What is the deviation ratio of an FM phone signal having a maximum frequency swing of plus or minus 7.5 kHz if the highest modulation frequency is 3.5 kHz?
The correct answer is A: 2.14. The deviation ratio of an FM phone signal with maximum frequency swing of ±7.5 kHz and highest modulation frequency of 3.5 kHz is 2.14. Deviation ratio = maximum frequency deviation / highest modulating frequency.
Calculation: Deviation ratio = 7.5 kHz / 3.5 kHz = 2.14. The deviation ratio tells you how many times the maximum deviation exceeds the highest modulating frequency. A deviation ratio of 2.14 means the carrier frequency can deviate up to 2.14 times the highest audio frequency, which affects the bandwidth and sideband structure of the FM signal.
Exam Tip
Deviation ratio = Deviation / Highest modulating frequency. Remember: Deviation ratio = maximum frequency deviation divided by highest modulating frequency. With ±7.5 kHz deviation and 3.5 kHz modulating frequency, ratio = 7.5 / 3.5 = 2.14.
Memory Aid
"**D**eviation **R**atio = **7**.**5** / **3**.**5** = **2**.**14** (think 'DR = 7.5/3.5 = 2.14')"
Real-World Application
You're transmitting FM with ±7.5 kHz peak deviation and the highest audio frequency is 3.5 kHz. The deviation ratio is 7.5 kHz / 3.5 kHz = 2.14. This means the maximum frequency deviation is 2.14 times the highest modulating frequency.
FCC Part 97.307Key Concepts
Why Other Options Are Wrong
Option B: Incorrect. 0.214 would be 3.5 / 7.5, which is the inverse of the correct calculation. Deviation ratio = deviation / modulating frequency.
Option C: Incorrect. 0.47 would be approximately 3.5 / 7.5, still the inverse. The correct value is 7.5 / 3.5 = 2.14.
Option D: Incorrect. 47 is way too large and doesn't match any reasonable calculation.
题目解析
The correct answer is A: 2.14. The deviation ratio of an FM phone signal with maximum frequency swing of ±7.5 kHz and highest modulation frequency of 3.5 kHz is 2.14. Deviation ratio = maximum frequency deviation / highest modulating frequency. Calculation: Deviation ratio = 7.5 kHz / 3.5 kHz = 2.14. The deviation ratio tells you how many times the maximum deviation exceeds the highest modulating frequency. A deviation ratio of 2.14 means the carrier frequency can deviate up to 2.14 times the highest audio frequency, which affects the bandwidth and sideband structure of the FM signal.
考试技巧
Deviation ratio = Deviation / Highest modulating frequency. Remember: Deviation ratio = maximum frequency deviation divided by highest modulating frequency. With ±7.5 kHz deviation and 3.5 kHz modulating frequency, ratio = 7.5 / 3.5 = 2.14.
记忆口诀
**D**eviation **R**atio = **7**.**5** / **3**.**5** = **2**.**14** (think 'DR = 7.5/3.5 = 2.14')
实际应用示例
You're transmitting FM with ±7.5 kHz peak deviation and the highest audio frequency is 3.5 kHz. The deviation ratio is 7.5 kHz / 3.5 kHz = 2.14. This means the maximum frequency deviation is 2.14 times the highest modulating frequency.
错误选项分析
Option B: Incorrect. 0.214 would be 3.5 / 7.5, which is the inverse of the correct calculation. Deviation ratio = deviation / modulating frequency. Option C: Incorrect. 0.47 would be approximately 3.5 / 7.5, still the inverse. The correct value is 7.5 / 3.5 = 2.14. Option D: Incorrect. 47 is way too large and doesn't match any reasonable calculation.
知识点
Deviation ratio, FM calculation, Frequency deviation, Modulating frequency
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.