# NCERT Physics Class 12 Exemplar Ch 15 Communication Systems Part 1

## Very Short Answer

Q 15 Which of the following would produce analog signals and which would produce digital signals?

(i) A vibrating tuning fork.

(ii) Musical sound due to a vibrating sitar string.

(iii) Light pulse.

(iv) Output of NAND gate.

Ans:

• analog

• analog

• digital

• digital

Q 16 Would sky waves be suitable for transmission of TV signals of 60 MHz frequency?

Ans:

No, signals of frequency greater than 30 MHz will not be reflected by the ionosphere, but will penetrate through the ionosphere.

Q 17 Two waves A and B of frequencies 2 MHz and 3 MHz, respectively are beamed in the same direction for communication via sky wave. Which one of these is likely to travel longer distance in the ionosphere before suffering total internal reflection?

Ans:

• The refractive index increases with increase in frequency which implies that for higher frequency waves, angle of refraction is less, i.e. bending is less.

• Hence, the condition of total internal reelection is attained after travelling larger distance (by 3MHz wave).

Q 18 The maximum amplitude of an A.M. wave is found to be while its minimum amplitude is found to be. What is the modulation index?

Ans:

Q 19 Compute the LC product of a tuned amplifier circuit required to generate a carrier wave of 1 MHz for amplitude modulation.

Ans:

Q 20 Why is a AM signal likely to be more noisy than a FM signal upon transmission through a channel?

Ans:

• In AM, the carrier waves instantaneous voltage is varied by modulating waves voltage.

• On transmission, noise signals can also be added and receiver assumes noise a part of the modulating signal.

• However in FM, the carriers frequency is changed as per modulating waves instantaneous voltage.

• This can only be done at the mixing/ modulating stage and not while signal is transmitting in channel.

• Hence, noise doesn’t effect FM signal.

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