1. Presented by Subhalaxmi Chakraborty Assistant Professor ,ECE
Amplitude Modulation
Presented by
Subhalaxmi Chakraborty
Assistant Professor ,ECE
1/3/2019

2. Subhalaxmi Chakraborty
Prerequisites:
Concepts of Signal and System
Reference Books:
Analog and Digital Communication , Simon . S. Haykin
Modern Digital and Analog Communication, B.P. Lathi
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Subhalaxmi Chakraborty

3. Block Diagram Of Communication System
m(t)
Carrier Signal
C(t)
Modulation takes place at the transmitter block and demodulation takes place at the receiver block.
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Subhalaxmi Chakraborty

4. Subhalaxmi Chakraborty
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Subhalaxmi Chakraborty

5. Types of Amplitude Modulation (AM)
It is the process of modulation by which the amplitude of carrier signal is varied in accordance with the message signal.
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6. Mathematical Expression of AM
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7. Subhalaxmi Chakraborty
(1)
Rearranging Eq. (1), we get
(2)
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8. Subhalaxmi Chakraborty
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9. Subhalaxmi Chakraborty
The modulation index or modulation depth is often denoted in percentage called as Percentage of Modulation. We will get the percentage of modulation, just by multiplying the modulation index value with 100.
For a perfect modulation, the value of modulation index should be 1, which implies the percentage of modulation should be 100%.
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Time Domain representation of AM
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16. Subhalaxmi Chakraborty
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Numerical Problems:
Q1. A 400 watt carrier is modulated to a depth of 75%.Find the total power in the amplitude modulated wave. Assume the modulating signal to be a sinusoidal signal.
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Subhalaxmi Chakraborty

18. Double Side Band Suppressed Carrier
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19. Basic concept of AM
In the process of Amplitude Modulation, the modulated wave consists of the carrier wave and two sidebands. The modulated wave has the information only in the sidebands.
The transmission of a signal, which contains a carrier along with two sidebands can be termed as Double Sideband Full Carrier
However, such a transmission is inefficient. Because, two-thirds of the power is being wasted in the carrier, which carries no information. 
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20. Mathematical Expression Of DSB-SC
Let us consider the same mathematical expressions for modulating and carrier signals
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21. The DSBSC modulated wave has only two frequencies
The DSBSC modulated wave has only two frequencies. So, the maximum and minimum frequencies are fc+fm and fc−fm respectively.
i.e.,
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22. Power Calculations of DSBSC Wave
Power of DSBSC wave is equal to the sum of powers of upper sideband and lower sideband frequency components.
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26. DSBSC Modulators The following two modulators generate DSBSC wave.
DSBSC modulators are also called as product modulators as they produce the output, which is the product of two input signals.
Balanced modulator
Ring modulator
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27. Balanced Modulator
 BLOCK DIAGRAM OF THE BALANCED MODULATOR
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28. Balanced modulator consists of two identical AM modulators
These two modulators are arranged in a balanced configuration in order to suppress the carrier signal. Hence, it is called as Balanced
 c(t)=Ac cos(2πfct) 
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31. Ring Modulator
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32. Mode 1 Carrier Suppression
Let us assume that the modulating signal is absent and only the  carrier signal is applied.
Hence x(t) = 0
Operations as follows:
i. Operation in the Positive half-cycle of Carrier
ii. Operation in the Negative half-cycle of Carrier
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33. i.
Diodes   D1  and  D2 are forward biased and the diodes  D3  and  D4 are reverse biased .
The direction of currents flowing through the primary windings of output transformer  T2  are equal and opposite to each other .
Therefore, the magnetic fields produced by these currents are equal and opposite and cancel each other .
Hence, the induced voltage in secondary winding is zero .
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34. ii.
In the negative half-cycle of the carrier, the diodes D3  and  D4 are forward biased and the diodes   D1  and  D2 are reverse biased .
The direction of currents flowing through the primary windings of output transformer  T2  are equal and opposite to each other .
Therefore, the magnetic fields produced by these currents are equal and opposite and cancel each other .
Hence, the induced voltage in secondary winding is zero . Thus, the carrier is supported in the negative half-cycle .
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35. Mode 2 : Operation in Presence of Modulating Signal
(i) Operation in the positive half-cycle of Modulating Signal
(ii) Operation in the Negative half-cycle of Modulating Signal
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36. (i)
The low frequency modulating signal through the input audio transformer T1 
In the positive half-cycle of the carrier, D1  and  D2 are ON and secondary of T1 is applied as it is across the primary of T2. Hence, during the positive half cycle of carrier, the output of T2 is positive
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37. In the negative half-cycle of the carrier,  the diodes D3  and  D4 are turned ON and the secondary of  T1 is applied in a  reversed manner across the primary of T2
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38. Subhalaxmi Chakraborty
THANK YOU…..
1/3/2019
Subhalaxmi Chakraborty