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Title Web for Principle of Communication - B. Tech 4th Sem Frequency Modulation Analog To Digital Converter Ellipse Signal To Noise Ratio Apsis 556.1 KB 36
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Page 1

Communication Engineering

Chapter 2: Amplitude Modulation

Example 8

An amplitude-modulated wave has a power content of 800W at its carrier frequency. Calculate

the power content of each of the sidebands for 80% modulation.

Solution:

Pc = 800W

m = .8

2

128
4

c

LSB USB

m P
P P W  

Example 10

A carrier wave Vc = A sin c

t is amplitude modulated by an audio wave

  
a a a a

B B
V Bsinω t sin3ω t sin5ω t

3 5

Determine the upper and lower bands. Sketch the complete spectrum of the modulated wave.

Estimate the total power in the side bands in terms of the carrier power if the modulated index

B/A = 0.6.

Solution:

After modulated, the frequencies will be c  a, c  3a, and c  5a, respectively. The frequency

spectrum is shown below.

2

MA

6

MA

10

MA

c
-

5

a

c
-

3

a

c
-

a

c

a

c
+

a

c
+

3

a

c
+

5

a
L

A

Page 2

Total power in side bands,

2 2 2

1 2 3

2 2 2

C C C

SBT

P m P m P m
P   

m1 = m = 0.6

2

0.6
0.2

3 3

m
m   

and
3

0.6
0.12

5 5

m
m   

2 2 2

SBT 1 2 3

C

P

P 2

m m m 
 

0.36 0.04 0.0144

2

 

= 0.2072

% Power = 0.2072  100 = 20.72%

Example 17

Determine the percent increase in the effective value of the antenna current when the

modulation of a transmitter is increased to 65% from 0%.

Solution:

We know

2
2

1
2

t

c

I m

I

 
  

 

Where It = modulated current (or total)

Ic = unmodulated current

m = modulation index

2
(0.65) 0.4225

1 1 1.2112
2 2

    

1.2112 1.1t

c

I

I
   or It = 1.1 Ic

So increase in antenna current = (It – Ic)

= 1.1 Ic – Ic = 0.1 Ic