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8/18/2019 Amplitude Modulation Technique
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Amplitude modulation (AM) radio is a commonplace
technology today, and is standard in any type of
commercial stereo device. Because of the low cost of the
parts necessary to implement AM transmission and the
simplicity of the underlying technology, using amplitude
modulations is a cheap and effective way to perform many
tasks that require wireless communication.
The most wellknown application of an AM transmitter is in
radio. Am radio receivers are availa!le in numerous
devices, from automo!ile stereos to clock radios.
"owever, the usage of AM transmitters is not restricted to
professional radio stations
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Transmit information !earing (message) or !ase!and signal
(voice music) through a communication channel
Base!and # is a range of frequency signal to be transmitted.
eg: Audio (0 - 4 kHz), ideo (0 - ! "Hz).
$ommunication channel% #ransmission $it%out frequency s%ifting.
#ransmission t%roug% t$isted &air cable, coa'ial cable and fibero&tic cable.
ignificant &o$er for $%ole range of frequencies. ot suitable for radio*micro$a+e and satellite communication.
$arrier communication se tec%nique of modulation to s%ift t%e frequency. %ange t%e carrier signal c%aracteristics (am&litude,
frequency and &%ase) follo$ing t%e modulatingsignal am&litude.
uitable for radio*micro$a+e and satellitecommunication.
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the instantaneous amplitude of a carrier wave is varied in
accordance with the instantaneous amplitude of the
modulating signal. Main advantages of AM are small
!andwidth and simple transmitter and receiver designs.
Amplitude modulation is implemented !y mi&ing the
carrier wave in a nonlinear device with the modulating
signal. This produces upper and lower side!ands, which
are the sum and difference frequencies of the carrier
wave and modulating signal. The carrier signal is represented !y
c(t) # A cos(wct)
The modulating signal is represented !y m(t) # B sin(wmt)
Then the final modulated signal is
' m(t)* c(t) # A ' m(t)* cos(wct)
# A ' B sin(wmt)* cos(wct)
# A cos(wct) A m+ (cos((wcwm)t)) A m+ (cos((wcwm)t))
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--B-B -/01A
--B2-B -/01A
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--B signal frequency spectrum
--B-B signal
--B2-B signal
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"artley modulator A direct approach for creating a single side!and AM signal (--BAM)
is to remove either the upper or lower -B !y filtering the 3-B-$AM signal (frequency discriminator method)
--B modulator using the frequency discrimination approach
Magnitude spectra % (a) !ase!and 4 (!) 3-B-$AM4 (c) upper --B4 (d) lower --B
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5uadrature modulator can !e used to create a --BAM
signal !y selecting the quadrature signal to coherently
cancel either the upper +lower -B from the inphase
channel --BAM signal given%
2sing the minus sign in equation () results in upper--B,whereas selection of the plus sign yields lower --B
The hil!ert transform is a wide!and 678 phase shifter
()
()
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"artley modulator for --B4 sign gives upper --B, signgives lower --B