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COMMUNICATION SYSTEM EEEB453 Chapter 2 AMPLITUDE MODULATION

COMMUNICATION SYSTEM EEEB453 Chapter 2 AMPLITUDE MODULATION. Dept of Electrical Engineering Universiti Tenaga Nasional. Comment on AM-DSBFC. Both modulator and demodulator have simple structure (low cost and reliable) DSB is wasteful of Power (carrier does not carry any information)

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COMMUNICATION SYSTEM EEEB453 Chapter 2 AMPLITUDE MODULATION

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  1. COMMUNICATION SYSTEM EEEB453Chapter 2AMPLITUDE MODULATION Dept of Electrical Engineering Universiti Tenaga Nasional

  2. Comment on AM-DSBFC • Both modulator and demodulator have simple structure (low cost and reliable) • DSB is wasteful of Power (carrier does not carry any information) • DSB is wasteful of Bandwidth (B vs 2B)

  3. DSBFC is wasteful of Power • 2/3 of total transmitted power taken up by carrier. • In transmitting 1320W of the total power, the carrier contains 1000W and does not contain any information being transmitted. The side freq each have 160W and each carries a copy of the same info signal. • So, 1320W is being used in order to transmit only 160W.

  4. DSB is wasteful of Bandwidth • DSB has a Wide Bandwidth • wasteful BW usage i.e info in USB = info in LSB • If so much of the transmitted wave is not required, then why transmit it? ANY ALTERNATIVE? DOUBLE SIDEBAND SUPPRESSED CARRIER?

  5. DSB Suppressed Carrier (DSBSC) • Generated by circuit called balanced modulator where it produces sum (fusb) and difference (flsb) freq but cancel or balance out the carrier (fc). • DSBSC helps in reducing power but bandwidth still the same as DSBFC.

  6. Single Side Band (SSB) System • Motivation: Both DSBFC and DSBSC occupy a bandwidth of 2B. How can we reduce the bandwidth requirements? • Due the symmetric condition (info in USB = info in LSB), one of the sidebands is sufficient to provide the complete information in the original signal.

  7. Single Side Band (SSB) System • AM Single Sideband Full Carrier (SSBFC) • Carrier is transmitted at full power with only one of the sidebands. • Half as much bandwidth will be required (BWSSBFC=1/2BWDSBFC).

  8. Single Side Band (SSB) System • AM Single Sideband Suppressed Carrier (SSBSC) • The carrier is totally removed together with one of the sidebands. • Half as much bandwidth is required ( BWSSBFC=1/2BWDSBFC).

  9. Single Side Band (SSB) System • AM Single Sideband Reduced Carrier (SSBRC) • Conserve BW and considerably power • One sideband is totally removed and carrier voltage is reduced to approx. 10 % of its unmodulated amplitudeor carrier power is reduced to approx. 1% of its unmodulated power • The carrier is totally suppressed during modulation and to be reinserted at reduced amplitude for the purpose of demodulation

  10. Single Side Band (SSB) System • AM Independent Sideband (ISB) • It is a form of DSB transmission in which the transmitter consists of two independent SSBSC modulators. • Output consists of two totally independent sidebands each of different information, with suppressed carrier. • It conserves both power and BW as two info sources are transmitted within the same freq spectrum.

  11. Single Side Band (SSB) System • AM Vestigal Sideband (VSB) • The carrier and one complete SB are transmitted, but only part (a vestige) of the second SB is transmitted. The carrier is transmitted at full power (Carrier and full 1st SB & part of 2nd SB). • The BW is typically 25% greater than that of SSBSC.

  12. DSBFC AM Wave

  13. SSBFC AM Wave PEAK CHANGE IN THE ENVELOPE IS HALF THAT OF THE DSB WAVE (ONLY ONE SIDEBAND) 100% modulated SSBFC wave with a single frequency modulating wave

  14. SSBSC AM Wave THE WAVEFORM IS NOT AN ENVELOPE; IT IS A SINE WAVE AT A SINGLE FREQUENCY EQUAL TO THE CARRIER FREQUENCY PLUS/MINUS THE MODULATING SIGNAL FREQUENCY

  15. ISB AM Wave WAVE IS SIMILAR TO A DSBSC WAVE BUT WITH A REPETITION RATE TWICE THAT OF THE MODULATING SIGNAL FREQUENCY

  16. Advantages of SSB Transmission • Bandwidth conservation - Only half the bandwidth is required. • Power conservation - Only one sideband with carrier removed or suppressed. Hence total transmitted power will be less. This allows smaller transmitters to be used. • Selective fading- In double sideband, the two sidebands may experience different impairments as the propagate along different paths in the medium. This could result in carrier phase shift. This cannot happen if only one sideband is transmitted. • Noise Reduction - Thermal noise is reduced to half, because the bandwidth is also half.

  17. Disadvantages of SSB Transmission • Complex receivers – Require more expensive receivers because envelope detection cannot be used • Tuning Difficulties - More difficult to tune than conventional AM receivers. Receivers need a precise tuning.

  18. Revision • Refer to Figure above, determine • Upper and lower side frequencies [1 marks] • Modulation coefficient and percent modulation [1 marks] • Peak amplitude of the modulated carrier and the upper and lower side frequency voltages [2 marks] • Maximum and minimum amplitude of the envelope [2 marks] • Expression of the modulated wave [1 marks] • The total average power required for a load resistance of 10Ω [2 marks] • Efficiency • If the waveform goes further modification, • Find the total average power required for transmitting a DSBSC wave [1 marks] • Find the total average power required for transmitting a SSBSC wave [1 marks] • Briefly explain two disadvantages of SSBSC compared to DSBSC [2 marks]

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