Deep Dive: E8B01
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.
Why Other Answers 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.
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 Example
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.
Source & Coverage
Question Pool: 2024-2028 Question Pool
Subelement: E8B
Reference: FCC Part 97.307
Key Concepts
Verified Content
Question from the official FCC Extra Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the E8B topic.