Modulation and Signal Types Flashcards

An FM transmitter has a maximum frequency deviation of 6kHz and a maximum audio frequency of 3kHz. What is the FM modulation index?

Answer: 2.0

FM modulation index = Δf / fm = 6000 / 3000 = 2.0.

Using Carson's rule, what is the approximate bandwidth of an FM broadcast signal with 75kHz maximum deviation and 15kHz maximum audio frequency?

Answer: 180kHz

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

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

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.

What is the 'capture effect' unique to FM receivers?

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.

What is the deviation ratio of a standard FM broadcast station?

Answer: 5.0

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

How does the FM modulation index compare to the PM modulation index as audio frequency increases with constant audio amplitude?

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.

What is the standard maximum frequency deviation for narrowband FM (NBFM) used in VHF/UHF land mobile radio?

Answer: 5kHz

NBFM uses a maximum deviation of ±5kHz to fit within the 15kHz channel spacing assigned to land mobile radio systems.