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##### Analog Communications

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**Analog Communications**Amplitude Modulation T.Srinivasa Rao**Double Side Band Suppressed Carrier**T.Srinivasa Rao**Contents**• Theory • Implementation • Transmitter • Detector • Synchronous • Square • Power analysis • Summary T.Srinivasa Rao**Carrier**USB LSB c - m c c + m Single frequency Double Side Band Suppressed Carrier From AM spectrum: • Carries signal c carries no information m. • Carries signal consumes a lot of power more than 50% Question: Why transmit carrier at all? Ans: Question: Can one suppress the carrier? Ans.:Yes, just transmit two side bands (i.e DSB-SC) But what is the penalty? System complexity at the receiver T.Srinivasa Rao**upper side band**lower side band DSB-SC - Theory General expression: Let k1 = 1, C = 0 and c = 0, the modulated carrier signal, therefore: Information signal m(t) = Am cos 2πfmt Thus T.Srinivasa Rao**B = 2m**Notice: No carrier frequency DSB-SC - Waveforms Mixer (Multiplier) T.Srinivasa Rao**Ac (1+ 0.5 m(t) cos 2 π fct**0.5 m(t) + Carrier Ac cos 2πfct DSB-SC Acm(t) cos 2 π fct - -0.5 m(t) Ac (1- 0.5 m(t) cos 2 π fct DSB-SC - Implementation • Balanced modulator AM modulator + AM modulator • Ring modulator • Square-law modulator T.Srinivasa Rao**Low pass**filter DSB-SC Local oscillator c(t) = cos 2πfct information Low pass filter m 2c-m 2c+m DSB-SC - Detection • Synchronous detection Multiplier Message signal • Condition: • Local oscillator has the same • frequency and phase as that of the • carrier signal at the transmitter. high frequency T.Srinivasa Rao**Low pass**filter DSB-SC Multiplier Message signal Local oscillator c(t) = cos(ct+) information Low pass filter m 2c-m 2c+m DSB-SC - Synchronous Detection • Case 1 - Phase error • Condition: • Local oscillator has the same • frequency but differentphase • compared to carrier signal at the • transmitter. high frequency T.Srinivasa Rao**In-phase**yip(t) 0.5Ec m(t) cos X DSB-SC LPF Ec cos (ct+) Vphase(t) Phase discriminator VCO 90o phase shift Ec sin (ct+) 0.5Ec m(t) sin X. LPF yqp(t) Quadrature-phase Phase Synchronisation -Costa Loop Recovered signal • When there is no phase error. The quadrature component is zero • When 0, yip(t) decreases, while yqp(t) increases • The out put of the phase discriminator is proportional to T.Srinivasa Rao**Low pass**filter DSB-SC Multiplier Message signal Local oscillator c(t)=Eccos(ct+) information Low pass filter m 2c-m 2c+m DSB-SC - Synchronous Detection • Case 1 - Frequency error • Condition: • Local oscillator has the same • phase but different frequency • compared to carrier signal at the • transmitter. high frequency T.Srinivasa Rao**g(t)**Squaring circuit g =x2 Band pass filter C 2c DSB-SC - Square Detection y(t) DSB-SC Si(t) by 2 Regenerated carrier z(t) Multiplier Message signal Low pass filter g(t) = Si2(t) = B2 cos2mt cos2ct = B2 (½ + ½ cos 2 mt )(½ + ½ cos 2 ct ) = B2/4 [1 + ½ cos 2(c + m)t + ½ cos 2(c - m)t + cos 2mt + cos 2 ct ] y(t) = B2/4 cos 2wct z(t) = B2/4 cos wct T.Srinivasa Rao**DSB-SC - Power**• The total power (or average power): • The maximum and peak envelop power T.Srinivasa Rao**DSB-SC - Summary**• Advantages: • Lower power consumption • Disadvantage: • - Complex detection • Applications: • - Analogue TV systems: to transmit colour information • - For transmitting stereo information in FM sound broadcast • at VHF T.Srinivasa Rao