Deep Dive: E8B03
The correct answer is A: 3. The modulation index of an FM phone signal with maximum frequency deviation of 3000 Hz and highest modulating frequency of 1000 Hz is 3. Modulation index = frequency deviation / modulating frequency. Calculation: Modulation index = 3000 Hz / 1000 Hz = 3. The modulation index tells you how many times the carrier frequency deviates relative to the modulating frequency. A modulation index of 3 means the carrier frequency varies 3 times the modulating frequency. This modulation index will determine the bandwidth and number of significant sidebands in the FM signal.
Why Other Answers Are Wrong
Option B: Incorrect. 0.3 would be 1000 / 3000, which is the inverse of the correct calculation. Modulation index = deviation / modulating frequency. Option C: Incorrect. 6 would be 2 × 3, suggesting a calculation error. The correct value is 3000 / 1000 = 3. Option D: Incorrect. 0.6 doesn't match any reasonable calculation with these values.
Exam Tip
FM modulation index = Deviation / Modulating frequency. Remember: m = Δf / fm. With 3000 Hz deviation and 1000 Hz modulating frequency, m = 3000 / 1000 = 3.
Memory Aid
**M**odulation **I**ndex = **3**000 / **1**000 = **3** (think 'MI = 3000/1000 = 3')
Real-World Example
You're transmitting FM with 3000 Hz peak deviation and the highest audio frequency is 1000 Hz. The modulation index is 3000 / 1000 = 3. This means the carrier frequency varies 3 times the modulating frequency, which will create significant sidebands and determine your signal 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.