physics : class12 Radio-Wave Bandwidth : Radio Waves : Electromagnetic Waves MCQ Questions and Answers
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Radio-Wave Bandwidth
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MCQ Questions
1. An AM transmitter is modulated by an audio signal whose frequency spectrum extends from 300 Hz to 5.5 kHz. What is the minimum radio-frequency bandwidth required for distortionless transmission?
A.
5.2 kHz
B.
5.5 kHz
C.
11.0 kHz
D.
11.3 kHz
Correct Answer:
11.0 kHz
Explanation:
For conventional AM, the bandwidth is twice the highest modulating frequency: B = 2f_max = 2 × 5.5 kHz = 11.0 kHz. The lower audio-frequency limit does not determine the bandwidth.
2. A double-sideband suppressed-carrier radio signal is modulated by two sinusoidal tones of 4 kHz and 9 kHz. What is the total bandwidth occupied by the transmitted signal?
A.
5 kHz
B.
9 kHz
C.
13 kHz
D.
18 kHz
Correct Answer:
18 kHz
Explanation:
The sidebands occur at carrier frequencies displaced by ±4 kHz and ±9 kHz. The occupied spectrum extends from f_c − 9 kHz to f_c + 9 kHz, giving a bandwidth of 2 × 9 kHz = 18 kHz.
3. An FM broadcast has a maximum frequency deviation of 75 kHz and the highest modulating frequency is 15 kHz. Using Carson's rule, what bandwidth should be allocated to the transmission?
A.
90 kHz
B.
105 kHz
C.
150 kHz
D.
180 kHz
Correct Answer:
180 kHz
Explanation:
Carson's rule gives the approximate FM bandwidth as B = 2(Δf + f_m(max)). Therefore, B = 2(75 + 15) kHz = 180 kHz.
4. A conventional AM radio channel is assigned a bandwidth of 12 kHz. If the carrier frequency is 1.2 MHz, what is the highest audio frequency that can be transmitted without exceeding the assigned bandwidth?
A.
3 kHz
B.
6 kHz
C.
12 kHz
D.
24 kHz
Correct Answer:
6 kHz
Explanation:
For AM, the required bandwidth is B = 2f_m(max). Thus f_m(max) = B/2 = 12 kHz/2 = 6 kHz. The carrier frequency affects the signal location, not its bandwidth.
5. An FM signal has a maximum deviation of 40 kHz and must accommodate modulating frequencies up to 5 kHz. According to Carson's rule, what is its approximate bandwidth?
A.
45 kHz
B.
50 kHz
C.
80 kHz
D.
90 kHz
Correct Answer:
90 kHz
Explanation:
Using Carson's rule, B = 2(Δf + f_m(max)) = 2(40 + 5) kHz = 90 kHz.
6. A suppressed-carrier AM transmitter is required to transmit an audio spectrum extending from 0.5 kHz to 7 kHz. If only the upper sideband is transmitted, what bandwidth is necessary?
A.
6.5 kHz
B.
7.0 kHz
C.
13.0 kHz
D.
14.0 kHz
Correct Answer:
6.5 kHz
Explanation:
For a single-sideband signal, the bandwidth equals the width of the modulating spectrum, not twice its highest frequency. Hence B = 7 kHz − 0.5 kHz = 6.5 kHz.
7. An AM signal has a carrier frequency of 900 kHz and a bandwidth of 16 kHz. A receiver is tuned to the upper edge of the transmitted spectrum. What frequency does it receive?
A.
892 kHz
B.
900 kHz
C.
908 kHz
D.
916 kHz
Correct Answer:
908 kHz
Explanation:
The AM bandwidth is symmetrically distributed about the carrier. Each sideband extends B/2 = 8 kHz from the carrier, so the upper edge is 900 kHz + 8 kHz = 908 kHz.
8. Two AM stations use carrier frequencies of 1.000 MHz and 1.018 MHz. Each station transmits audio frequencies up to 8 kHz. What is the minimum frequency separation required to prevent overlap of their idealized sidebands, and are these stations sufficiently separated?
A.
8 kHz; yes
B.
16 kHz; yes
C.
16 kHz; no
D.
32 kHz; no
Correct Answer:
16 kHz; yes
Explanation:
Each AM signal occupies a bandwidth of 2 × 8 kHz = 16 kHz, extending 8 kHz on either side of its carrier. Therefore, the carrier separation must be at least 16 kHz. The actual separation is 18 kHz, so overlap does not occur in the ideal case.
9. An FM system has a maximum deviation of 12 kHz. Its modulating frequency varies from 2 kHz to 6 kHz, while the deviation remains fixed. What is the maximum approximate bandwidth according to Carson's rule?
A.
24 kHz
B.
28 kHz
C.
36 kHz
D.
48 kHz
Correct Answer:
36 kHz
Explanation:
The largest modulating frequency determines the maximum bandwidth. Thus B = 2(Δf + f_m(max)) = 2(12 + 6) kHz = 36 kHz.
10. A radio communication system must transmit an AM voice signal containing frequencies up to 4 kHz. To reduce the occupied bandwidth while retaining the same information, the carrier and one complete sideband are suppressed. What percentage reduction in bandwidth is obtained compared with conventional AM?
A.
25%
B.
50%
C.
75%
D.
100%
Correct Answer:
50%
Explanation:
Conventional AM requires bandwidth 2f_m = 8 kHz. Suppressing the carrier and one sideband produces SSB, requiring bandwidth f_m = 4 kHz. The reduction is (8 − 4)/8 × 100% = 50%.
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