1. Electrical Communications Systems ECE.09.433
Amplitude Modulation I
Dr. Shreek Mandayam
Electrical & Computer Engineering
Rowan University

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

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. Summary