From MATLAB® and Simulink® to
Real Time with TI DSPs
Amplitude Modulation
Content developed in partnership with
Tel-Aviv University
© 2007 Texas Instruments Inc,
0-1
Amplitude Modulation
• Amplitude Modulation is a process where the
amplitude of a carrier signal is altered according to
information in a message signal.
• The frequency of the carrier signal is usually much
greater than the highest frequency of the input
message signal.
© 2007 Texas Instruments Inc,
Slide 2
AM – Basic Definitions (cont.)
The Envelope:
5
4
st   Ac 1  k  mt 
The AM Signal
st   Ac 1  k  mt cosct
3
2
1
0
-1
-2
-3
-4
-5
© 2007 Texas Instruments Inc,
0
0.02 0.04
0.06 0.08
0.1
0.12 0.14
0.16 0.18
0.2
Slide 4
The AM Spectrum
s t 
m t 
1.5
1.5
Carrier
1
|M(f)|
|Y(f)|
1
0.5
0.5
0
Side
Bands
0
500
1000
1500
2000 2500 3000
Frequency (Hz)
© 2007 Texas Instruments Inc,
3500
4000
4500
5000
0
0
500
1000
1500
2000 2500 3000
Frequency (Hz)
3500
4000
4500
5000
Slide 5
AM Modulation Scheme
Ac cosct
m t 
k
+
X
k
s t 
1
© 2007 Texas Instruments Inc,
Slide 6
Square-Law Demodulation
s 2 t    Ac 1  k  mt cosct  
y t   .25Ac 1  k  mt 
2
.5 A 1  k  mt   .5 A 1  k  mt  cos 2ct
2
c
s t 
2
2
c
2

  mt   DCoffset
2

LPF

y t 
.5 Ac2 1  k  mt 
2
© 2007 Texas Instruments Inc,
Slide 7
Coeherent Demodulation
st    Ac 1  k  mt cos c t   cos c t 
 Ac 1  k  mt  cos 2 c t 
 1  cos 2c t 
 Ac 1  k  mt  

2


.5 Ac 1  k  mt   .5 Ac 1  k  mt cos 2c t
s t 
X
yt   .5 Ac 1  k  mt 
  mt   DCoffset 
LPF
y t 
cos ct
© 2007 Texas Instruments Inc,
Slide 8
Hands-On
• Simulation
• Implementation using the DSK6713
© 2007 Texas Instruments Inc,
Slide 9
The Simulation Environment
Virtual
Scope
Information
Signal
Modulator
© 2007 Texas Instruments Inc,
Demodulator
Demodulated
Signal
Slide 10
The Simulation Environment
© 2007 Texas Instruments Inc,
Slide 11
Real Time Environment
RTDX
PC
CODEC
s t 
A/D
(Right)
Signal
Generator
Line In
Line Out
Scope
y t 
D/A
(Right)
DSK6713
© 2007 Texas Instruments Inc,
AM
Modulator
D/A
(Left)
AM
Demodulator
A/D
(Left)
Line Out
Line In
TMS320C6713
Slide 12
GUI
© 2007 Texas Instruments Inc,
Slide 13
SQRT Detector – Real Time
© 2007 Texas Instruments Inc,
Slide 14
Coherent Detection – Real Time
© 2007 Texas Instruments Inc,
Slide 15
Configurable Carrier
fc
j
2

n

fc
fs
cos c nTs  cos 2 n  Re e

fs


© 2007 Texas Instruments Inc,
Slide 16