1. TE4201-Communication Electronics 1  Amplitude Modulation Fundamentals Amplitude Modulation Fundamentals Amplitude Modulation Fundamentals  Percentage Modulation Percentage Modulation Percentage Modulation  AM Analysis AM Analysis AM Analysis 5. AM Fundamentals and Analysis

2. TE4201-Communication Electronics 2 Amplitude Modulation Fundamentals e carrier e signal Linear summing of carrier and signal frequencies e carrier fcfcfcfc e signal fsfsfsfs e sum f c, f s 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 long distance. 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 long distance. AM modulator fsfsfsfs fcfcfcfc fsfsfsfs 2f s 3f s f f vovovovo v in vovovovo vovovovo fcfcfcfc f c - f s f c + f s 2f c Modulation of carrier and signal frequencies

3. 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

4. TE4201-Communication Electronics 4 fcfc f c - f s f c + f s AM envelope f c + f s fcfc f c - f s AM wave seen on oscilloscope screen Reproduction of AM wave by carrier and it ’ s two side freq.

5. TE4201-Communication Electronics 5 Percentage modulation of AM wave by envelope voltage vovovovo = v AM AB vsvsvsvs = v s (pp) (A-B)/2 v c (pp) AM wave over-modulation Percentage Modulation

6. TE4201-Communication Electronics 6 carrier modulated by different signal levels vovovovo = v AM vcvcvcvc vsvsvsvs AM modulator fcfcfcfc f c - f s f c + f s vcvcvcvc vsvsvsvs fcfcfcfc fsfsfsfs vovovovo = v AM M = 50% V s = 0.5V c V s < 0.5V c M < 50% V s = V c M = 100% AM wave over- modulation V s > V c M > 100%

7. TE4201-Communication Electronics 7 AM Analysis e c =E c sin  c t e s =E s sin  s t In AM, E c is varying with e s e c =(E c + e s ) sin  c t =(E c + E s sin  s t) sin  c t and since m = (E s /E c ) We have e c =E c (1 + m sin  s t) sin  c t = E c sin  c t + m E c (sin  c t)(sin  s t) But (sin x)(sin y) = (1/2)( cos (x-y) – cos (x+y) ) Then e c = E c sin  c t + (m E c /2) cos (  c t -  s t) - (m E c /2) cos (  c t +  s t) Carrier amplitude Lower frequency amplitude Upper frequency amplitude AM modulator fcfcfcfc fsfsfsfs fcfcfcfc f c - f s f c + f s EsEsEsEs EcEcEcEc mE c /2 EcEcEcEc Amplitude of AM wave components

8. TE4201-Communication Electronics 8 Power of AM wave components rms current of AM wave components