Electrical Communications Systems 0909'331'01 Spring 2005

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Electrical Communications Systems0909.331.01
Spring 2005
Lecture 4bFebruary 23, 2005

• Shreekanth Mandayam
• ECE Department
• Rowan University
• http//engineering.rowan.edu/shreek/spring05/ecom
  ms/

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Plan

• Amplitude Modulation
• AM Features
• AM Modifications
• DSB-SC

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ECOMMS Topics
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Recall AM Spectrum
Message Spectrum
AM Spectrum
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Single Tone AM
m(t) Amcos(2pfmt)
c(t) Accos(2pfct)
s(t) Ac1 Amcos(2pfmt)cos(2pfct)
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Spectrum of Single Tone AM
Instrument Demo
Matlab Demo
http//engineering.rowan.edu/shreek/spring03/ecom
ms/demos/am.m
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Percentage Modulation
Amax Ac1Am
s(t)
Amin Ac1-Am
Ac
t
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AM Bandwidth
Message Spectrum
AM Spectrum
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AM Power Efficiency
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AM Standards
See p. 308
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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

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DSB-SC Modulation
Message Spectrum
Matlab Demo
http//engineering.rowan.edu/shreek/spring05/ecom
ms/demos/dsbsc.m
f
DSB-SC Spectrum
S(f)
-fc-B -fc -fcB 0 fc-B fc
fcB
f
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Summary