What is the modulation index of an FM signal?
The correct answer is A: The ratio of frequency deviation to modulating signal frequency. The modulation index of an FM signal is the ratio of frequency deviation to modulating signal frequency. Modulation index (m) = Δf / fm, where Δf is the peak frequency deviation and fm is the modulating frequency.
The modulation index determines how much the carrier frequency varies relative to the modulating signal. A higher modulation index means more frequency deviation for a given modulating frequency. The modulation index affects the bandwidth of the FM signal and the number of significant sidebands. For example, with a modulation index of 5, the FM signal will have significant sidebands extending well beyond the modulating frequency. Modulation index is a key parameter in FM system design.
Exam Tip
FM modulation index = Deviation / Modulating frequency. Remember: Modulation index = frequency deviation divided by modulating signal frequency. m = Δf / fm.
Memory Aid
"**M**odulation **I**ndex = **D**eviation / **M**odulating **F**requency (think 'MI = D/MF')"
Real-World Application
You're transmitting FM with 5 kHz peak deviation and a 1 kHz modulating tone. The modulation index is 5 kHz / 1 kHz = 5. This tells you the carrier frequency varies 5 times the modulating frequency. A higher modulation index means more sidebands and wider bandwidth.
FCC Part 97.307Key Concepts
Why Other Options Are Wrong
Option B: Incorrect. This describes the inverse relationship. Modulation index is deviation divided by modulating frequency, not amplitude divided by deviation.
Option C: Incorrect. This describes bandwidth divided by modulating frequency, not the modulation index. Modulation index is about deviation, not bandwidth.
Option D: Incorrect. This describes bandwidth divided by modulating frequency, not the modulation index definition.
题目解析
The correct answer is A: The ratio of frequency deviation to modulating signal frequency. The modulation index of an FM signal is the ratio of frequency deviation to modulating signal frequency. Modulation index (m) = Δf / fm, where Δf is the peak frequency deviation and fm is the modulating frequency. The modulation index determines how much the carrier frequency varies relative to the modulating signal. A higher modulation index means more frequency deviation for a given modulating frequency. The modulation index affects the bandwidth of the FM signal and the number of significant sidebands. For example, with a modulation index of 5, the FM signal will have significant sidebands extending well beyond the modulating frequency. Modulation index is a key parameter in FM system design.
考试技巧
FM modulation index = Deviation / Modulating frequency. Remember: Modulation index = frequency deviation divided by modulating signal frequency. m = Δf / fm.
记忆口诀
**M**odulation **I**ndex = **D**eviation / **M**odulating **F**requency (think 'MI = D/MF')
实际应用示例
You're transmitting FM with 5 kHz peak deviation and a 1 kHz modulating tone. The modulation index is 5 kHz / 1 kHz = 5. This tells you the carrier frequency varies 5 times the modulating frequency. A higher modulation index means more sidebands and wider bandwidth.
错误选项分析
Option B: Incorrect. This describes the inverse relationship. Modulation index is deviation divided by modulating frequency, not amplitude divided by deviation. Option C: Incorrect. This describes bandwidth divided by modulating frequency, not the modulation index. Modulation index is about deviation, not bandwidth. Option D: Incorrect. This describes bandwidth divided by modulating frequency, not the modulation index definition.
知识点
Modulation index, FM modulation, Frequency deviation, Modulating frequency
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.