Efficiency of AM modulation

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Efficiency of AM modulation
 The Modulation Efficiency is the percentage of the total
power of the modulated signal that conveys information.
 Therefore given the equation for power of an AM waveform,
the efficiency is:
2

 
2
2
 100 %
 Highest efficiency for a 100% AM signal : 50% - square
wave modulation
 It can be seen from this equation that the efficiency of AM
modulation increases as the modulation index, μ (m),
increases.
Normalized Peak Envelope Power (PEP) of the AM signal:
2
PPEP 
Ac
2
1  max m 2
Importance of High Percentage Modulation
Let R be the resistance into which the current flow.
T hen,
H ence
It
 1
Ic
It = Ic
It
Ic
m
2
2
Pt I R  I t 
m


 1
Pc I R  I c 
2
2
t
2
c
m
2
2
1+
m
2
= Total transmitted current
= Carrier current
= Modulating index
2
Effective Modulation Index
If a carrier is modulated by more than a single sine wave,
the effective modulation index is given by:
m eff 
 m1  m 2  m 3  ...... 
2
2
2
Note that the total modulation index must not exceed 1 or
distortion will occur.
meff can be used in all previously developed equations
using m.
Usage
• Amplitude is susceptible to interference
– This technique in not normally used in modems
• Also used for two-way mobile radio communications,
such as citizens band (CB) radio.
• Use in commercial broadcasting of both audio and
video signals.
• A variation of this technique is used in AM radio
transmission
– Analog-to-analog modulation takes place
AM Advantages
• Low bandwidth
• Easy to modulate, demodulate signal
AM Disadvantages
• Must maintain linearity of message signal and transmitted
signal. Need to use special filters which are not as efficient as
those that do not maintain strict linearity; relates to battery
lifetime
• Interfering signals received additively
discriminated during demodulation
and
must
be
• Hard to lock frequency of receiver local oscillator to carrier
frequency (esp. in S.C. systems)
• Can use pilot tone AM to mitigate these effects
– Transmit known pilot tone either in-band or out-of-band
– PLL can detect pilot tone and lock the frequency and
amplitude of the local oscillator
Types of Amplitude Modulation
•
Double Sideband with full carrier (AM DSBFC): This is
the most widely used type of AM modulation. In fact, all
radio channels in the AM band use this type of modulation.
•
Double Sideband Suppressed Carrier (DSBSC): This is the
same as the AM modulation above but without the carrier.
•
Single Sideband (SSB): In this modulation, only half of the
signal of the DSBSC is used.
•
Vestigial Sideband (VSB): This is a modification of the SSB
to ease the generation and reception of the signal.
Linear carrier wave (CW) modulation
•
•
•
•
•
•
Bandpass systems and signals
Lowpass (LP) equivalents
Amplitude modulation (AM)
Double-sideband modulation (DSB)
Modulator techniques
Suppressed-sideband amplitude
modulation (LSB, USB)
• Detection techniques of linear modulation
– Coherent detection
– Non coherent detection
AM
DSB
LSB
USB
Question:
Find the percentage of an AM wave whose total
power content is 2500W and whose side bands
contain 400W.
Solution:
Given, Pt = 2500W, Plsb =Pusb = 400W
Pt = Pc + Plsb + Pusb
Pc = 1700W
Plsb =Pusb = m2Pc/4
Thus, m = 0.970
M= 0.970X100 = 97%
M=?
Generation of AM
There are two types of devices by which
amplitude modulated signals can be generated:
– The AM transmitter, which generates such high
powers that its prime requirement is efficiency, and
so is a quite complex means.
– The AM generator in lab, where AM is produced at
such a low power level that simplicity is more
important requirement than efficiency.
Basic Requirements
To generate the AM wave:
 It is necessary merely to apply the series of current
pulses to a tank (resonant) circuit.
 Each pulse initiates a damped oscillation in the tuned
circuit.
 The oscillation would have an initial amplitude
proportional to the size of the current pulse and decay
rate dependent on the time constant of the circuit.
 Since a train of pulses is fed to the tank circuit here,
each pulse will cause a complete sine wave
proportional in amplitude to the size of this pulse.
 This is followed by the next sine wave, proportional to
the size of the next applied pulse, and so on.
 A very good approximation of AM wave will result if
the original current pulses are made proportional to the
modulating voltage. This process is known as flywheel
effect of the tuned circuit, and it works best with a
tuned circuit whose Q is not too low.
 In AM transmitter, amplitude modulation can be
generated at any point after the radio frequency source.
AM Transmitters
A transmitter not only performs the modulation process,
but also raises the power level of a modulated signal to
the desired extent for effective radiation. The AM
transmitters are divided into two categories, which
depends on the transmitted circuit arrangements.
• High level: If the output stage in a transmitter is plate modulated
(or collector modulated in low power transmitter), the system is
called high level modulation.
• Low level: If modulation is applied at any other point, including
some other electrode of the output amplifier, then it is low level
modulation.

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