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Chapter 4. Amplitude Modulation

Chapter 4. Amplitude Modulation. Husheng Li The University of Tennessee. AM Signals and Spectra. An AM signal can be written as. Power of AM. The total transmit power is given by

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Chapter 4. Amplitude Modulation

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  1. Chapter 4. Amplitude Modulation Husheng Li The University of Tennessee

  2. AM Signals and Spectra An AM signal can be written as

  3. Power of AM The total transmit power is given by We can prove that, at least 50% (often close to 2/3) of the total transmitted power resides in a carrier term that is independent of the signal and thus conveys no message information. (what a waste!)

  4. DSB Signal Spectra We set u=1 and suppress the unmodulated component and obtain the modulated signal: DSB conserves power but requires complicated demodulation circuitry, whereas AM requires increased power to permit simple envelope detection.

  5. Homework 4 Deadline: Sept. 30, 2013

  6. Tone Modulation If the transmitted signal is a single tone signal, then the tone-modulated DSB waveform is given by

  7. Modulators Product modulators

  8. Modulators Spectrum of V_out Square-law and balanced modulators

  9. Balanced Modulator Perfect square-law devices are rare; high-frequency DSB is obtained in practice using two AM modulation arranged in a balanced configuration to cancel out the carrier.

  10. Ring Modulator Another commonly used modulator is the ring modulator, which uses a carrier to cause the diode to switch on and off.

  11. Switching Modulators Efficient high-level modulators are arranged so that undesired modulation products never fully develop and need not be filtered out.

  12. Suppressed Sideband AM Conventional AM is wasteful of both transmission power and bandwidth. Suppressing the carrier can reduce the transmit power, while suppressing one-sideband can reduce the bandwidth. For suppressing the sideband, we have either SSB or VSB.

  13. SSB in Time Domain In the time domain, the expression of SSB is given by

  14. Drawbacks of SSB

  15. Generation of SSB SSB requires perfect filter actions. But a perfect cutoff at f_c cannot be synthesized. Fortunately, many modulating Signals of practical interest have Little or no low-frequency content.

  16. Generation of SSB Two-step SSB generation phase-shift method

  17. V (Vestige) SB VSB achieves a tradeoff between SSB and DSB, whose signal is passed through the following filter

  18. Frequency Conversion Frequency converter (mixer) Satellite transponder The frequency conversion starts with multiplication by a sinusoid.

  19. Coherent Detection In coherent detection, the local oscillator of receiver is exactly synchronized with the carrier in both phase and frequency. We can pick off the pilot carrier by using a narrow bandpass filter, which is called homodyne detection.

  20. Coherent Demodulation of VSB For VSB, the sum of the vestige side band recovers the original frequency spectrum:

  21. Detected Signal For imperfect coherent detection, the detected signal is given by Summary

  22. Envelope Detection An envelope detection can only demodulate signals with a carrier.

  23. Homework 4 Deadline: Oct. 7, 2013

  24. Quiz 2 Problem 1. Write down the expression of SSB signal in the time domain, if the base band signal is x(t). Problem 2. Explain the principle of coherent demodulation of VSB signals.

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