Amplitude modulation (AM) is a modulation technique that varies the amplitude of a carrier wave according to the message signal. AM is commonly used for radio communication, where information such as audio is encoded in the amplitude of a radio wave. The carrier wave has a constant frequency and amplitude, while the message signal causes the amplitude of the carrier wave to vary. In AM, the amplitude of the carrier wave is directly proportional to the instantaneous amplitude of the message signal. The AM signal can be expressed as the sum of the carrier wave, upper sideband, and lower sideband frequencies. Demodulation of AM signals can be done using envelope detection or synchronous detection methods.

1. 1
Amplitude Modulation
By:
Sanjay Jangra
Assistant Professor
Deptt. Of Electronics & Communication
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2. 2
Introduction
• 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
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3. 3
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.
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4. 4
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
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5. 5
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
 
 

 
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6. 6
* AM Signal Math Expression*
• Mathematical expression for AM: time domain
• expanding this produces:
• In the frequency domain this gives:
( ) (1 cos ) cos
AM m c
S t k t t
 
 
( ) cos cos cos
c c
AM m
S t t k t t
  
 
 
)
cos(
)
cos(
cos
cos
:
using 2
1 B
A
B
A
B
A 



2 2
( ) cos cos( ) cos( )
c c c
k k
AM m m
S t t t t
    
    
frequency
k/2
k/2
Carrier, A=1.
upper sideband
lower
sideband
Amplitude
fc
fc-fm fc+fm
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7. 7
AM Power Frequency Spectrum
• AM Power frequency spectrum obtained by squaring
the amplitude:
• Total power for AM:
.
2 2
2
2
4 4
1
2
k k
A
k
  
 
freq
k2/4
k2/4
Carrier, A2=12 = 1
Power
fc
fc-fm fc+fm
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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).
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9. 9
AM Modulation – Example
• The information signal is usually not a single frequency but a
range of frequencies (band). For example, frequencies from
20Hz to 15KHz. If we use a carrier of 1.4MHz, what will be the
AM spectrum?
• 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). Bandwidth: 2x(25K-20)Hz.
frequency
1.4 MHz
1,385,000Hz to
1,399,980Hz
1,400,020Hz to
1,415,000Hz
fc
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10. 10
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.
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11. 11
Modulation Index of AM Signal
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12. 12
Modulation Index of AM Signal
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13. 13
Modulation Index of AM Signal
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14. 14
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
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15. 15
Demodulation of AM Signals
Demodulation extracting the baseband message from
the carrier.
•There are 2 main methods of AM Demodulation:
• Envelope or non-coherent detection or demodulation.
• Synchronised or coherent demodulation.
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16. 16
Envelope/Diode AM Detector
If the modulation depth is > 1, the distortion below occurs
K>1
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17. 17
Synchronous or Coherent
Demodulation
This is relatively more complex and more expensive. The Local
Oscillator (LO) must be synchronised or coherent, i.e. at the same
frequency and in phase with the carrier in the AM input signal.
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18. 18
Synchronous or Coherent
Demodulation
If the AM input contains carrier frequency, the LO or synchronous
carrier may be derived from the AM input.
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19. 19
Synchronous or Coherent
Demodulation
If we assume zero path delay between the modulator and
demodulator, then the ideal LO signal is cos(ct).
Analysing this for a AM input =  
   
t
ω
t
m
+
V c
DC cos
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20. 20
Coherent Detection
Assume zero path delay between the modulator and demodulator:
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21. 21
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22. 22
Coherent Detection
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23. 23
AM Transmitter and Receiver
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24. 24
AM Transmitter and Receiver
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25. 25
Summary
• Modulation, Amplitude Modulation
• Modulation Index, Modulation Depth
• Demodulation of AM signals
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26. SANJAY JANGRA
ASSISTANT PROFESSOR
sanjay_439@yahoo.co.in
Advanced Educational Institutions
70 km Milestone,
Delhi-Mathura Road, Dist. Palwal, Haryana-121105
Enquiry No: +91–1275–398400, 302222
Tele Fax: +91-1275-398406
E-mail: info@advancedinstitutions.com
Website: www.advanced.edu.in
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