12 physics communication system 02

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CBSE TEST PAPER-02

CLASS - XII PHYSICS (Communication System)

[ANSWERS]

Ans1: 2 2 6400 1000 300d Rh= = × × ×

62 .d km=

Ans2: Ground waves are not suitable for propagation high frequencies because signals

having frequency more than 1500 KHz are greatly absorbed by the surface of the

earth and cannot be transmitted.

Ans3: Amplitude Modulation.

Ans4: F2 is the topmost layer of ionosphere. Its height is upto 400km and is called as a

reflecting layer for high frequency radio wave.

Ans5: m

c

A

Aµ =

7512

100

m c

m

A A

A

µ= ×

= ×

Ans6: Basic communication system consist of an information source, a transmitter, a link

and a receiver.

9mA V=



Ans7:

Ans8: (a) Ground wave propagation – Radio travel along the surface of the earth and are

called ground waves and the propagation along the surface of the earth is called

ground wave propagation. It is limited to a frequency below 1.5 MHz

(b) Space wave propagation – The radio waves which are reflected back to the

earth by ionosphere are known as sky waves and this mode of propagation of

sky waves is known as sky wave propagation.

(c) Space wave propagation – High frequency waves which cannot be reflected

back to the earth by transmitting antenna to receiving antenna by the mode

called line of sight communication. It is also called as space wave propagation.

Ans9: Satellite communication and line of sight (LOS) communication make use of space

waves.

Now 1 12d Rh=

2 22d Rh=

For maximum range

Analog communication Digital communication

(1) In analog communication analog

signal is used which continuous

varies with time.

(2)

(1) In digital communication digital

signal is used which has only two

levels i.e. high and low.

(2)



1 2

1 2

2 2m

m

d Rh Rh

d d d d

= += + =

Given 1 2h h h+ =

Let 1h x= then 2h h x= −

2 2 ( )md Rx R h x= + −

Differentiating wr.t x

2 21 2

1 10 . .

2 2( ) 2 2( )m

x

h h

dd R Ri e

d x h x x h x

x h h

− = = =− −

⇒ = ⇒ = =

Ans10: Given e = 10 sin ( )85 10 6sin1000t× + +

Compare if with general equation

e = E sin (wct + mf sin wmt)

Carries frequency 85 10

2 2 3.14c

c

wνπ

×= =×

79.62c MHzν =

Modulating signal frequency

100

15.922 2 3.14m

WmHzν

π= = =

×

Power dissipated ( ) ( )22 10

2 0.5 100

ErmsP Watts

R= = =

12 physics communication system 02

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