AMPLITUDE MODULATION
1. DEFINING AM
• A carrier frequency whose amplitude is varied
in proportion to the instantaneous amplitude of
a modulating voltage
2. GENERATING THE AM
nonlinear device: diode or transistor biased in its
nonlinear region
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3. ANALYSIS OF THE AM WAVE
vc  Vc sin 2f c t
m
m
v  Vc sin 2f c t  Vc cos 2 ( f c  f s )t  Vc cos 2 ( f c  f s )t
2
2
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4. Different Carriers and AM
Carriers are spaced at 20 kHz, beginning at 100kHz.
Each carrier is modulated by a signal with 5kHz
bandwidth. Is there interference from sideband overlap?
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5. MODULATION INDEX AND SIGNAL POWER
Vm
Vmax  Vmin
m

Vc
Vmax  Vmin
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Moduiation Index and Power
2
carr
2
V
2
c
(Vc / 2 )
V
Pc 


R
R
2R
PLSB  PUSB
2
Vc
m


2R
4
2
C
2
2
2
PT
m  2(  1)
PC
V
m
m
PT 
(1 
)  Pc (1 
)
2R
2
2
PT
m
 1
Pc
2
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Current Calculations
2
IT
m
 1
Ic
2
Example
A carrier of 1000 W is modulated with a resulting
modulation index of 0. 8. What is the total power?
What is the carrier power if the total power is 1000 W
and the modulation index is 0.95?
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6. SUPPRESSED CARRIERS AND SINGLE
SIDEBAND (SSB)
6.1 Signal Magnitudes and Ranges
decibel (dB)
PI
dB  10log
PO
2
I
2
O
V
VI
dB(for volts) 10log  20log
V
VO
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The dB gain is
Ratio
1
10
100
1,000
10,000
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Power
(dB)
0
10
20
30
40
Voltage
(dB)
0
20
40
60
80
10
dB REFERENCE VALUES
• dBm (reference is 1 mW),
• dBV (reference is 1 V),
• dBW (reference is 1 W).
• Example:
A signal of 200 mW could be expressed in dBm
as
An audio amplifier might have an output of 7 dBW.
This corresponds to
A signal of 8.2 V corresponds to
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6.2 Double Sideband Suppressed Carrier
(DSBSC)
When the carrier is reduced, this is called doublesideband suppressed-carrier AM, or DSB-SC. If the
carrier could somehow be removed or reduced, the
transmitted signal would consist of two informationbearing sidebands, and the total transmitted power
would be information
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• For a 100-W total available power, compare the
power in the sidebands when modulation is
standard AM with a modulation index of 100%,
versus a suppressed carrier design where 90%
of the carrier power is suppressed. How many
times greater is the sideband power in the
suppressed carrier case?
• A 500-W DSB-SC system with 100%
modulation suppresses 50% of the carrier, and
the suppressed carrier power goes to the
sidebands. How much power is in the
sidebands, and how much is in the carrier? By
how many dB has the sideband power
increased?
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6.3 Single-Sideband (SSB)
• suppressing the carrier and one of the
sidebands
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6.4 Filtering the SSB LSB or USB
• Dual Conversion: up-converting the modulating frequency twice and selecting the upper
or lower sideband for transmission.
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7. Filtering the SSB LSB or USB
Their quality or "Q" factor:
A filter is said to have a high Q if it selects or rejects a
narrow range of frequencies compared with its centre
frequency. Q is defined as centre frequency fo/3dB
bandwidth.
Type of LC filters ceramic mechanical crystal
the filters
Order of
Qs
200
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10,000
50,000
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Dual Conversion
up-converting the modulating frequency twice and
selecting the upper or lower sideband for
transmission.
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8. VESTIGIAL SIDEBAND
• In VSB, a portion (or vestige) of the lower
sideband is transmitted along with the full
carrier power and upper-sideband content.
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AM: Features and Drawbacks:
the AM signal is greatly affected by noise
impossible to determine absolutely the
original signal level
conventional AM is not efficient in the use of
transmitter power
AM is useful where a simple, low-cost receiver
and detector is desired
The noise is either not a problem or can be
tolerated in the application.
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