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