Question
What is the deviation ratio of an FM phone signal having a maximum frequency swing of plus or minus 5 kHz if the highest modulation frequency is 3 kHz?
Answer Options
- A) 6
- B) 0.167
- C) 0.6
- D) 1.67
Correct Answer: D
Explanation
The deviation ratio is a specific term for the modulation index when applied to a system where the maximum frequency deviation and the maximum modulating frequency are set by system design standards (not the input signal characteristics). For the purpose of amateur radio licensing, the calculation is identical to the modulation index calculation.
Given the maximum frequency deviation is 5 kHz and the highest modulation frequency is 3 kHz, the deviation ratio is calculated as 5 kHz / 3 kHz \approx \mathbf{1.67}. This value is typical for NBFM (narrowband FM) communication systems that are designed to limit the FM signal bandwidth to a narrow channel.
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