower and upper sideband at audio frequencies.
The complex mixer directly converts the incoming signal to audio
frequencies ands creates I and Q components (zero-IF or direct
conversion receiver). The complex mixer stage consists of a a sine
and cosine local oscillator (e.g. implemented by a phase shift of
90°) and two mixers. The following low-pass filters with a width of
one sideband determines the final bandwidth. After filtering a
Hilbert transformer shifts the Q component by 90° before it can
be added or subtracted to the I component to select one of the
two sidebands.

63
3. The Weaver Demodulator (The “Third Method”):
The Weaver demodulator (also called the “Third Method”, besides
filtering and phasing techniques) for SSB reception has been introduced
by D. Weaver in “A Third Method of Generation and Detection of Single-
Sideband Signals”. It differs a little bit from the first two methods since
it does not resolve any superposition of lower and upper sideband.
Instead the Weaver simply converts a portion of the spectrum to audio
frequencies without any ambiguities. The Weaver demodulator uses two
complex mixing stages. The first mixer stages translates the signal in
order to center the wanted sideband at zero frequency. There
the bandwidth is selected by low-pass filters with half the bandwidth of
a . A second mixer stage translates the signal in order to align its
frequencies with the audio frequencies. Summing or subtracting I and Q
selects the orientation of the output spectrum (normal or inverted).

Vestigial Side-Band(VSB) Modulation
65
Vestigial Sideband:
Both of the sidebands are not required for the transmission, as it is a
waste. But a single band if transmitted, leads to loss of information.
Hence, this technique has evolved.
Vestigial Sideband Modulation or VSB Modulation is the process where a
part of the signal called as vestige is modulated, along with one sideband.
A VSB signal can be plotted as shown in the following figure.

66
Along with the upper sideband, a part of the lower sideband is also being
transmitted in this technique. A guard band of very small width is laid on
either side of VSB in order to avoid the interferences. VSB modulation is
mostly used in television transmissions.
VSB Modulation − Advantages:
Following are the advantages of VSB −
•Highly efficient.
•Reduction in bandwidth.
•Filter design is easy as high accuracy is not needed.
•The transmission of low frequency components is possible,
without difficulty.
•Possesses good phase characteristics.

67
VSB Modulation − Disadvantages:
Following are the disadvantages of VSB −
•Bandwidth when compared to SSB is greater.
•Demodulation is complex.
AD
VSB Modulation − Application:
The most prominent and standard application of VSB is for
the transmission of television signals. Also, this is the most
convenient and efficient technique when bandwidth usage is
considered.

Frequency Division Multiplexing
68
Definition: Frequency division multiplexing is a multiplexing technique in
which multiple separate information signals can be transmitted over a single
communication channel by occupying different frequency slots within common
channel bandwidth.
FDM technique is basically used for muxing of analog signals.
Look at the figure given below to understand how multiple signals can be
transmitted over a common channel.

69
In Frequency division multiplexing simultaneous transmission of the signal
takes place over a common channel in which the channel bandwidth is
divided into various sub-channels. These sub-channels comprised of different
frequency slots for carrying individual signal during transmission.

70
When we talk about the transmission of various signals to a long distance
through FDM technique, the required bandwidth needed for transmission must
also be large. Now, the question arises how can we provide multiple signals,
various frequency slots under the same channel bandwidth?
The answer to the above question is modulation. The different input signal
modulates different carriers having different frequencies. These signals are then
mixed to form a hybrid signal for transmitting it over a single channel.
When we talk about a common communication channel the thing that first
comes to our mind is overlapping of signals. But here the total available
bandwidth is divided into various non-overlapping frequency bands which will
carry different input signal thus preventing them from causing interference. We
can use the full allotted time for transmission, the only need is the division of
frequency. Sometimes this transmission during the same time interval leads
to crosstalk.

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