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Electrical Communications Systems ECE.09.331 Spring 2011

Electrical Communications Systems ECE.09.331 Spring 2011. Lecture 4b February 9, 2011. Shreekanth Mandayam ECE Department Rowan University http://engineering.rowan.edu/~shreek/spring11/ecomms/. Plan. Amplitude Modulation (AM) DSB-SC Modulation Complex Envelope Representation.

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Electrical Communications Systems ECE.09.331 Spring 2011

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  1. Electrical Communications SystemsECE.09.331Spring 2011 Lecture 4bFebruary 9, 2011 Shreekanth Mandayam ECE Department Rowan University http://engineering.rowan.edu/~shreek/spring11/ecomms/

  2. Plan • Amplitude Modulation (AM) • DSB-SC Modulation • Complex Envelope Representation Signals Systems Modulation Index Efficiency Bandwidth Noise Time Domain Complex Envelope Spectrum Power Performance Transmitters Receivers Standards

  3. ECOMMS: Topics

  4. Information Modulating Signal Message Signal m(t) (Baseband) Modulated Signal s(t) (Bandpass) Modulation Frequency Translation message: m(t) carrier: c(t) Modulator s(t): radio signal Recall: Modulation What is it? How is it done?

  5. Signals Systems Complex Envelope Time Domain Spectrum Power Performance Transmitters Receivers Standards Modulation Index Efficiency Bandwidth Noise Amplitude Modulation (AM)

  6. |M(f)| |S(f)| -B 0+B f -fc-B -fc -fc+B 0fc-B fc fc+B f AM Spectrum Message Spectrum AM Spectrum

  7. m(t) Am t c(t) Ac t Ac[1+Am] s(t) Ac t Single Tone AM m(t) = Amcos(2pfmt) c(t) = Accos(2pfct) s(t) = Ac[1 + Amcos(2pfmt)]cos(2pfct)

  8. Spectrum of Single Tone AM Instrument Demo Matlab Demo http://engineering.rowan.edu/~shreek/spring11/ecomms/demos/am.m

  9. Percentage Modulation Amax = Ac[1+Am] s(t) Amin = Ac[1-Am] Ac t

  10. |M(f)| |S(f)| -B 0+B f -fc-B -fc -fc+B 0fc-B fc fc+B f AM Bandwidth Message Spectrum AM Spectrum

  11. AM Power & Efficiency

  12. AM Standards See p. 308

  13. AM: Features • AM system is very cheap to build and maintain • AM is wasteful of power - max efficiency is 33% • AM is wasteful of bandwidth - twice the message bandwidth is required • AM Modifications: • DSB-SC • SSB • VSB

  14. |M(f)| -B 0+B Signals Systems Modulation Index Efficiency Bandwidth Noise Performance Transmitters Receivers Standards Time Domain Complex Envelope Spectrum Power DSB-SC Modulation Message Spectrum Matlab Demo http://engineering.rowan.edu/~shreek/spring11/ecomms/demos/dsbsc.m f DSB-SC Spectrum |S(f)| -fc-B -fc -fc+B 0fc-B fc fc+B f

  15. n(t) m(t) g(t) s(t) r(t) Signal P rocessing Signal processing Carrier circuits Carrier circuits Channel Complex Envelope Representation • We want a common mathematical representation for • information transfer from baseband to bandpass • any bandpass signal • any time of modulation • signal/noise and signal+noise

  16. message: m(t) carrier: c(t) Modulator s(t): radio signal Complex Envelope Representation • We want a common mathematical representation for • information transfer from baseband to bandpass • any bandpass signal • any time of modulation • signal/noise and signal+noise

  17. Summary

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