## WBCHSE Class 12 Physics Communication System Multiple Choice Questions

**Question 1. Which of the following frequencies will be suitable for beyond-the-horizon communication using sky waves?**

- 10 kHz
- 10 MHz
- 1 GHz
- 1000 GHz

**Answer:** 2. 10 MHz

Frequency 10 MHz will suffer from antenna problems. Frequency of 1 GHz and 1000 GHz have so high penetration power that the earth would absorb them

**Question 2. Frequencies in the UHF range normally propagate using**

- Ground waves
- Surface waves
- Sky waves
- Space waves

**Answer:** 4. Space waves

**Question 3. Digital signals**

- Do not provide a continuous set of values,
- Represent values as discrete steps,
- Can utilize only binary systems, and
- Can utilize decimal as well as binary systems

**Question 4. Which ofthe above statements are true?**

- (1) and (2) only
- (2) and (3) only
- (1), (2), and (3) but not (4)
- All of (1), (2), (3) and (4)

**Answer:** 3. (1), (2), and (3) but not (4)

**Read and Learn More Class 12 Physics Multiple Choice Questions**

**Question 5. Is it necessary for a transmitting antenna to be at the same Ac 6 height as that of the receiving antenna for line-of-sight communication? A TV transmitting antenna is 81 m all How much service area can it cover if the receiving antenna is at the ground level?**

**Answer:**

Two antennas don’t need to be of height

Required area = \(\pi d^2=\pi(\sqrt{2 \times h \times R})^2\)

= \(\frac{22}{7} \times 2 \times 81 \times 6.4 \times 10^6\)

= 3258 km²

**Question 6. A carrier wave of peak voltage 12 V is used to transmit a message signal. What should be the peak voltage of the modulating signal in order to have a modulation index of 75%?**

**Answer:**

μ = \(\frac{A_m}{A_c}\)

∴ A_{m }= μ A_{c }

= \(\frac{75}{100} \times 12\)

= 9V

**Question 7. For an amplitude-modulated wave, the maximum amplitude is found to be 10 V while minimum amplitude is found to be 2 V. Determine the modulation index H . What would be the value of μ if the minimum amplitude is zero?**

**Answer:**

A_{max} = A_{c}+A_{m} and A_{min}= A_{c}– A_{m}

∴ A_{c }= \(\frac{A_{\max }+A_{\min }}{2}\) = \(\frac{10+2}{2}\)

= 6V

A_{m }= \(\frac{A_{\max }-A_{\min }}{2}\) = \(\frac{10-2}{2}\)

= 4V

∴ μ = \(\frac{A_m}{A_c}=\frac{4}{6}\)

= 0.67

If A_{min}= 0 , then A_{m} = A_{c} and μ = 1 .

**Question 8. Three waves A, B, and C of frequencies 1600 kHz, 5 MHz carrier wave of frequency (oc to get modulated wave (AM ). The frequency of the AM wave will and 60 MHz respectively are to be transmitted from one place to another. Which of the following is the most appropriate mode of communication?**

- A is transmitted via space waves while B and C are transmitted via sky waves
- A is transmitted via ground wave, B via sky wave and C via space wave
- B and C are transmitted via ground wave while A is transmitted via sky wave
- B is transmitted via ground wave while A and C are transmitted via space wave

**Answer:** 1. A is transmitted via space waves while B and C are transmitted via sky waves

**Question 9. A 100-long antenna is mounted on a 500 m tall building The complex can become a transmitting tower for waves with A**

- 400m
- 25m
- 150m
- 2400m

**Answer:** 1. 400m

**Question 10. A 1 kW signal is transmitted using a communication channel that provides attenuation at the rate of -2dBperkmrIf the communication channel has a total length of 5 km, the power of the signal received is [gain in dB = 10Iog(P _{0}/P_{t})**

- 900 W
- 100 W
- 990W
- 1010W

**Answer:** 2. 100 W

**Question 11. A speech signal of 3 kHz is used to modulate a carrier signal of frequency 1MHz, using amplitude modulation. The frequencies ofthe sidebands will be**

- 3001 kHZ and 2997 kHz
- 1.003 MHz and 0.997 MHz
- 1003 kHz and 1000 kHz
- 1 MHz and 0.997 MHz

**Answer:** 2. 1.003 MHz and 0.997 MHz

**Question 12. A message signal of frequency com is superposed on a carrier wave of frequency to get an amplitude-modulated wave (AM). The frequency of the AM wave will.**

- ω
_{m} - ω
_{c} - \(\frac{\omega_c+\omega_m}{2}\)
- \(\frac{\omega_c-\omega_m}{2}\)

**Answer:** 3. \(\frac{\omega_c+\omega_m}{2}\)

**Question 13. I-V characteristics of four devices are shown. Identify devices that can be used for modulation**

- (1) and (3)
- Only (3)
- (2) and some regions of (4)
- All the devices can be used

**Answer:** 3.

**Question 14. A male voice after modulation transmission sounds like that of a female to the receiver. The problem is due to**

- Poor selection of modulation index (selected 0 < m < 1)
- Poor bandwidth selection of amplifiers
- Poor selection of carrier frequency
- Loss of energy in transmission

**Answer:** 2. Poor bandwidth selection of amplifiers

**Question 15. Identify the mathematical expression for the amplitude-modulated wave**

- \(A_c \sin \left[\left\{\omega_c+k_1 v_m(t)\right\} t+\phi\right]\)
- \(A_c \sin \left\{\omega_c t+\phi+k_2 v_m(t)\right\}\)
- \(\left\{A_c+k_2 v_m(t)\right\} \sin \left(\omega_c t+\phi\right)\)
- \(A_c v_m(t) \sin \left(\omega_c t+\phi\right)\)

**Answer:** 3. \(\left\{A_c+k_2 v_m(t)\right\} \sin \left(\omega_c t+\phi\right)\)

**Question 16. An audio signal of 15 kHz frequency cannot be transmitted over long distances without modulation because**

- The size of the required antenna would be at least 5 km which is inconvenient
- The audio signal cannot be transmitted through sky waves
- The size of the required antenna would be at least 20 km which is inconvenient
- Effective power transmitted would be very low if the size of the antenna is less than 5 km

**Answer:** 1,2 And 4

**Question 17. Audio sine waves of 3 kHz frequency are used to amplitude modulate a carrier** **signal of 1.5 MHz. Which of the following statements are true?**

- The sideband frequencies are 1506 kHz and 1494 kHz
- The bandwidth required for amplitude modulation is 6 kHz
- The bandwidth required for amplitude modulation is 3 MHz
- The sideband frequencies are 1503 kHz and 1497 kHz

**Answer:** 2 And 4

**Question 18. In amplitude modulation, the modulation index m, is kept less than or equal to 1 because**

- m > 1 will result in interference between frequency and message frequency, distortion
- m> 1 will result in overlapping of both sidebands resulting in loss of information
- m> 1 will result in a change in phase between the carrier signal and the message signal
- m> 1 indicates amplitude of the message signal greater than the amplitude of the carrier signal resulting in distortion

**Answer:** 2 And 4

**Question 19. A TV transmission tower has a height of 240 m. Signals broadcast from this tower will be received by LOS communication at a distance of(assume the radius of the earth to be 6.4 × 10 ^{6} m )**

- 100 km
- 24 km
- 55km
- 50 km

**Answer:** 2,3 And 4

**Question 20. Out of the following, which is an essential element of a communication system?**

- Transistor
- Transmitter
- Computer
- Both 1 and 3

**Answer:** 3. Computer

**Question 21. The distortion in the transmission and processing of signals is called**

- Interference
- Diffraction
- Noise
- Scattering

**Answer:** 2. Diffraction

**Question 22. If the maximum frequency of an audio signal is f, then what would be the bandwidth of an Amplitude Modulated (AM) wave?**

- \(\frac{f}{2}\)
- f
- 2f
- 4f

**Answer:** 3.2f

**Question 23. If the frequencies of carrier waves for AM and FM are f _{A} and f_{F} respectively, then**

- f
_{A}≈ f_{F} - f
_{A}< f_{p} - f
_{A}>f_{p} - f
_{A}>f_{p}

**Answer:** 2. f_{A} < f_{p}

**Question 24. If the modulation frequency of an FM wave is f, then the modulation index will be directly proportional to:**

- \(\frac{1}{f}\)
- f
- \(\frac{1}{f^2}\)
- f²

**Answer:** 1.\(\frac{1}{f}\)

**Question 25. In amplitude modulation modulation index of a transmitted carrier wave is 0. The increase in power dissipation would be directly proportional to**

- β
- β²
- 1+β²
- 1+β²/2

**Answer:** 2. β²

**Question 26. The rate of energy dissipation in a carrier wave transmission is 10 kW. What would be the rate of energy dissipated if the wave is frequency modulated to a 10% level?**

- 10 kW
- 10.05 kW
- 10.1 kW
- 10.5 kW

**Answer:** 1. 10 kW

**Question 27. In an amplitude-modulated wave, the circular frequency of the carrier wave is ft and that of the data signal lies in the range between (ω’ -ω) Then the width of a single sideband is**

- Ω +ω’
- Ω + (ω’ -ω)
- 2ω’
- (ω’ -ω)

**Answer:** 4.(ω’ -ω)

**Question 28. A MODEM is used**

- To superimpose a data signal on a carrier wave
- To retrieve a data signal from its mixture with a carrier wave
- To amplify a data signal
- To convert an analog signal to a digital signal and vice-versa

**Answer:** 4. To convert an analog signal to a digital signal and vice-versa

**Question 29. The process in which the amplitude of the carrier wave is made proportional to the instantaneous amplitude ofthe signal wave is called**

- Amplitude modulation
- Demodulation
- Rectification
- Amplification

**Answer:** 1. Amplitude modulation

**Question 30. Radio waves of low frequencies cannot be transmitted to long distances. So these are. superposed on a high-frequency carrier signal. This process is known as**

- Amplification
- Modulation
- Rectification
- Demodulation

**Answer:** 2. Modulation

**Question 31. Long-distance transmission is not possible using ground waves due to which characteristics of electromagnetic waves?**

- Scattering
- Diffraction
- Interference
- Polarisation

**Answer:** 4. Polarisation

**Question 32. A transmitting antenna and a receiving antenna, both of, height H, are used for a space wave transmission. At; another place, a similar transmission uses a transmitting antenna of height 2H, but the receiving antenna is set very near the ground. The ratio of the radio horizons for these two cases is
Answer:**

- 1: 1
- 2: 1
- √2: 1
- 1: √2

**Answer: ** 3. √2: 1

**Question 33. An example of employing electromagnetic wave, with a range of frequency from 30 MHz to very high 300 MHz, as carrier wave is**

- Radio transmission of short-distance
- Transmission of FM radio
- Television transmission
- Radar system

**Answer:** 2 And 3

**Question 34. In distant communication, the data signal is not transmitted directly without the help of a carrier wave. The reason is**

- An extremely large transmitting antenna is necessary
- The transmitting antenna is to be installed at a very high altitude
- The chance of mixing noise in data signal increased
- Due to the overlapping of many data signals, the data at the receiving end becomes very unclear

**Answer:** 1 And 4

**Question 35. If a properly modulated carrier wave is transmitted through an antenna, then rate of energy dissipated at the antenna**

- Remains the same for AM wave
- Increases in AM wave
- Remains the same for FM wave
- Increases in FM wave

**Answer:** 2 And 3

**Question 36. Long-distance communication is possible through space wave**

- Due to reflection at the ionosphere
- Due to reflection at artificial satellite
- Through successive reflections from the earth’s surface
- By Installing many antennas one after another at a specific distance apart

**Answer:** 2 And 4

**Question 37. Which of the following is used as carrier wave, in a communication system?**

- Ultraviolet wave
- Infrared wave
- Microwave
- Radio wave

**Answer:** 3 and 4

**Question 38. In long-distance communication, the similar potential wave produced from a carrier wave is V – V _{0 }sin Ω t. On the other side, the potential wave corresponding to data signal**

Is, v = v_{0 }sin ω t, v_{0 }<< V_{0} and ω <<Ω. Superimposing data signal on a carrier wave, generally two types of modulated waves are produced; they are:

**1. Amplitude Modulated (AM) wave:**

V_{AM} = \(V_0\left(1+\beta_1 \sin \omega t\right) \sin \Omega t\)

Where β_{1 }= \(k_1 \frac{v_0}{V_0}\) = Modulation index

k_{1 } = constant

**2. Frequency Modulated (FM) wave:**

V_{FM} = \(V_0 \sin \left(\Omega t-\beta_2 \cos \omega t\right)\)

Where, \(\beta_2=k_2 \frac{v_0}{\omega}\)

Mathematical analysis of two modulated waves shows that, for the AM wave, apart from Ω frequency, there are two sine wave components of frequencies Ω – ω and Ω + ω. As a result, the bandwidth of the waves becomes (Ω – ω)- (Ω + ω). = 2ω). On the other hand, in FM wave, the number of single wave components is infinite and their frequencies are Ω ±ω, Ω ±2ω, ……….

1000 Hz

Besides that, if the dissipated power for a transmitting antenna to transmit a carrier wave be P_{C} and the dissipated power for transmitting AM and FM waves be P_{AM} and P_{FM}, then

P_{AM} = \(\left(1+\frac{\beta^2}{2}\right)\)

P_{FM} = P_{C}

**1. The unit of constant K _{1}**

- Does not have any unit
- V.s
^{-1} - V
^{-1}. s - V
^{-1}. s^{-1}

**Answer:** 1. Does not have any unit

**2. The unit of constant K _{2}**

- 1
- V.s
^{-1} - V
^{-1}. s - V
^{-1}. s^{-1}

**Answer: ** 4. V^{-1} . s^{-1}

**Question 39. What Would Be The Width OF SIdeband On Either side of the amplitude modulated (AM) wave, if there is a mixing of frequency of 1000 Hz to 1500 Hz in the data signal?**

- 500 Hz
- 1500 Hz
- 2000 Hz
- 3000 Hz

**Answer:** 4. 3000 Hz

**Question 40. What would be the width of the sideband on either side of the amplitude-modulated (AM) wave, if there is a mixing frequency of 1000 Hz to 1500 Hz in the data signal?**

- 500 Hz
- 1000 Hz
- 1500 Hz
- 2000 Hz

**Answer:** 4. 2000 Hz

**Question 41. In converting a carrier wave into an amplitude-modulated (AM) wave, the modulation index was 0.05. Then, what would be the percentage increase in dissipated power at the transmitting antenna**

- 2.5
- 1.25
- 0.25
- 0.125

**Answer:** 4. 0.125

**Question 42. As radio waves of low frequencies cannot be transmitted to long distances, a high-frequency carrier signal is superposed on it. This process is called**

- Amplification
- Rectification
- Modulation
- Oscillation

**Answer:** 3. Modulation

**Question 43. Satellite communication is done with**

- Ground wave
- Skywave
- Space Wave
- None of these

**Answer:** 3. Space Wave

**Question 44. A signal of 5 kHz frequency is amplitude modulated carrier wave of frequency 2 MHz. The frequencies of the resultant signal is/are**

- 2 MHz only
- 2005 kHz and 1995 kHz
- 2005 kHz, 2000 kHz and 1995 kHz
- 2000 kHz and 1995 kHz

**Answer:** 3. 2005 kHz, 2000 kHz and 1995 kHz

The two sidebands are also present along with the main frequency ofthe carrier wave

**Question 45. Choose the correct statement:**

- In amplitude modulation, the amplitude of the high-frequency carrier wave is made to vary in proportion to the amplitude of the audio signal
- In amplitude modulation, the frequency of the high-frequency carrier wave is made to vary in proportion to the amplitude of the audio signal
- In frequency modulation the amplitude of the high. frequency carrier wave is made to vary in proportion to the amplitude of the audio signal
- In frequency modulation, the amplitude of the high-frequency carrier wave is made to vary in proportion to the frequency of the audio signal

**Answer:** 1. In amplitude modulation the amplitude of the high-frequency carrier wave is made to vary in proportion to the amplitude of the audio signal