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1 Double Side Band Suppressed Carrier Professor Z Ghassemlooy Electronics and IT Division School of Engineering SheffieldHallamUniversity U.K.

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2 Contents Theory Implementation –Transmitter –Detector Synchronous Square Power analysis Summary

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3 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% c - m c c + m Carrier USB LSB Single frequency Question: Why transmit carrier at all? Ans: Question: Can one suppress the carrier? Ans.: Yes, just transmit two side bands (i.e DSB-SC) System complexity at the receiver But what is the penalty?

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4 DSB-SC - Theory General expression: Let k 1 = 1, C = 0 and c = 0, the modulated carrier signal, therefore: Information signal m(t) = E m cos m t Thus upper side band lower side band

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5 DSB-SC - Waveforms B = 2 m Mixer (Multiplier) Notice: No carrier frequency

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6 DSB-SC - Implementation AM mod. AM mod. Carrier E c cos c t 0.5 m(t) -0.5 m(t) DSB-SC E c m(t) cos c t + E c ( m(t) cos c t E c ( m(t) cos c t Balanced modulator Ring modulator Square-law modulator AM mod. AM mod.

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7 DSB-SC - Detection Synchronous detection Multiplier Low pass filter Low pass filter Message signal DSB-SC Local oscillator c(t) = cos c t Local oscillator c(t) = cos c t Condition: Local oscillator has the same frequency and phase as that of the carrier signal at the transmitter. mm 2c+m2c+m 2c-m2c-m Low pass filter high frequency information

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8 DSB-SC - Synchronous Detection Case 1 - Phase error Multiplier Low pass filter Low pass filter Message signal DSB-SC Local oscillator c(t) = cos( c t+ ) Local oscillator c(t) = cos( c t+ ) Condition: Local oscillator has the same frequency but different phase compared to carrier signal at the transmitter. mm 2c+m2c+m 2c-m2c-m Low pass filter high frequency information

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9 Phase Synchronisation - Costa Loop Recovered signal When there is no phase error. The quadrature component is zero When 0, y ip (t) decreases, while y qp (t) increases X X VCO DSB-SC E c cos ( c t+ ) LPF Phase discriminator Phase discriminator In-phase 0.5E c m(t) cos V phase (t) y ip (t) X. 0.5E c m(t) sin 90 o phase shift LPF E c sin ( c t+ ) Quadrature-phase y qp (t) The out put of the phase discriminator is proportional to

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10 DSB-SC - Synchronous Detection Case 1 - Frequency error Multiplier Low pass filter Low pass filter Message signal DSB-SC Local oscillator c(t)=E c cos( c t+ ) Local oscillator c(t)=E c cos( c t+ ) Condition: Local oscillator has the same phase but different frequency compared to carrier signal at the transmitter. mm 2c+m2c+m 2c-m2c-m Low pass filter high frequency information

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11 DSB-SC - Square Detection Regenerated carrier z(t) Band pass filter DSB-SC S i (t) C Squaring circuit g =x 2 Squaring circuit g =x 2 g(t)g(t) by 2 Multiplier Low pass filter Message signal y(t)y(t) 2c2c g(t) = S i 2 (t) = B 2 cos 2 m t cos 2 c t = B 2 (½ + ½ cos 2 m t )(½ + ½ cos 2 c t ) = B 2 /4 [1 + ½ cos 2( c + m )t + ½ cos 2( c - m )t + cos 2 m t + cos 2 c t ] y(t) = B 2 /4 cos 2w c tz(t) = B 2 /4 cos w c t

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12 DSB-SC - Power The total power (or average power): The maximum and peak envelop power

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13 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

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