Amplitude Modulation MCQ

31. The original spectrum of a message contains 100 Hz, 200 Hz frequency components. It is amplitude modulated by a carrier of 0.9 kHz. Which frequency components are contained in the amplitude modulated signal spectrum?

  1. 900, 1000 and 1100 Hz
  2. 700, 800 and 900 Hz
  3. 700, 800, 900, 1000 and 1100 Hz
  4. 100, 200 and 900 Hz
Answer. c

32. A DSB-SC signal is generated using the carrier signal cos (ωct + θ) and modulating signal m(t). What is the envelope detector output of this DSB-SC signal?

  1. | m(t) |
  2. m(t) cosθ
  3. m(t) sineθ
  4. m(t) tanθ
Answer. a

33. A given AM broadcast station transmits an average carrier power output of 40 kW and uses a modulation index of 0.707 for sine wave modulation. What is the maximum (peak) amplitude of the output if the antenna is represented by a 50 Ω resistive load?

  1. 50 V
  2. 50 kV
  3. 28.28 kV
  4. 3.414 kV
Answer. d

34. A broadcast AM radio transmitter radiates 125 kW when the modulation percentage is 70. How much of this is carrier power?

  1. ≈ 25 kW
  2. ≈ 50 kW
  3. ≈ 100 kW
  4. ≈ 75 kW
Answer. c

35. Which of the following modulated signals can be detected by an envelope detector?

  1. DSB-suppressed carrier
  2. DSB-full carrier
  3. Frequency modulated signal
  4. SSB-suppressed carrier
Answer. b

36. An AM voltage signal s(t), with a carrier frequency of 1.15 GHz has a complex envelope g(t) = Ac [1 + m(t)], Ac = 500 V, and the modulation is a 1 kHz sinusoidal test tone described by m(t) = 0.8 sin(2π × 103t), appears across a 50 Ω resistive load. What is the actual power dissipated in the load?

  1. 82.5 kW
  2. 165 kW
  3. 6.6 kW
  4. 3.3 kW
Answer. d

37. Which one of the following statements is correct?

The capacitor charging time in the AM envelope demodulator is based on the time for one

  1. half-cycle of the carrier frequency
  2. quarter cycle of the carrier frequency
  3. half cycle of the lowest audio frequency
  4. quarter cycle of the highest audio frequency
Answer. a

38. Which one of the following statements is correct?

The type of modulation used generally in TV transmission for video and audio signals, respectively, are

  1. FM and FM
  2. FM and AM
  3. AM and FM
  4. AM and AM
Answer. c

39. In amplitude modulation, the modulation envelope has a peak value which is double the unmodulated carrier value. What is the value of the modulation index?

  1. 50%
  2. 25%
  3. 100%
  4. 75%
Answer. c

40. The detection-gain for coherent DSB demodulator is

  1. 4
  2. 2
  3. 16
  4. 8
Answer. b
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