communications _ amplitute modulation
TRANSCRIPT
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MODULATION
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Modulation is the process of putting information
onto a high frequency carrier for transmission
(frequency translation).
1. What is modulation?
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Once this information is received, the low frequency
information must be removed from the high frequency
carrier. This process is known as Demodulation.
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2. What are the reasons formodulation?
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1. Frequency division multiplexing (To support multiple
transmissions via a single channel)
To avoid interference
2. What are the reasons formodulation?
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f
M1(f)
0
f
M2(f)
0
f
M(f)
0 f1 f2
Multiplexed
signal
+
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2. Practicality of Antennas
Transmitting very low frequencies require antennas with
miles in wavelength
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3.What are the Different ofModulation Methods?
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1. Analogue modulation- The modulating signal and
carrier both are analogue signals
Examples: Amplitude Modulation (AM) , Frequency
Modulation (FM) , Phase Modulation (PM)
2. Pulse modulation- The modulating signal is an
analogue signal but Carrier is a train of pulses
Examples : Pulse amplitude modulation (PAM), Pulse
width modulation (PWM), Pulse position modulation
(PPM)
3. What are the Different ofModulation Methods?
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3. Digital to Analogue modulation- The modulating
signal is a digital signal , but the carrier is an
analogue signal.
Examples: Amplitude Shift Keying (ASK), FSK, Phase
Shift Keying (PSK)
4. Digital modulation -
Examples: Pulse Code Modulation, Delta
Modulation,Adaptive Delta Modulation
3.What are the Different ofModulation Methods?
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ANALOG AND DIGITALANALOG AND DIGITAL
Analog-to-analog conversion is the representation ofAnalog-to-analog conversion is the representation ofanalog information by an analog signal. One may askanalog information by an analog signal. One may ask
why we need to modulate an analog signal; it is alreadywhy we need to modulate an analog signal; it is already
analog. Modulation is needed if the medium isanalog. Modulation is needed if the medium is
bandpass in nature or if only a bandpass channel isbandpass in nature or if only a bandpass channel is
available to us.available to us.
Amplitude Modulation
Frequency Modulation
Phase Modulation
Topics discussed in this section:Topics discussed in this section:
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Figure Types of analog-to-analog modulation
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Figure Amplitude modulation
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The total bandwidth required for AM
can be determined
from the bandwidth of the audio
signal: BAM = 2B.
Note
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Figure AM band allocation
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The total bandwidth required for FM canbe determined from the bandwidth
of the audio signal: BFM = 2(1 + )B.
Note
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Figure Frequency modulation
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Figure FM band allocation
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Figure Phase modulation
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The total bandwidth required for PM can
be determined from the bandwidthand maximum amplitude of the
modulating signal:
BPM = 2(1 + )B.
Note
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4. What are the Basic Types ofAnalogue Modulation Methods ?
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Consider the carrier signal below:
sc(t ) = Ac(t) cos( 2fc t + )
1. Changing of the carrier amplitude Ac(t)produces
Amplitude Modulation signal(AM)
2. Changing of the carrier frequency fc produces
Frequency Modulation signal(FM)
3. Changing of the carrier phase producesPhase Modulation signal(PM)
4. What are the Basic Types ofAnalogue Modulation Methods ?
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Analogue Modulation Methods
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5 Wh t th diff t F
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5. What are the different Formsof Amplitude Modulation ?
5 Wh t th diff t F
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1. Conventional Amplitude Modulation (DSB-LC)
(Alternatively known as Full AM or Double Sideband
with Large carrier (DSB-LC) modulation
2. Double Side Band Suppressed Carrier(DSB-SC)
modulation
3. Single Sideband(SSB) modulation
4. Vestigial Sideband(VSB) modulation
5. What are the different Formsof Amplitude Modulation ?
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Conventional Amplitude Modulation
(Full AM)
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6. Derive the Frequency Spectrum for Full-AM
Modulation (DSB-LC)
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1 The carrier signal is
ccccc
ftAts 2where)cos()( ==
2 In the same way, a modulating signal (information
signal) can also be expressed as
tAtsmmm
cos)( =
6. Derive the Frequency Spectrum for Full-AM
Modulation (DSB-LC)
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3 The amplitude-modulated wave can be expressed as
[ ] )cos()()( ttsAts cmc +=
[ ] )cos()cos()( ttAAts cmmc +=
4 By substitution
c
m
A
Am =
5 The modulation index.
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6 Therefore The full AM signal may be
written as
)cos())cos(1()( ttmAts cmc +=
)]cos()[cos(2/1coscos BABABA ++=
tmA
tmA
tAtsmc
c
mc
c
cc)cos(
2)cos(
2)(cos)( +++=
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7. Draw the Frequency Spectrum of the above AM
signal and calculate the Bandwidth
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fC fc+fmfc-fm
2fm
7. Draw the Frequency Spectrum of the above AM
signal and calculate the Bandwidth
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8. Draw Frequency Spectrum for a complex input
signal with AM
8 S f i
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8. Draw Frequency Spectrum for a complex input
signal with AM
fcfc-fm fc+fm
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The frequency spectrum of AM waveform containsthree parts:
1. A component at the carrierfrequency fc
2. An upper side band (USB), whose highest frequency
component is at fc+fm
3. A lower side band (LSB), whose highest frequencycomponent is at fc-fm
The bandwidth of the modulated waveform is twice the
information signal bandwidth.
Frequency Spectrum of an AM signal
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Because of the two side bands in the frequency spectrum its
often called Double Sideband with Large Carrier.(DSB-LC)
The information in the base band (information) signal is
duplicated in the LSB and USB and the carrier conveys no
information.
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ExampleExample
We have an audio signal with a bandwidth of 5 KHz.
What is the bandwidth needed if we modulate the signal
using AM?
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ExampleExample
We have an audio signal with a bandwidth of 5 KHz.
What is the bandwidth needed if we modulate the signal
using AM?
SolutionSolution
An AM signal requires twice the bandwidth of the original
signal:
BW = 2 x 5 KHz = 10 KHz
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AM Radio Band
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Modulation Index (m)
m is merely defined as a parameter, which determines the
amount of modulation.
What is the degree of modulation required to establish a
desirable AM communication link?
Answer is to maintain m
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Modulation Index (m)9. What is the significance of modulation index ?
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If the amplitude of the modulating signal is higher than the
carrier amplitude, which in turn implies the modulation
index . This will cause severe distortion to the
modulated signal.
%)100(0.1m
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The ratio of useful power, power efficiency :
2
2
2
2
22/12/
mm
mm
powertotalpowersidebands
+=
+=
In terms of power efficiency, for m=1 modulation, only33% power efficiency is achieved which tells us that only
one-third of the transmitted power carries the useful
information.
10. Calculate the power efficiency of AM signals
Double Side Band Suppressed Carrier
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The carrier component in full AM or DSB-LC does not convey any
information. Hence it may be removed or suppressed during the
modulation process to attain higher power efficiency.
The trade off of achieving a higher power efficiency using DSB-SC is
at the expense of requiring a complex and expensive receiver due to
the absence of carrier in order to maintain transmitter/receiversynchronization.
Double Side Band Suppressed Carrier(DSB-SC) Modulation
11 Derive the Frequency Spectrum for Double Sideband
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1 Consider the carrier
ccccc ftAts 2where)cos()( ==
2 modulated by a single sinusoidal signal
mmmm ftAts 2wherecos)( m ==3 The modulated signal is simply the product of these two
( )
LSB
mc
cm
USB
mc
cm
mcmc
mmcc
tAA
tAA
BABABA
ttAA
tAtAts
)cos(
2
)cos(
2
)cos()cos(2
1coscossince
)cos()cos(
)cos()cos()(
++=
++=
=
=
11. Derive the Frequency Spectrum for Double Sideband
Suppressed Carrier Modulation (DSB-SC)
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tAts mmm cos)( =
tAts ccc cos)( =
)cos()cos()( tAtAts mmcc =X
fcfc-fm fc+fm
Frequency Spectrum of a DSB-SC AM Signal
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All the transmitted power is contained in the two sidebands
(no carrier present).
The bandwidth is twice the modulating signal bandwidth.
USB displays the positive components ofsm(t) and LSB
displays the negative components ofsm(t).
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Generation and Detection of DSB-SC
The simplest method of generating a DSB-SC signal is
merely to filterout the carrier portion of a full AM (or DSB-
LC) waveform.
Given carrier reference, modulation and demodulation
(detection) can be implemented using product devices or
balanced modulators.
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BALANCED MODULATOR
AM Modulator 1
AM Modulator 2
Carrier
Sm(t)
Sm(t)
-Sm(t)
Accos(ct)
Accos(ct)
S2(t)
S1(t)
S(t)
DSB-SC
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The two modulators are identical except for the sign reversal
of the input to one of them. Thus,
)cos())cos(1()(1 ttmAts cmc +=
)cos())cos(1()(2 ttmAts cmc =
)cos()cos(2
)()()( 21
ttmA
tststs
cmc =
=
COHERENT (SYNCHRONOUS) DETECTOR OR
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DSB-SC Signal s(t)
Local Oscillator
LPFX
Cosct
v(t) vo(t)
COHERENT (SYNCHRONOUS) DETECTOR OR
DSB-SC (PRODUCT DETECTOR)
Since the carrier is suppressed the envelope no longer
represents the modulating signal and hence envelope
detector which is of the non-coherent type cannot be used.
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[ ]
2cos)
)cos()(since
)2cos()cos()cos(
2
2cos1)cos(2
)(cos)cos(2
)cos()cos()cos(2)cos()()(
2
d by LPF)erm(removeUnwanted t
cmm
mmm
cmmmm
cmm
cmc
c
m
ccmcc
t)((ts(t)s
tAts
ttAtA
ttA
ttAA
A
tttmAttstv
+=
=+=
+=
=
==
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It is necessary to have synchronization in both frequency
and phase between the transmitter (modulator) & receiver
(demodulator), when DSB-SC modulation ,which is of the
coherent type, is used.Both phase and frequency must be known to demodulate
DSB-SC waveforms.
LACK OF PHASE SYNCHRONISATION
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LACK OF PHASE SYNCHRONISATION
Let the received DSB-SC signal be( ) ccmSCDSB Attsts += cos)()(
if is unknown,
( )
( )[ ]
++=
+==
tts
A
tttsA
ttstv
cm
c
ccmc
cSCDSB
2coscos)(2
coscos)(
cos)()(
Output of LPF
cos)(2)( ts
A
tv mc
o=
But we want just
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But we want just
)(
2
)( tsA
tv mc
o =
Due to lack of phase synchronization, we will see that the
wanted signal at the output of LPF will be attenuated by an
amount of cos .In other words, phase error causes an attenuation of the
output signal proportional to the cosine of the phase error.
The worst scenario is when = /2, which will give rise tozero or no output at the output of the LPF.
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LACK OF FREQUENCY SYNCHRONISATION
Suppose that the local oscillator is not stable at fc but at
fc+ f,then ( )( )
( )[ ]
++=
+=
+=
tttsA
tttsA
ttstv
cmc
ccmc
cSCDSB
2coscos)(2
coscos)(
cos)()(
Output of LPF
ttsAtv mc
o = cos)(2
)(
Thus, the recovered baseband information signal will vary
sinusoidal according to cos t
This problem can be overcome by adding an extra
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This problem can be overcome by adding an extra
synchronization circuitry which is required to detect and t and by providing the carrier signal to the receiver.A synchronizer is introduced to curb the synchronization
problem exhibited in a coherent system.
Let the baseband signal be
tAtsmmm
cos)(=
Received DSB-SC signal
ttsAts cmc cos)()( =
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( )2 PLL BPF 2
SYNCHRONISER
Mathematical analysis of the synchronizer is shown below:
[ ][ ]
[ ]
( ) ( )
+++++=
+++=
++=
=
ttttAA
ttttAA
ttAA
ttAAts
mcmccmmc
cmcmmc
cmmc
cmmc
2cos2
12cos
2
12cos2cos1
4
2cos2cos2cos2cos14
2cos12cos14
coscos)(
22
22
22
22222
Output of BPF
t
AA
c
mc
2cos4
22
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Output of frequency dividertk ccos
where kis a constant of proportionality.
DISADVANTAGE OF USING COHERENT SYSTEMS
The frequency and phase of the local oscillator signal must
be very precise which is very difficult to achieve.
It requires additional circuitry such as synchronizer circuit
and hence the cost is higher.
Single-Sideband Modulation
Single Side Band Modulation (SSB)
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g
How to generate SSB signal?
Generate DSB-SC signal
Band-pass filter to pass only one of the sidebandand suppress the other.
For the generation of an SSB modulated signal
to be possible, the message spectrum must have
an energy gap centered at the origin.
Single Side Band Modulation (SSB)
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Example of signal with -300 Hz ~ 300 Hz energy gap
Voice : A band of 300 to 3100 Hz gives good articulation Also required for SSB modulation is a highly selective filter
Vestigial Sideband Modulation
Vestigial Side Band Modulation (VSB)
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Instead of transmitting only one sideband as SSB, VSB
modulation transmits a partially suppressed sideband and avestige of the other sideband.
Vestigial Side Band Modulation (VSB)
C i f A li d M d l i h d
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Comparison of Amplitude Modulation methods
C i f A lit d M d l ti th d
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Full AM (or DSB-LC)
- Sidebands are transmitted in full with the carrier.
- Simple to demodulate / detect
- Poor power efficiency
- Wide bandwidth ( twice the bandwidth of the information
signal)- Used in commercial AM radio broadcasting, one transmitter
and many receivers.
Comparison of Amplitude Modulation methods
C i f A lit d M d l ti th d
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DSB-SC
- Less transmitted power than full AM and all the transmitted
power is useful.- Requires a coherent carrier at the receiver; This results in
increased complexity in the detector(i.e. synchroniser)
- Suited for point to point communication involving one
transmitter and onereceiver which would justify the use of
increased receiver complexity.
Comparison of Amplitude Modulation methods
Comparison of Amplitude Modulation methods
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SSB
- Good bandwidth utilization (message signal bandwidth =
modulated signal bandwidth)- Good power efficiency
- Demodulation is harder as compares to full AM; Exact filter
design and coherent demodulation are required
- Preferred in long distance transmission of voice signals
Comparison of Amplitude Modulation methods
Comparison of Amplit de Mod lation methods
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VSB
- Offers a compromise between SSB and DSB-SC
- VSB is standard for transmission of TV and similar signals- Bandwidth saving can be significant if modulating signals
are of large bandwidth as in TV and wide band data signals.
For example with TV the bandwidth of the modulatingsignal can extend up to 5.5MHz; with full AM the
bandwidth required is 11MHz
Comparison of Amplitude Modulation methods