# Physics Questions and Answers – Communication Systems – Modulation and its Necessity

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This set of Physics Question Papers for IIT JEE Exam focuses on “Communication Systems – Modulation and its Necessity”.

1. What is the modulation used to?
a) Reduce the bandwidth used
b) Separate the transmissions of different users
c) Ensure that intelligence may be transmitted to long distances
d) Allow the use of practical antennas

Explanation: Modulation is used to reduce the bandwidth used. The use of high-frequency carrier waves reduces bandwidth. Modulation allows us to send a signal over a bandpass frequency range.

2. Identify the limitation of amplitude modification.
a) The quality of the audio signal is high
b) Amplitude modulation suffers from noise
c) The efficiency of AM transmission is high
d) It does not require a wider channel

Explanation: Amplitude modification suffers from noise, the quality of the audio signal is poor, and also the efficiency of the amplitude modification transmission is low. The other statements are not valid.

3. Why audio signals cannot be transmitted directly?
a) The signal has more noise
b) The signal cannot be amplified for distance communication
c) The transmitting antenna length is very small to design
d) The transmitting antenna length is very large and impracticable

Explanation: Audio signals require a transmitting antenna of very large and impracticable length. This is because of the high wavelength of the sound waves produced. Therefore, audio signals cannot be transmitted directly.

4. What happens in a frequency modulated wave?
a) Frequency varies with time
b) Amplitude varies with time
c) Both frequency and amplitude varies with time
d) Both frequency and amplitude are constant

Explanation: In a frequency modulated wave, the frequency of the carrier wave is considered to vary corresponding to the modulating signal. At the resonant frequency, the modulated wave is resistive in nature.

5. An oscillator is producing FM waves of frequency 5 kHz with a variation of 15 kHz. What is the modulation index?
a) 1.00
b) 3.00
c) 0.5
d) 5.00

Explanation: Modulation index = $$\frac {\triangle f}{f}$$
Modulation index = $$\frac {15 kHz}{5 kHz}$$
Modulation index = 3.

6. Modulation is a process of the superposing high-frequency audio signal on low-frequency radiowaves.
a) True
b) False

Explanation: No, this is a false statement. Modulation refers to the superposition of low-frequency audio signals on a high-frequency radiowave. The frequency of the modulated wave is considered to vary with the amplitude of the modulating wave.

7. Why is AM used for broadcasting in the communication systems?
a) Its use avoids receiver complexity
b) It is more noise immune than other modulation systems
c) It requires less transmitting power
d) No other modulation system can give the necessary bandwidth for faithful transmission

Explanation: In the communication systems, amplitude modification is used for broadcasting because its use avoids receiver complexity. Only a diode and a capacitor are sufficient to separate the audio signal from the amplitude modified wave.

8. If the highest modulating frequency of the wave is 10 kHz, what is the number of stations that can be accommodated in a bandwidth of 200 kHz?
a) 10
b) 20
c) 5
d) 15

Explanation: Number of stations = $$\frac {Total \, bandwidth}{(Bandwidth \times station)}$$.
Number of stations = $$\frac {200}{(2 \times 10)}$$
Number of stations = 10

9. What is the maximum distance up to which a TV transmission can be received if the TV tower has a height of 50 m?
a) 28284 km
b) 20.8 km
c) 28.2 km
d) 45 km

Explanation: d = √2hR.
d = √(2 × 50 × 8 × 106)
d = 28284.27 m ≈ 28284 m
d = 28.2 km.
Therefore, the maximum distance up to which the TV transmission can be received is 28.2 km.

10. What happens in frequency modulation?
a) The amplitude of the modulated wave varies as the frequency of the carrier wave
b) The frequency of the modulated wave varies as the amplitude of the modulating wave
c) The amplitude of the modulated wave varies as the amplitude of the carrier wave
d) The frequency of modulated wave varies as the frequency of modulating wave