Introduction to Modulation:
Amplitude Modulation(AM)
Sharlene Katz
James Flynn
Overview
  Modulation
Overview
  Basics of Amplitude Modulation (AM)
  AM Demonstration
  GRC Exercise
2
Flynn/Katz 7/8/10
Why do we need Modulation/Demodulation?
Example: Radio transmission
 
Voice
Microphone
Transmitter
Electric signal,
20 Hz – 20
KHz
Antenna:
Size requirement
> 1/10 wavelength
c 3 ×10 8
5
λ
=
=
=
10
= 100km
3
At 3 KHz:
f 3 ×10
⇒ .1λ = 10km
3
Antenna too large!
Use modulation to
transfer
information to a
higher frequency
Flynn/Katz 7/8/10
Why do we need Modulation/Demodulation?
(cont’d)
 
 
 
 
 
 
4
Frequency Assignment
Reduction of noise/interference
Multiplexing
Bandwidth limitations of equipment
Frequency characteristics of antennas
Atmospheric/cable properties
Flynn/Katz 7/8/10
Basic Concept of Modulation
The information source
 
 
 
Typically a low frequency signal
Referred to as the “baseband signal”
x(t)
X(f)
t
f
Carrier
 
 
 
A higher frequency sinusoid
Example: cos(2π10000t)
baseband
carrier
Modulator
Modulated
signal
Modulated Signal
 
 
5
Some parameter of the carrier (amplitude, frequency, phase) is varied in
accordance with the baseband signal
Flynn/Katz 7/8/10
Types of Modulation
 
Analog Modulation
 
 
 
 
Digital Modulation
 
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 
6
Amplitude Modulation, AM
Frequency Modulation, FM
Double and Single Sideband, DSB and SSB
Phase Shift Keying: BPSK, QPSK, MSK
Frequency Shift Keying, FSK
Quadrature Amplitude Modulation, QAM
Flynn/Katz
7/8/10
Amplitude Modulation (AM)
Block Diagram
 
x(t)
m
Time Domain
 
x
xAM(t)=Ac [1+mx(t)]cos wct
+
Ac cos wct
Frequency Domain
 
XAM(f)
X(f)
-fm
7
fm
Signal information is
contained in the
sidebands
f
f
-fc
fc
Flynn/Katz 7/8/10
Traditional AM Demodulators
Synchronous Detector
 
xAM(t)=Ac [1+mx(t)]cos wct
XAM(f)
-fc
fc
f
Envelope Detector
 
8
Low Pass
Filter
x
cos wct
-2fc
2fc
f
Flynn/Katz 7/8/10
Receiving AM with SDR
 
USRP and Computer with GNU Radio Software
USRP
Basic Rx
64Ms/s
A/D
or
256Ks/s
FPGA
Shifts by -710KHz
Computer
(GNU Radio)
Stored USRP File
9
Flynn/Katz 7/8/10
AM Modulation
 
Demonstration
 
 
 
Receiving off-air signals
Using a captured file source
GRC Exercise
10
Flynn/Katz 7/8/10
Questions?
11
Flynn/Katz 7/8/10