GROL Modulation and Signal Types 3

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?

Accepted Answer

2.0

Answer

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?

Accepted Answer

180kHz

Answer

Carson's rule: BW ≈ 2(Δf + fm) = 2(75 + 15) = 180kHz.

Question 3

What is the purpose of pre-emphasis in FM broadcasting?

Accepted Answer

To boost high audio frequencies before transmission to improve signal-to-noise ratio

Answer

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?

Accepted Answer

An FM receiver locks onto the stronger of two co-channel signals and suppresses the weaker one

Answer

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?

Accepted Answer

5.0

Answer

Deviation ratio = maximum deviation / maximum audio frequency = 75kHz / 15kHz = 5.0 for standard FM broadcasting.

(GROL) General Radiotelephone Operator License Practice Test