What Is Amplitude Modulation
What Is Amplitude Modulation
What Is Amplitude Modulation
Amplitude Modulation
Amplitude modulation is a type of modulation where the
amplitude (signal strength) of the carrier signal is varied in
accordance with the amplitude (signal strength) of the message
signal.
Or
Or
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The above figures show the amplitude modulation.
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The first figure shows the modulating signal or message signal which
contains information, the second figure shows the high frequency carrier
signal which contains no information and the last figure shows the
resultant amplitude modulated signal.
The third figure shows that the amplitude of both the positive and
negative half cycles of the carrier wave is varied in accordance with the
instant amplitude of the message signal. It can be observed that the
positive and negative peaks of the amplitude modulated (AM) wave are
interconnected with an imaginary line. This imaginary line on the AM
wave is called envelope. The shape of the envelope of AM wave looks
same as the message signal. Therefore, this envelope helps in recreating
the exact shape of the message signal.
The carrier signal does not contain any information so even if we change
the amplitude of the carrier signal, no information loss will occur.
However, if we change the characteristics (amplitude, frequency, or
phase) of the message signal, information loss will occur because the
message signal contains the information. So, the characteristics of the
message signal should not be changed.
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Mathematical Expression
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Modulation index of amplitude modulation
Where,
Am is the maximum amplitude of the message signal
Ac is the maximum amplitude of the carrier signal
The maximum amplitude of the message signal must be less than the
maximum amplitude of the carrier signal to avoid any distortion in the
modulated signal. For example, if the carrier signal amplitude is 5 volts
then the message signal amplitude must be less than 5 volts. The
maximum value of the modulation index will be equal to one
when Am = Ac. The minimum value of the modulation index will be
zero. If modulation index is higher than 1, then it is called
overmodulation. In overmodulation, the data loss will occur. When
modulation index is expressed in percentage, it is also called percentage
modulation.
Calculation of Modulation Index from Amplitude Modulated (AM)
waveform
The below figure shows the amplitude modulated (AM) waveform
through which we can calculate the modulation index.
It is clear from the below figure that the modulating signal rides above
the carrier signal.
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From the above figure, we can write,
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Modulation Index or Modulation Depth Examples
The maximum amplitude of the message signal must be less than (or
equal to) the maximum amplitude of the carrier signal to avoid any
than (or equal to) 5 volts. Hence, the maximum value of the modulation
index will be less than one or equal to one (Mi<=1) when Am <= Ac.
1. Perfect-Modulation
2. Under-Modulation
3. Over-Modulation
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Perfect-Modulation:
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Under-Modulation:
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Over-Modulation:
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Frequency Spectrum of Amplitude Modulation
The sidebands that are created above the carrier frequency are called
upper sidebands and the sidebands that are created below the carrier
frequency are called lower sidebands.
To see how it works, take the example of a carrier of 800 kHz frequency
which is modulated by a message signal (audio signal) of 10 kHz
frequency. The process of modulating a carrier signal with message
signal is same as mixing two signals together. As a result of modulation,
two sideband frequencies are produced.
One sideband frequencies are created above the carrier frequency. These
sidebands are known as upper sidebands or sum frequencies. The upper
sidebands are created due to the addition of carrier signal frequency (800
kHz) with the message signal frequency (10 kHz)
I.e. 800 kHz + 10 kHz = 810 kHz.
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Another sideband frequencies are created below the carrier
frequency. These sidebands are known as lower sidebands or difference
frequencies. The lower sidebands are created due to the subtraction of
message signal frequency (10kHz) with the carrier signal frequency (800
kHz) I.e. 800 kHz – 10 kHz = 790 kHz.
I.e. 10 kHz frequency is produced above and below the carrier.
Consider the expression of AM (amplitude modulated) wave given by
equation (6)
a = (Ac + Am sin ωmt) sin ωct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (1)
We know that Mi = Am / Ac. Hence we have Am = Mi Ac.
Putting this value of Am in above equation (1) we get,
a = (Ac + Mi Ac sin ωmt) sin ωct
= Ac (1 + Mi sin ωmt) sin ωct
= Ac sin ωct + Ac Mi sin ωmt sin ωct . . . . . . . . . . . . . . . . . . . . . . . . . . . .
(2)
In the above equation, the first term represents unmodulated carrier, the
second term represents lower sideband and the last term represents upper
sideband.
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Note that ωc = 2πfc and ωm = 2πfm. Hence, the above equation (3) can
also be written as
From these above equations (4) and (5), we can prepare the frequency
spectrum of AM wave as shown in the below figure.
This contains the full carrier and both the sidebands. Hence, it is also
called Double Sideband Full Carrier (DSBFC) system.
BW = fUSB – fLSB
= (fc + fm) – (fc – fm)
BW = 2 fm
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Advantages of Amplitude Modulation
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