Double Side Band Supressed Carrier

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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
• Transmitter
• Detector
• Synchronous
• Square
• Power analysis
• Summary

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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
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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
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DSB-SC - Waveforms
Mixer (Multiplier)
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DSB-SC - Implementation

• Balanced modulator

AM mod.

AM mod.

• Ring modulator

• Square-law modulator

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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
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DSB-SC - Synchronous Detection

• Case 1 - Phase error

• Condition
• Local oscillator has the same
• frequency but different phase
• compared to carrier signal at the
• transmitter.

high frequency
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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 ?

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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
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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 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
2?mt 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