1. Electrical Communications Systems ECE.09.433 Spring 2019
Lecture 2b January 31, 2019
Shreekanth Mandayam
ECE Department
Rowan University

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

3. ECOMMS: Topics

4. 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 fm
W(f) f
- fc fc
W(f) f
Carrier
Frequency
AF Signals
RF Signals

5. 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

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

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

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

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

10. 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)

11. Spectrum of Single Tone AM
Matlab Demo

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

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

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. 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

18. Summary