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CBSE Class 12-science Answered

What is amplitude modulation? Explain the block diagram of a practical AM transmitter.

Asked by tanyagupta1874 | 17 Jan, 2022, 10:16: PM
Expert Answer
In amplitude modulation the amplitude of the carrier is varied in accordance with the information signal.
 
Let us explain amplitude modulation process using a sinusoidal signal as the modulating signal.
 
 
Let us consider carrier wave c(t) , which is marked as (a) in figure  is represented by the equation below
 
c(t) = Ac sin ωct   ................................(1)
 
Where Ac is amplitude of carrier wave and ωc = 2π fc , is angular frequency of carrier wave.
Carrier wave is shown as (a) in the figure
 
Let us consider a message m(t) or modulating signal, which is marked as (b) in figure  is represented by the equation below
 
m(t) = Am sin ωmt ................................ (2)
 
Where Am is amplitude of modulating signal and ωc = 2π fm , is angular frequency of modulating signal .

The modulated signal cm (t) can be written as
 
cm (t) = Ac sin ωct + Am sin ωmt sin ωct
 
cm (t) = Ac sin ωct + μ Ac sin ωmt sin ωct ............................(3)
 
Here μ = Am/Ac is the modulation index; in practice,μ is kept ≤ 1 to avoid distortion. 
 
It is to be noted that the modulated signal now contains the message signal. This can also be seen from
the given Figure.  Modulated signal is marked as (c).
 
Using the trignomatric relation sinA sinB = ½ (cos(A – B) – cos (A + B), we can write cm (t) of Eq. ( 3) as
 
 
begin mathsize 14px style c m left parenthesis t right parenthesis space equals space A subscript c space sin omega subscript c t space space plus space fraction numerator mu space A subscript c over denominator 2 end fraction cos left parenthesis omega subscript c minus omega subscript m right parenthesis t space minus space fraction numerator mu space A subscript c over denominator 2 end fraction cos left parenthesis omega subscript c plus omega subscript m right parenthesis t end style
       Here ωc – ωm and ωc + ωm are respectively called the lower side and upper side frequencies.
The modulated signal now consists of the carrier wave of frequency ωc plus two sinusoidal waves each with a frequency slightly
different from, known as side bands.

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A simplified block diagram for Amplitude modulation is shown above.
 
Message signal and carrier wave are given as input to the Amplitude modulator device that gives
modulated signal as output. The modulator is to be followed by a power amplifier which
provides the necessary power and then the modulated signal is fed to an
antenna of appropriate size for radiation as shown in Figure
 




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