250+ TOP MCQs on Amplitde Modulation and Answers

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Electronic Devices and Circuits Multiple Choice Questions on “Amplitde Modulation”.

(Q.1-Q.3) An AM signal is represented by x(t) = (20 + 4sin(500πt)) cos(2πt x 105)V.

1. The modulation index is
A. 20
B. 4
C. 0.2
D. 10
Answer: C
Clarification: 20 + 4sin(500πt) = 20(1 + 0.2sin(500πt)), hence the modulation index is 0.2.

2. Total signal power is
A. 208 W
B. 204 W
C. 408 W
D. 416 W
Answer: B
Explantion: Pc = 20×20/2 or 200 W. Pt = Pc(1 + 0.2×0.2/4) or 204 W.

3. Total sideband power is
A. 4 W
B. 8 W
C. 16 W
D. 2 W
Answer: A
Clarification: Pt – Pc = 204 – 200 = 4W.

4. An AM broadcast station operates at its maximum allowed total output of 50 kW with 80% modulation. The power in the intelligence part is
A. 12.12 kW
B. 31.12 kW
C. 6.42 kW
D. none of the mentioned
Answer: A
Clarification: Pi = Pt – Pc = 50 – 37.88 kW or 12.12 kW.

5. The aerial current of an AM transmitter is 18 A when unmodulated but increases to 20 A when modulateC.The modulation index is
A. 0.68
B. 0.73
C. 0.89
D. 0.95
Answer: A
Clarification: 400/326 = 1 + (α2)/2, therefore α is 0.68.

6. A modulating signal is amplified by a 80% efficiency amplifier before being combined with a 20 kW carrier to generate an AM signal. The required DC input power to the amplifier, for the system to operate at 100% modulation, would be
A. 5 kW
B. 8.46 kW
C. 12.5 kW
D. 6.25 kW
Answer: C
Clarification: Pi = Pt -Pc = 30 – 20 = 10 kW. DC input = 10/0.8 or 12.5 kW.

7. A 2 MHz carrier is amplitude modulated by a 500 Hz modulating signal to a depth of 70%. If the unmodulated carrier power is 2 kW, the power of the modulated signal is
A. 2.23 kW
B. 2.36 Kw
C. 1.18 kW
D. 1.26 kW
Answer: A
Clarification:Pt = Pc (1 + 0.49/2).

8. A carrier is simultaneously modulated by two sine waves with modulation indices of 0.4 and 0.3. The resultant modulation index will be
A. 1.0
B. 0.7
C. 0.5
D. 0.35
Answer: C
Clarification: α2 = 0.32 + 0.42 = 0.52 or α = 0.5.

9. In a DSB-SC system with 100% modulation, the power saving is
A. 100%
B. 55%
C. 75%
D. 100%
Answer: B
Clarification: This is so because the power is suppressed by two thirds of the total. hence the power saving is 66%.

10. A 10 kW carrier is sinusoidally modulated by two carriers corresponding to a modulation index of 30% and 40% respectively. The total radiated power is
A. 11.25 kW
B. 12.5 kW
C. 15 kW
D. 17 kW
Answer: A
Clarification: The required answer is 1 (1 + 0.42 + 0.32) or 11.25 kW.