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Electrical Communications Systems ECE.09.331 Spring 2010

Electrical Communications Systems ECE.09.331 Spring 2010. Lecture 5a February 16, 2010. Shreekanth Mandayam ECE Department Rowan University http://engineering.rowan.edu/~shreek/spring10/ecomms/. Plan. Baseband and Bandpass Signals Modulation Battle Plan for Analyzing Comm Systems

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Electrical Communications Systems ECE.09.331 Spring 2010

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  1. Electrical Communications SystemsECE.09.331Spring 2010 Lecture 5aFebruary 16, 2010 Shreekanth Mandayam ECE Department Rowan University http://engineering.rowan.edu/~shreek/spring10/ecomms/

  2. Plan • Baseband and Bandpass Signals • Modulation • Battle Plan for Analyzing Comm Systems • Amplitude Modulation (AM)

  3. ECOMMS: Topics

  4. Baseband signals: spectral magnitude is non-zero only near the origin and is zero (or negligible) elsewhere Bandpass signals: spectral magnitude is non-zero only near the vicinity of f = ± fc, were fc >> 0 W(f) W(f) - fm 0 +fm f - fc 0 +fc f Carrier Frequency Baseband and Bandpass Signals AF Signals RF Signals

  5. 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 Modulation What is it? How is it done?

  6. Why Modulate? • Antenna size considerations • Narrow banding • Frequency multiplexing • Common processing

  7. Signals Systems Complex Envelope Time Domain Spectrum Power Performance Transmitters Receivers Standards Modulation Index Efficiency Bandwidth Noise Battle Plan for Analyzing any Comm. Sys.

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

  9. |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

  10. 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)

  11. Spectrum of Single Tone AM Instrument Demo Matlab Demo http://engineering.rowan.edu/~shreek/spring10/ecomms/demos/am.m

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

  13. |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

  14. AM Power & Efficiency

  15. AM Standards See p. 308

  16. 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

  17. |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/spring09/ecomms/demos/dsbsc.m f DSB-SC Spectrum |S(f)| -fc-B -fc -fc+B 0fc-B fc fc+B f

  18. 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

  19. 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

  20. Summary

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