Frequency Modulation MCQs

Practice Frequency Modulation MCQs with four options, answers and explanations for focused Electronics & Telecommunication Engineering revision.

Question 1

Communication SystemsFrequency Modulation

An FM signal has peak frequency deviation 75 kHz and maximum modulating frequency 15 kHz. Its modulation index beta is?

A0.2
B5
C15
D90

Show AnswerHide Answer Report an error

Correct Answer: B. 5

Explanation: For FM, beta=Delta f/fm=75/15=5.

Reference: Standard communication systems: frequency modulation

Question 2

Communication SystemsFrequency Modulation

Using Carson’s rule, the approximate bandwidth of an FM signal with peak deviation 75 kHz and maximum modulating frequency 15 kHz is?

A90 kHz
B150 kHz
C180 kHz
D300 kHz

Show AnswerHide Answer Report an error

Correct Answer: C. 180 kHz

Explanation: Carson's rule gives B≈2(Delta f+fm)=2(75+15)=180 kHz.

Reference: Standard communication systems: frequency modulation

Question 3

Communication SystemsFrequency Modulation

A major advantage of FM over conventional AM in many analog radio applications is better?

ANoise immunity
BDC power conversion
CCarrier suppression
DAntenna elimination

Show AnswerHide Answer Report an error

Correct Answer: A. Noise immunity

Explanation: Because information is carried in frequency rather than amplitude, FM can provide improved rejection of amplitude noise and better noise performance under suitable conditions.

Reference: Standard communication systems: frequency modulation

Question 4

Communication SystemsFrequency Modulation

Pre-emphasis and de-emphasis in FM systems are used primarily to improve?

AHigh-frequency noise performance
BDC supply regulation
CAntenna mechanical strength
DCarrier power efficiency to 100%

Show AnswerHide Answer Report an error

Correct Answer: A. High-frequency noise performance

Explanation: Pre-emphasis boosts higher message frequencies before transmission and de-emphasis restores the response at the receiver while reducing high-frequency noise.

Reference: Standard communication systems: frequency modulation

Question 5

Communication SystemsFrequency Modulation

For an FM signal with frequency deviation 50 kHz and modulating frequency 10 kHz, the modulation index is?

A0.2
B5
C10
D50

Show AnswerHide Answer Report an error

Correct Answer: B. 5

Explanation: beta=Delta f/fm=50/10=5.

Reference: Standard communication systems: frequency modulation

Question 6

Communication SystemsFrequency Modulation

Using Carson’s rule, an FM signal with Delta f=50 kHz and fm(max)=10 kHz occupies approximately?

A60 kHz
B100 kHz
C120 kHz
D500 kHz

Show AnswerHide Answer Report an error

Correct Answer: C. 120 kHz

Explanation: B≈2(Delta f+fm)=2(50+10)=120 kHz.

Reference: Standard communication systems: frequency modulation

Question 7

Communication SystemsFrequency Modulation

An FM signal has beta=2 and fm=5 kHz. Its peak frequency deviation is?

A2.5 kHz
B5 kHz
C10 kHz
D20 kHz

Show AnswerHide Answer Report an error

Correct Answer: C. 10 kHz

Explanation: Delta f=beta fm=2×5 kHz=10 kHz.

Reference: Standard communication systems: frequency modulation

Question 8

Communication SystemsFrequency Modulation

For an FM wave, if frequency deviation is doubled while modulating frequency remains unchanged, the FM modulation index becomes?

AHalf
BUnchanged
CDouble
DFour times

Show AnswerHide Answer Report an error

Correct Answer: C. Double

Explanation: FM modulation index beta=Delta f/fm. Doubling Delta f with fm fixed doubles beta.

Reference: Standard communication systems: FM modulation index

Question 9

Communication SystemsFrequency Modulation

An FM signal has frequency deviation 20 kHz and modulating frequency 10 kHz. Using Carson’s rule, its approximate bandwidth is?

A20 kHz
B40 kHz
C60 kHz
D80 kHz

Show AnswerHide Answer Report an error

Correct Answer: C. 60 kHz

Explanation: Carson's rule gives B≈2(Delta f+fm)=2(20+10)=60 kHz.

Reference: Standard communication systems: Carson rule numerical

Question 10

Communication SystemsFrequency Modulation

For narrowband FM, the modulation index beta is generally?

AMuch less than 1
BExactly 1 in every case
CMuch greater than 1
DInfinite

Show AnswerHide Answer Report an error

Correct Answer: A. Much less than 1

Explanation: Narrowband FM is characterized by a small modulation index, beta<<1, so higher-order sidebands are negligible.

Reference: Standard communication systems: narrowband FM