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TE4201-Communication Electronics 3 AM wave modulated by a band of frequencies AM wave modulated by a single frequency AM modulator fcfcfcfc fsfsfsfs fcfcfcfc f c - f s f c + f s Frequency spectrum of output signal Frequency spectrum of input signal fcfcfcfc AM modulator f sL f sH f c + f sH f c -f sL f c - f sH fcfcfcfc f c + f sL Frequency spectrum of output signal Frequency spectrum of input signal
Citation preview
1TE4201-Communication Electronics
Amplitude Modulation Fundamentals
Percentage Modulation
AM Analysis
5. AM Fundamentals and Analysis5. AM Fundamentals and Analysis
2TE4201-Communication Electronics
Amplitude Modulation FundamentalsAmplitude Modulation Fundamentals
eecarriercarrier
eesignalsignal
Linear summing of carrier and signal Linear summing of carrier and signal frequenciesfrequencies
eecarriercarrier
ffcc
eesignalsignaleesignalsignal
ffss
eesumsum
ffcc , f , fss
• Radio Frequency can travel long distance whereas Signal can not. Radio Frequency can travel long distance whereas Signal can not. Modulation is to make the carrier to carry the signal along with it for a Modulation is to make the carrier to carry the signal along with it for a long distance.long distance.
AM AM modulatomodulato
rr
ffss ffcc ffss2f2fss
3f3fss
ff ffvvoo
vvinin
vvoovvininvvinin vvoo
ffccffcc- f- fss ffcc+ f+ fss
2f2fcc
Modulation of carrier and signal frequenciesModulation of carrier and signal frequencies
3TE4201-Communication Electronics
AM wave modulated by a band of frequenciesAM wave modulated by a band of frequencies
AM wave modulated by a single frequencyAM wave modulated by a single frequency
AM AM modulatomodulato
rrffccffss
ffccffcc- f- fss ffcc+ f+ fss
Frequency spectrum of Frequency spectrum of output signaloutput signal
Frequency spectrum of Frequency spectrum of input signalinput signal
ffcc
AM AM modulatomodulato
rrffsLsL ffsHsH ffcc+ f+ fsHsHffcc-f-fsLsL
ffcc- f- fsHsH
ffcc
ffcc+ f+ fsLsL
Frequency spectrum of Frequency spectrum of output signaloutput signal
Frequency spectrum of Frequency spectrum of input signalinput signal
4TE4201-Communication Electronics
fc
fc- fs
fc+ fs
AM envelope
fc+ fs fc fc- fs
AM wave seen on oscilloscope screen
Reproduction of AM wave by carrier and it’s two side freq.
5TE4201-Communication Electronics
Percentage modulation of AM wave by envelope voltagePercentage modulation of AM wave by envelope voltage
vvoo = = vvAMAM
AA BB
vvss = v= vs s (pp)(pp)(A-B)/2(A-B)/2
vvc c (pp)(pp) 100%ABABm%
or
100%EEm%
cs
AM wave over-modulationAM wave over-modulation
modulationOverEEif100100%EEm% cs
c
s
Percentage ModulationPercentage Modulation
6TE4201-Communication Electronics
carrier modulated by different signal levelscarrier modulated by different signal levels
vvoo = = vvAMAM
vvcc
vvss
AM AM modulatomodulato
rr ffccffcc- f- fss ffcc+ f+ fss
vvccvvss
ffcc ffss
vvoo = = vvAMAM
M = 50%M = 50%
VVss = = 0.5V0.5Vcc
VVss < < 0.5V0.5Vcc
M < 50%M < 50%
VVss = V = Vcc
M = 100%M = 100%
AM wave over-AM wave over-modulationmodulation
VVss > V > Vcc
M > 100%M > 100%
7TE4201-Communication Electronics
AM AnalysisAM Analysis
eecc=E=Ecc sin sin cctt
eess=E=Ess sin sin sstt
In AM, EIn AM, Ecc is varying with e is varying with ess
eecc=(E=(Ecc+ e+ ess) sin ) sin cct =(Et =(Ecc+ E+ Es s sin sin sst) sin t) sin cct and since m = (Et and since m = (Ess/E/Ecc))We have eWe have ecc=E=Ecc (1 + m sin (1 + m sin sst) sin t) sin cct = Et = Ecc sin sin cct + m Et + m Ecc (sin (sin cct)(sin t)(sin sst)t)But (sin x)(sin y) = (1/2)( cos (x-y) – cos (x+y) ) But (sin x)(sin y) = (1/2)( cos (x-y) – cos (x+y) )
Then ec= Ec sin ct + (m Ec/2) cos (ct - st) - (m Ec/2) cos (ct + st)
Carrier Carrier amplitudeamplitude
Lower Lower frequency frequency amplitudeamplitude
Upper Upper frequency frequency amplitudeamplitude
AM AM modulatomodulato
rrffccffss
ffccffcc- f- fss ffcc+ f+ fss
EEssEEcc
mEmEcc/2/2 mEmEcc/2/2
EEcc
Amplitude of AM wave componentsAmplitude of AM wave components
8TE4201-Communication Electronics
Power of AM wave componentsPower of AM wave components
2m1p2
mPP
2m
R2
E
R2
E
R22
mE2R
2E
PPPP
R22
mEPPandR
2E
P
2c
2cc
22
c2
c2
c2
c
USBLSBcAM
2c
USBLSB
2c
c
rms current of AM wave componentsrms current of AM wave components
2m1II2
m1IIor
2m1RI2
m1PRIP
2cAM
22c
2AM
22c
2c
2AMAM