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**Double Side Band Suppressed Carrier**

Professor Z Ghassemlooy Electronics and IT Division School of Engineering Sheffield Hallam University U.K.

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**Contents Theory Implementation Power analysis Summary Transmitter**

Detector Synchronous Square Power analysis Summary

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**Double Side Band Suppressed Carrier**

c - m c c + m Carrier USB LSB Single frequency 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

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**DSB-SC - Theory General expression:**

Let k1 = 1, C = 0 and c = 0, the modulated carrier signal, therefore: Information signal m(t) = Em cos mt Thus upper side band lower side band

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**DSB-SC - Waveforms Mixer (Multiplier) Notice: No carrier frequency**

B = 2m Notice: No carrier frequency

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**DSB-SC - Implementation**

Balanced modulator + Ec ( m(t) cos ct AM mod. 0.5 m(t) Carrier Ec cos ct - DSB-SC Ec m(t) cos ct + Ec ( m(t) cos ct AM mod. -0.5 m(t) Ring modulator Square-law modulator

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**DSB-SC - Detection Synchronous detection Multiplier Low pass filter**

Message signal Local oscillator c(t) = cos ct Condition: Local oscillator has the same frequency and phase as that of the carrier signal at the transmitter. information high frequency m 2c-m 2c+m Low pass filter

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**DSB-SC - Synchronous Detection**

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

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**Phase Synchronisation - Costa Loop**

X VCO DSB-SC Ec cos (ct+) LPF Phase discriminator In-phase 0.5Ec m(t) cos Vphase(t) yip(t) Recovered signal X. 0.5Ec m(t) sin 90o phase shift LPF Ec sin (ct+) Quadrature-phase yqp(t) 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

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**DSB-SC - Synchronous Detection**

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

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**DSB-SC - Square Detection**

Squaring circuit g =x2 g(t) y(t) DSB-SC Si(t) Band pass filter by 2 2c Regenerated carrier z(t) Multiplier Message signal Low pass filter g(t) = Si2(t) = B2 cos2 mt cos2 ct = 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

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**DSB-SC - Power The total power (or average power):**

The maximum and peak envelop power

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**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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