Amplitude modulation (AM) is a process where the amplitude of a carrier signal is altered according to an information-containing message signal. The carrier signal's frequency is usually much higher than the highest frequency of the message signal. There are two main types of AM demodulation: square-law demodulation and coherent demodulation. Square-law demodulation uses a diode and low-pass filter to extract the message signal from the AM signal. Coherent demodulation multiplies the AM signal with a reference carrier signal to directly extract the message signal. The document then describes using software simulation and a DSK6713 board to implement and observe AM modulation and these two demodulation techniques in real-time.

1. Slide 1
© 2007 Texas Instruments Inc,
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.

2. Slide 3
© 2007 Texas Instruments Inc,
AM – Basic Definitions (cont.)
The Envelope:
The AM Signal
   
  t
t
m
k
A
t
s c
c 
cos


 1
   
 
t
m
k
A
t
s c 

 1
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
-5
-4
-3
-2
-1
0
1
2
3
4
5

3. Slide 4
© 2007 Texas Instruments Inc,
The AM Spectrum
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
0
0.5
1
1.5
Frequency (Hz)
|M(f)|
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
0
0.5
1
1.5
Frequency (Hz)
|Y(f)|
 
t
m  
t
s
Carrier
Side
Bands

4. Slide 5
© 2007 Texas Instruments Inc,
AM Modulation Scheme
t
A c
c 
cos k
1
 
t
m  
t
s
+ X
k

5. Slide 6
© 2007 Texas Instruments Inc,
Square-Law Demodulation
 2

   
 
 
 
   
  t
t
m
k
A
t
m
k
A
t
t
m
k
A
t
s
c
c
c
c
c


2
1
5
1
5
1
2
2
2
2
2
2
cos
.
.
cos









LPF  

 
t
s
   
 
 
 
offset
c
DC
t
m
t
m
k
A
t
y






1
25
.
 
 2
2
1
5 t
m
k
Ac 

.
 
t
y

6. Slide 7
© 2007 Texas Instruments Inc,
Coeherent Demodulation
t
c

cos
   
 
 
 
 
 
 
 
   
  t
t
m
k
A
t
m
k
A
t
t
m
k
A
t
t
m
k
A
t
t
t
m
k
A
t
s
c
c
c
c
c
c
c
c
c
c





2
cos
1
5
.
1
5
.
2
2
cos
1
1
cos
1
cos
cos
1
2











 














LPF
 
t
s
   
 
 
 
offset
c
DC
t
m
t
m
k
A
t
y






1
5
.
 
t
y
X

7. Slide 8
© 2007 Texas Instruments Inc,
Hands-On
• Simulation
• Implementation using the DSK6713

8. Slide 9
© 2007 Texas Instruments Inc,
The Simulation Environment
Modulator
Virtual
Scope
Demodulator
Information
Signal
Demodulated
Signal

9. Slide 10
© 2007 Texas Instruments Inc,
The Simulation Environment

10. Slide 11
© 2007 Texas Instruments Inc,
Real Time Environment
DSK6713
CODEC
TMS320C6713
AM
Modulator
D/A
(Left)
AM
Demodulator
A/D
(Left)
A/D
(Right)
D/A
(Right)
 
t
s
 
t
y
Line In
Line In
Line Out
Line Out
Signal
Generator
Scope
PC
RTDX

11. Slide 12
© 2007 Texas Instruments Inc,
GUI

12. Slide 13
© 2007 Texas Instruments Inc,
SQRT Detector – Real Time

13. Slide 14
© 2007 Texas Instruments Inc,
Coherent Detection – Real Time

14. Slide 15
© 2007 Texas Instruments Inc,
Configurable Carrier










n
f
f
j
s
c
s
c
s
c
e
n
f
f
nT



2
Re
2
cos
cos