CHAPTER 2: AMPLITUDE MODULATION 1
May 25, 2015
CHAPTER 2: AMPLITUDE MODULATION
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AM RADIO/RECEIVER
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TOPICS
Need for Modulation Principles of AM Modulation Index and Signal Power AM Circuits Single Sideband Suppressed Carrier
(SSBSC) SSB Circuits
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NEED FOR MODULATION It is because modulation makes the information
signal more compatible with the medium.
Modulation = Imposing information at low frequency onto a higher frequency signal.
A technique for transmitting information efficiently from one place to another.
Simplest form of modulation is the amplitude modulation.
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PRINCIPLES OF AMPLITUDE MODULATION
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PRINCIPLES OF AM AM is defined as:
Amplitude of carrier frequency change proportionately to the value of the modulation signal.
Advantages: Simple modulator circuits Cheap :low-quality form of modulation used for
commercial broadcasting of audio & video signal.
Disadvantages: Poor performance due to noise Inefficient use of transmitter power.
Application: 2 way radio communications, broadcasting, aircraft
comm. & citizen band (CB) radio.6
AM modulators are nonlinear devices 2 input and 1 output:
modulating signal and carrier signal.
Several types of amplitude modulation AM DSBFC DSB-SC SSB
AM generation is shown in Figure 2.1
Modulated wave = AM envelope as shown in Figure 2.2
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Figure 2.1: Block diagram of Amplitude Modulation
Information, Vm(t)
Carrier, Vc(t)
ModulatorVAM (t)
8Figure 2.2 AM signal with the envelope
AM IN ACTION
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AM begins with carrier vc , a sine wave with frequency c & amplitude Vc:
Modulating signal:
Then AM is:
DERIVATION OF AM EQUATION
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Where m (modulation index) is defined as Vm/Vc, hence:
The voltage resulting AM wave envelope at any instant is:
tfmVVV
VVmtfVVV
vVV
mccenv
cmmmcenv
mcenv
2sin
]/[2sin
)2sin1( tfmVV mcenv
tftfmV
tfVv
cmc
cenv
2sin)2sin1(
2sin
This yield, the upper and lower sidebands – frequency & amplitude. 11
tffV
tffV
tfVv mcm
mcm
ccAM )(2cos2
)(2cos2
2sin
Carrier LSB USB
tffVm
tffVm
tfVv
mcc
mcc
cc
)(2cos2
)(2cos2
2sin
)cos()cos(2/1))(sin(sin bababa
Using Trigo ID
AM FREQUENCY SPECTRUM & BANDWIDTH
AM modulators are non-linear device => non-linear mixing occurs.
Output envelope is complex wave made up of DC voltage, carrier frequency, the sum (fm + fc) & difference (fc – fm) frequencies.
AM spectrum contains frequency component spaced fm Hz on either side of the carrier.
Figure 2.3 shows the frequency spectrum of AM wave.
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Figure 2.3: Frequency spectrum for AM wave
fc fc + fmfc - fm
Bandwidth = 2fm
Vc
Vm /2Vm /2
SPECTRUM PARAMETERS
Center frequency = Carrier frequency =
Upper sideband freq. = carrier freq. + modulating freq.
Lower sideband freq. = carrier freq. - modulating freq.
mcUSB fff
mcLSB fff
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Center frequency peak amplitude:
Upper and lower sideband voltages:
Bandwidth = Maximum freq. - minimum freq.
2m
LSBUSB
VVV
cV
m
mcmc
lsbusb
f
ffff
ff
ffBW
2
)()(
minmax
cf
EXAMPLE 2.1
Q. Modulating signal fm =3 kHz frequency and a carrier frequency fc =1 MHz. What is the upper & lower sideband frequency? Then find the bandwidth of the modulated signal.
A. 997 kHz, 1003 kHz, 6 kHz.
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EXAMPLE 2.2
Q. A 1.4 MHz carrier is modulated by a signal with frequencies from 20Hz & 10KHz. Determine the range of frequencies generated for the upper and lower sidebands?
A. USB = 1.400020Hz, 1.410000Hz, LSB = 1.390000Hz, 1.399980Hz
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