Deep Dive: E8B04
The correct answer is B: 3. The modulation index of an FM phone signal with maximum carrier deviation of plus or minus 6 kHz and highest modulating frequency of 2 kHz is 3. Modulation index = frequency deviation / modulating frequency. Calculation: Peak deviation = 6 kHz, modulating frequency = 2 kHz. Modulation index = 6 kHz / 2 kHz = 3. The 'plus or minus 6 kHz' means the peak deviation is 6 kHz in either direction from the carrier. The modulation index of 3 means the carrier frequency varies 3 times the modulating frequency, which determines the bandwidth and sideband structure of the FM signal.
Why Other Answers Are Wrong
Option A: Incorrect. 0.3 would be 2 / 6, which is the inverse of the correct calculation. Modulation index = deviation / modulating frequency. Option C: Incorrect. 0.6 would be 2 / 3.33 or some other incorrect calculation. The correct value is 6 / 2 = 3. Option D: Incorrect. 6 would be the deviation value itself, not the modulation index. The index is deviation divided by modulating frequency.
Exam Tip
FM modulation index = Deviation / Modulating frequency. Remember: m = Δf / fm. With ±6 kHz deviation and 2 kHz modulating frequency, m = 6 / 2 = 3.
Memory Aid
**M**odulation **I**ndex = **6** kHz / **2** kHz = **3** (think 'MI = 6/2 = 3')
Real-World Example
You're transmitting FM with ±6 kHz peak deviation and the highest audio frequency is 2 kHz. The modulation index is 6 kHz / 2 kHz = 3. This modulation index determines how many significant sidebands your FM signal will have and what bandwidth it occupies.
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.