This document discusses amplitude modulation (AM) and its implementation using Texas Instruments DSPs. It defines AM as altering the amplitude of a carrier signal by an input message signal. The frequency of the carrier is usually much greater than the highest frequency of the message signal. The document covers the AM envelope, spectrum, modulation scheme, and demodulation techniques including square-law demodulation and coherent demodulation. It also describes simulating and implementing AM modulation and demodulation on a DSK6713 board in real time.

1. 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-

2. 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

3. AM – Basic Definitions (cont.)
The Envelope: 5
4
3
s t Ac 1 k m t
2
1
The AM Signal
0
s t Ac 1 k m t cos ct
-1
-2
-3
-4
-5
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
© 2007 Texas Instruments Inc, Slide

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

5. AM Modulation Scheme
Ac cos ct
k
m t k + X s t
1
© 2007 Texas Instruments Inc, Slide

6. Square-Law Demodulation
2 2
s t Ac 1 k m t cos ct
y t . 25 Ac 1 k m t
2 2 2 2
.5 A 1 k m t .5 A 1 k m t cos 2 t
c c c m t DC offset
2
s t LPF y t
2 2
. 5 Ac 1 k m t
© 2007 Texas Instruments Inc, Slide

7. Coeherent Demodulation
s t Ac 1 k m t cos c
t cos c
t
2
Ac 1 k m t cos 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 y t . 5 Ac 1 k m t
m t DC offset
s t X LPF y t
cos c
t
© 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 Demodulated
Modulator Demodulator
Signal Signal
© 2007 Texas Instruments Inc, Slide

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

11. Real Time Environment
RTDX
PC
CODEC
s t
Signal A/D AM D/A
Generator (Right) (Left)
Line In
Modulator Line Out
Line Out Line In
Scope
D/A AM A/D
(Right) Demodulator (Left)
y t
TMS320C6713
DSK6713
© 2007 Texas Instruments Inc, 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
j2 n
fc fs
cos c
nT s cos 2 n Re e
fs
© 2007 Texas Instruments Inc, Slide