GROL Modulation and Signal Types 3 — Questions and Answers
Question 1: An FM transmitter has a maximum frequency deviation of 6kHz and a maximum audio frequency of 3kHz. What is the FM modulation index?
- 0.5
- 2.0 (Correct answer)
- 18
- 9kHz
Correct answer: 2.0
FM modulation index = Δf / fm = 6000 / 3000 = 2.0.
Question 2: Using Carson's rule, what is the approximate bandwidth of an FM broadcast signal with 75kHz maximum deviation and 15kHz maximum audio frequency?
- 90kHz
- 150kHz
- 180kHz (Correct answer)
- 300kHz
Correct answer: 180kHz
Carson's rule: BW ≈ 2(Δf + fm) = 2(75 + 15) = 180kHz.
Question 3: What is the purpose of pre-emphasis in FM broadcasting?
- To increase total carrier power
- To boost high audio frequencies before transmission to improve signal-to-noise ratio (Correct answer)
- To reduce frequency deviation on strong signals
- To filter out adjacent-channel interference
Correct answer: To boost high audio frequencies before transmission to improve signal-to-noise ratio
Pre-emphasis amplifies high-frequency audio components before transmission so they overcome FM noise, which naturally increases with frequency.
Question 4: What is the 'capture effect' unique to FM receivers?
- An FM receiver locks onto the stronger of two co-channel signals and suppresses the weaker one (Correct answer)
- The ability of FM to receive signals below the noise floor
- A squelch action that mutes the receiver between transmissions
- Bandwidth narrowing that occurs with very strong FM signals
Correct answer: An FM receiver locks onto the stronger of two co-channel signals and suppresses the weaker one
The capture effect causes an FM receiver to demodulate only the dominant signal when two signals share the same frequency, rejecting the weaker one.
Question 5: What is the deviation ratio of a standard FM broadcast station?
- 2.5
- 5.0 (Correct answer)
- 75
- 200
Correct answer: 5.0
Deviation ratio = maximum deviation / maximum audio frequency = 75kHz / 15kHz = 5.0 for standard FM broadcasting.
Question 6: How does the FM modulation index compare to the PM modulation index as audio frequency increases with constant audio amplitude?
- FM modulation index decreases; PM modulation index remains constant (Correct answer)
- FM modulation index increases; PM modulation index remains constant
- Both increase proportionally with audio frequency
- Both remain constant regardless of audio frequency
Correct answer: FM modulation index decreases; PM modulation index remains constant
FM modulation index = Δf/fm, so it decreases as fm rises; PM phase deviation is proportional only to signal amplitude and is independent of modulating frequency.
Question 7: What is the standard maximum frequency deviation for narrowband FM (NBFM) used in VHF/UHF land mobile radio?
- 75kHz
- 15kHz
- 5kHz (Correct answer)
- 25kHz
Correct answer: 5kHz
NBFM uses a maximum deviation of ±5kHz to fit within the 15kHz channel spacing assigned to land mobile radio systems.
GROL Modulation and Signal Types 3
An FM transmitter has a maximum frequency deviation of 6kHz and a maximum audio frequency of 3kHz.
What is the FM modulation index?