Amplitute Modulation
Post on 22-Dec-2015
1 Views
Preview:
DESCRIPTION
Transcript
MODULATION
1. What is modulation?
• Modulation is the process of putting information
onto a high frequency carrier for transmission
(frequency translation).
1. What is modulation?
• Once this information is received, the low frequency
information must be removed from the high frequency
carrier. This process is known as “ Demodulation”.
2. What are the reasons for modulation?
1. Frequency division multiplexing (To support multiple
transmissions via a single channel)
To avoid interference
2. What are the reasons for modulation?
f
M1(f)
0
f
M2(f)
0
f
M(f)
0 f1 f2
Multiplexed signal
+
2. Practicality of Antennas
Transmitting very low frequencies require antennas with
miles in wavelength
3.What are the Different of Modulation Methods?
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 of Modulation Methods?
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 of Modulation Methods?
ANALOG AND DIGITALANALOG AND DIGITAL
Analog-to-analog conversion is the representation of Analog-to-analog conversion is the representation of analog information by an analog signal. One may ask analog information by an analog signal. One may ask why we need to modulate an analog signal; it is why we need to modulate an analog signal; it is already analog. Modulation is needed if the medium is already analog. Modulation is needed if the medium is bandpass in nature or if only a bandpass channel is bandpass in nature or if only a bandpass channel is available to us. available to us.
Amplitude ModulationFrequency ModulationPhase Modulation
Topics discussed in this section:Topics discussed in this section:
Figure Types of analog-to-analog modulation
Figure Amplitude modulation
The total bandwidth required for AM can be determined
from the bandwidth of the audio signal: BAM = 2B.
Note
Figure AM band allocation
The total bandwidth required for FM can be determined from the bandwidth of the audio signal: BFM = 2(1 + β)B.
Note
Figure Frequency modulation
Figure FM band allocation
Figure Phase modulation
The total bandwidth required for PM can be determined from the bandwidth
and maximum amplitude of the modulating signal:
BPM = 2(1 + β)B.
Note
4. What are the Basic Types of Analogue Modulation Methods ?
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 produces
Phase Modulation signal (PM)
4. What are the Basic Types of Analogue Modulation Methods ?
Analogue Modulation Methods
5. What are the different Forms of Amplitude
Modulation ?
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 Forms of Amplitude
Modulation ?
Conventional Amplitude Modulation (Full AM)
6. Derive the Frequency Spectrum for Full-AM Modulation (DSB-LC)
1 The carrier signal is
ccccc ftAts 2 where)cos()(
2 In the same way, a modulating signal (information
signal) can also be expressed as
tAts mmm cos)(
6. Derive the Frequency Spectrum for Full-AM Modulation (DSB-LC)
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.
6 Therefore The full AM signal may be written as
)cos())cos(1()( ttmAts cmc
)]cos()[cos(2/1coscos BABABA
tmA
tmA
tAts mcc
mcc
cc )cos(2
)cos(2
)(cos)(
7. Draw the Frequency Spectrum of the above AM signal and calculate the Bandwidth
fC fc+fmfc-fm
2fm
7. Draw the Frequency Spectrum of the above AM signal and calculate the Bandwidth
8. Draw Frequency Spectrum for a complex input signal with AM
8. Draw Frequency Spectrum for a complex input signal with AM
fcfc-fm fc+fm
The frequency spectrum of AM waveform contains three parts:
1. A component at the carrier frequency fc
2. An upper side band (USB), whose highest frequency component is at fc+fm
3. A lower side band (LSB), whose highest frequency component is at fc-fm
The bandwidth of the modulated waveform is twice the information signal bandwidth.
Frequency Spectrum of an AM signal
• 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.
ExampleExample
We have an audio signal with a bandwidth of 5 KHz. What is the bandwidth needed if we modulate the signal using AM?
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
AM Radio Band
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<1.0 (m<100%).
• This is important for successful retrieval of the original
transmitted information at the receiver end.
9. What is the significance of modulation index ?
Modulation Index (m)9. What is the significance of modulation index ?
• 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
Power distribution in full AM10. Calculate the power efficiency of AM signals
• The ratio of useful power, power efficiency :
2
2
2
2
22/1
2/
m
m
m
m
powertotal
powersidebands
• In terms of power efficiency, for m=1 modulation, only
33% 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
• 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/receiver
synchronization.
Double Side Band Suppressed Carrier (DSB-SC) Modulation
1 Consider the carrier
ccccc ftAts 2 where)cos()( 2 modulated by a single sinusoidal signal
mmmm ftAts 2 wherecos)( m 3 The modulated signal is simply the product of these two
LSB
mccm
USB
mccm
mcmc
mmcc
tAA
tAA
BABABA
ttAA
tAtAts
)cos(2
)cos(2
)cos()cos(2
1coscos since
)cos()cos(
)cos()cos()(
11. Derive the Frequency Spectrum for Double Sideband Suppressed Carrier Modulation (DSB-SC)
tAts mmm cos)(
tAts ccc cos)(
)cos()cos()( tAtAts mmcc X
fcfc-fm fc+fm
Frequency Spectrum of a DSB-SC AM Signal
• 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 of sm(t) and LSB
displays the negative components of sm(t).
Generation and Detection of DSB-SC
• The simplest method of generating a DSB-SC signal is
merely to filter out 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.
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
• 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
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.
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
cmcc
m
ccmcc
t)((ts(t)s
tAts
ttAtA
ttA
ttAA
A
tttmAttstv
• 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
Let the received DSB-SC signal be
ccmSCDSB Attsts cos)()(if is unknown,
ttsA
tttsA
ttstv
cmc
ccmc
cSCDSB
2coscos)(2
coscos)(
cos)()(
Output of LPF
cos)(2
)( tsA
tv mc
o
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 to
zero or no output at the output of the LPF.
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
ttsA
tv 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
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
tAts mmm cos)( Received DSB-SC signal
ttsAts cmc cos)()(
( )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
AAc
mc 2cos4
22
Output of frequency divider tk ccos
where k is 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
How to generate SSB signal?
• Generate DSB-SC signal
• Band-pass filter to pass only one of the sideband
and 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)
• Example of signal with -300 Hz ~ 300 Hz energy gap
Voice : A band of 300 to 3100 Hz gives goodarticulation
• Also required for SSB modulation is a highly selective filter
• Vestigial Sideband Modulation
Instead of transmitting only one sideband as SSB, VSB modulation transmits a partially suppressed sideband and a vestige of the other sideband.
Vestigial Side Band Modulation (VSB)
Comparison of Amplitude Modulation methods
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
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 one receiver which would justify the use of increased receiver complexity.
Comparison of Amplitude Modulation methods
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
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 modulating signal can extend up to 5.5MHz; with full AM the bandwidth required is 11MHz
Comparison of Amplitude Modulation methods
top related