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 Amplitude Modulation (AM)

ECOMMS: Topics

Baseband and Bandpass Signals 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 - fm 0 +fm W(f) f - fc 0 +fc W(f) f Carrier Frequency AF Signals RF Signals

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

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

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

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

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

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

Spectrum of Single Tone AM Matlab Demo http://engineering.rowan.edu/~shreek/spring19/ecomms/demos/am.m

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

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

AM Power & Efficiency

AM Standards See p. 320

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

Complex Envelope Representation Modulator message: m(t) carrier: c(t) s(t): radio signal 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

Summary