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Electrical Communications Systems ECE.09.433 Spring 2019

Electrical Communications Systems ECE.09.433 Spring 2019. Lecture 2b January 31, 2019. Shreekanth Mandayam ECE Department Rowan University http://engineering.rowan.edu/~shreek/spring19/ecomms/. Plan. Baseband and Bandpass Signals Modulation Battle Plan for Analyzing Comm Systems

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Electrical Communications Systems ECE.09.433 Spring 2019

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  1. Electrical Communications SystemsECE.09.433Spring 2019 Lecture 2bJanuary 31, 2019 Shreekanth Mandayam ECE Department Rowan University http://engineering.rowan.edu/~shreek/spring19/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 Matlab Demo http://engineering.rowan.edu/~shreek/spring19/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. 320

  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. 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

  18. Summary

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