The document explains amplitude modulation (AM), a process in which the amplitude of a high-frequency carrier signal is varied according to the instantaneous amplitude of an information signal. It outlines the principles of modulation, its applications in broadcasting and communications, and the importance of maintaining an optimal modulation index to maximize signal power without causing over-modulation. It also discusses the calculation of modulation parameters and the structure of AM signals in both time and frequency domains.

1. 1
Amplitude Modulation

2. 2
Content
• What is Modulation
• Amplitude Modulation (AM)
• Demodulation of AM signals
• Calculation and Examples
• Summary

3. 3
What is Modulation
• Modulation
– In the modulation process, some characteristic
of a high-frequency carrier signal (bandpass), is
changed according to the instantaneous
amplitude of the information (baseband) signal.
• Why Modulation
– Suitable for signal transmission (distance…etc)
– Multiple signals transmitted on the same channel
– Capacitive or inductive devices require high
frequency AC input (carrier) to operate.
– Stability and noise rejection

4. 4
About Modulation
• Application Examples
– broadcasting of both audio
and video signals.
– Mobile radio communications,
such as cell phone.
• Basic Modulation Types
– Amplitude Modulation: changes the amplitude.
– Frequency Modulation: changes the frequency.
– Phase Modulation: changes the phase.

5. 5
AM Modulation/Demodulation
Modulator Demodulator
Baseband Signal
with frequency
fm
(Modulating Signal)
Bandpass Signal
with frequency
fc
(Modulated Signal)
Channel
Original Signal
with frequency
fm
Source Sink
fc >> fm
Voice: 300-3400Hz GSM Cell phone: 900/1800MHz

6. Example Amplitude modulation

7. 7
Amplitude Modulation
• The amplitude of high-carrier signal is
varied according to the instantaneous
amplitude of the modulating message
signal m(t).
Carrier Signal: or
Modulating Message Signal: or
The AM Signal:
cos(2 ) cos( )
( ): cos(2 ) cos( )
( ) [ ( )]cos(2 )
c c
m m
AM c c
f t t
m t f t t
s t A m t f t
 
 

 

8. 8
Amplitude Modulation
• The AM signal is generated using a
multiplier.
• All info is carried in the amplitude of
the carrier, AM carrier signal has
time-varying envelope.
• In frequency domain the AM
waveform are the lower-side
frequency/band (fc - fm), the carrier
frequency fc, the upper-side
frequency/band (fc + fm).

9. 9
Modulation Index of AM Signal
m
c
A
k
A

)
2
cos(
)
( t
f
A
t
m m
m 

Carrier Signal: cos(2 ) DC:
c C
f t A

For a sinusoidal message signal
Modulation Index is defined as:
Modulated Signal: ( ) [ cos(2 )]cos(2 )
[1 cos(2 )]cos(2 )
AM c m m c
c m c
S t A A f t f t
A k f t f t
 
 
 
 
Modulation index k is a measure of the extent to
which a carrier voltage is varied by the modulating
signal. When k=0 no modulation, when k=1 100%
modulation, when k>1 over modulation.

10. 10
Modulation Index of AM Signal

11. CSULB May 22, 2006 11
Modulation Index of AM Signal

12. 12
Modulation Index of AM Signal

13. 13
High Percentage Modulation
• It is important to use as high percentage of
modulation as possible (k=1) while ensuring
that over modulation (k>1) does not occur.
• The sidebands contain the information and
have maximum power at 100% modulation.
• Useful equation
Pt = Pc(1 + k2/2)
Pt =Total transmitted power (sidebands and
carrier)
Pc = Carrier power