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AMPLITUDE MODULATION
Unit -1Semester :5th
Department:electronics and Instrumentation
Reference books: Communication systems- Taub Schiling Communication systems –Sanjay sharma
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INTRODUCTION
• Amplitude Modulation is the simplest and earliest form of transmitters
• AM applications include broadcasting in medium- and high-frequency applications, CB radio, and aircraft communications
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Amplitude Modulation
Amplitude Modulation is a process where the amplitude of a carrier signal is altered according to information in a message signal.
The frequency of the carrier signal is usually much greater than the highest frequency of the input message signal.
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AM – Basic Definitions
The AM signal
The modulating signal:
The Carrier Signal:
ttmkAts cc cos 1
tm
tAtc cc cos
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-5
0
5
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-1
0
1
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-1
0
1
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AM – Basic Definitions(cont.)The information signal
varies the instantaneous amplitude of the carrier
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AM – Basic Definitions(cont.)
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-5
-4
-3
-2
-1
0
1
2
3
4
5
tmkAts c 1
The Envelope:
The AM Signal
ttmkAts cc cos 1
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AMPLITUDE SENSITIVITY
Modulation Index - The ratio between the amplitudes between the amplitudes of the modulating signal and carrier, expressed by the equation:
c
m
EEm =
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AM – Percentage Modulation
Under modulated (<100%) 100% modulated
Envelope Detector
Can be used
Envelope Detector
Gives Distorted signal
Over Modulated (>100%)
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NEED OF THE MODULATION PROCESS
To reduce the height of the antennaTo avoid mixing signalsTo increase the range of communicationTo improve quality of receptionLastly to increase over all strength
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Bandwidth Signal bandwidth is an important
characteristic of any modulation scheme In general, a narrow bandwidth is desirableBandwidth is calculated by:
mFB 2
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Power Relationships
Power in a transmitter is important, but the most important power measurement is that of the portion that transmits the information
AM carriers remain unchanged with modulation and therefore are wasteful
Power in an AM transmitter is calculated according to the formula at the right
21
2mPP ct
12
Modulation Index of AM Signal
13
Double-Sideband Suppressed-Carrier AM
• A double-sideband, suppressed-carrier (DSB-SC) AM signal is obtained by multiplying the message signal m(t) with the carrier signal c(t) =
Accos(2fct)
• Amplitude-modulated signal
• An example of the message signal m(t), the carrier c(t), and the modulated signal u (t) are shown in Figure 3.1
• This figure shows that a relatively slowly varying message signal m(t) is changed into a rapidly varying modulated signal u(t), and due to its rapid changes with time, it contains higher frequency components
• At the same time, the modulated signal retains the main characteristics of the message signal; therefore, it can be used to retrieve the message signal at the receiver
)2cos()()()()( tftmAtctmtu cc
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Double-Sideband Suppressed-Carrier AM
• Figure 3.1 An example of message, carrier, and DSB-SC modulated signals
Single-Sideband AM• The two sidebands of an AM signal are mirror images of one
another• As a result, one of the sidebands is redundant• Using single-sideband suppressed-carrier transmission results
in reduced bandwidth and therefore twice as many signals may be transmitted in the same spectrum allotment
• Typically, a 3dB improvement in signal-to-noise ratio is achieved as a result of SSBSC
16
Single-Sideband AM• A method, illustrated in Figure
3.16, generates a DSB-SC AM
signal and then employs a filter
that selects either the upper
sideband or the lower sideband of
the double-sideband AM signal
.,
.
Figure 3.16 Generation of a single-sideband AM signal by filtering one of the sidebands of a DSB-SC AM signal.
Sideband and carrier power• Carrier term does not carry information, and
hence the carrier power is wasted
• The carrier power is the mean sq. value of which is • The sideband power is the mean sq. value
• of which is
sidebandscarrierttmtAt ccAM cos)(cos)(cP
tA ccos 2/2AsP
ttm ccos)( 2/)(2 tm
Power Efficiency
• The power efficiency
• For the special case of tone modulation
• Hence
tAtm m cos)(
%100)(
)(22
2
tmA
tmPP
P
sc
s
%1002
%1002/
2/2
2
22
2
AA
APP
P
sc
s
2/)( 22 Atm
%33,1 max
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Single-Sideband AM• A DSB-SC AM signal transmits two sidebands and required
a channel bandwidth of Bc = 2W Hz– However, the two sidebands are redundant
• The transmission of either sideband is sufficient to reconstruct the message signal m(t) at the receiver
• Thus, we reduce the bandwidth of the transmitted signal from 2W to W
• In the appendix 3A, a single-sideband (SSB) AM signal is represented mathematically as
– where is the Hilbert transform of m(t)– The plus sign indicates the lower sideband and the minus sign
indicates the upper sideband)(ˆ tm
)2sin()(ˆ)2cos()()( tftmAtftmAtu cccc
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Single-Sideband AM• The SSB-AM signal u(t) may be
generated by using the system
configuration as shown in right. (Generation of a lower SSB-AM)
• Another method (“filter method”)
generates a DSB AM signal and
then employs a filter that selects
either the upper sideband or the
lower sideband of the DSB AM.
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Demodulation of SSB-AM Signals• To recover the message signal m(t) in the received SSB-AM signal, we
require a phase-coherent or synchronous demodulator• For the USSB signal
• By passing the product signal in above equation through an lowpass filter, the double-frequency components are eliminated. Then
– Note that the phase offset not only reduces the amplitude of the desired signal m(t) by cos, but it also results in an undesirable sideband signal due to the presence of in yl(t)
– The latter term was not present in the demodulation of a DSBSC signal– It contributes to the distortion of the demodulated SSB signal
)(ˆ tm
terms.freq. double )sin()(ˆ)cos()()2cos()()2cos()(
21
21
tmAtmAtftutftr
cc
cc
)sin()(ˆ)cos()()( 21
21 tmAtmAty ccl
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Demodulation of SSB-AM Signals• The transmission of a pilot tone at the carrier frequency is a
very effective for providing a phase-coherent reference signal • However, a portion of the transmitted power must be
allocated to the transmission of the carrier• The spectral efficiency of SSB AM is very attractive in voice
communications over telephone channels• “Filter method”, which selects one of the two signal sidebands
for transmission, is difficult to implement when the message signal m(t) has a large power concentrated around f = 0– In such a case, the sideband filter must have an extremely sharp
cutoff around the carrier in order to reject the sideband– Such filter characteristics are very difficult to implement in
practice
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Vestigial-Sideband AM• The stringent-frequency response requirements on the
sideband filter in an SSB-AM system can be relaxed by allowing vestige, which is a portion of the unwanted sideband, to appear at the output of the modulator
• Thus, we simplify the design of the sideband filter at the cost of a small increase in the channel bandwidth required to transmit the signal
• The resulting signal is called vestigial-sideband (VSB) AM– This type of modulation is appropriate for signals that have a strong
low-frequency component, such as video signals– That is why this type of modulation is used in standard TV
broadcasting
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Vestigial-Sideband AM• To generate a VSB-AM signal, we generate a DSB-SC AM
signal and pass it through a sideband filter with the frequency response H( f ), as shown in below
• In the time domain, the VSB signal may be expressed as
– where h(t) is the impulse response of the VSB filter
• In the frequency domain, the corresponding expression is (eq. 1)
)(]2cos)([)( thtftmA tu cc
)()()(2
)( fHffMffMAfU cncnc
Generation of vestigial-sideband AM signal.
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Vestigial-Sideband AM• To determine the frequency-response characteristics of the
filter, we will consider the demodulation of the VSB signal u(t).• We multiply u(t) by the carrier component cos2fct and pass
the result through an ideal lowpass filter, as shown in below.– Thus, the product signal is or
)(t
tftutv c2cos)()( )()(21)( cc ffUffUfV
Demodulation of VSB signal.
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Vestigial-Sideband AM• If we substitute U( f ) from eq. (1) into V(f) , we obtain
• The lowpass filter rejects the double-frequency terms and passes only the components in the frequency range | f|W
• Hence, the signal spectrum at the output of the ideal lowpass filter is
• The message signal at the output of the lowpass filter must be undistorted– Hence, the VSB-filter characteristic must satisfy the condition
)()2()(4
)()()2(4
)( ccc
ccc ffHffMfMAffHfMffMAfV
)()()(4
)( ccc
l ffHffHfMAfV
WfffHffH cc || constant )()(
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Vestigial-Sideband AM
• We note that H(f) selects the upper sideband and a vestige of the lower sideband
• It has odd symmetry about the carrier frequency fc in the frequency range fc - fa < f < fc + fa, where fa is a conveniently selected frequency that is some small fraction of W, i.e., fa << W
• Thus, we obtain an undistorted version of the transmitted signal
VSB-filter characteristics.
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Vestigial-Sideband AM• The frequency response of a VSB filter that selects the lower
sideband and a vestige of the upper sideband is shown in below
• In practice, the VSB filter is designed to have some specified phase characteristic
• To avoid distortion of the message signal, VSB filter should have a linear phase over its passband fc - fa | f | fc + W
Frequency response of the VSB filter for selecting the lower sideband of the message signals.
Advantages/disadvantagesAdvantages of Amplitude Modulation, AMThere are several advantages of amplitude modulation, and some of these reasons have
meant that it is still in widespread use today:• It is simple to implement• it can be demodulated using a circuit consisting of very few components• AM receivers are very cheap as no specialized components are needed.
Disadvantages of amplitude modulationAmplitude modulation is a very basic form of modulation, and although its simplicity is one of
its major advantages, other more sophisticated systems provide a number of advantages. Accordingly it is worth looking at some of the disadvantages of amplitude modulation.
• It is not efficient in terms of its power usage• It is not efficient in terms of its use of bandwidth, requiring a bandwidth equal to twice
that of the highest audio frequency• It is prone to high levels of noise because most noise is amplitude based and obviously
AM detectors are sensitive to it.