How does the modulation index of a phase-modulated emission vary with RF carrier frequency?
The correct answer is D: It does not depend on the RF carrier frequency. The modulation index of a phase-modulated emission does not depend on the RF carrier frequency. The modulation index is determined by the phase deviation and modulating frequency, not the carrier frequency.
In phase modulation (PM), the modulation index is the peak phase deviation in radians. This is determined by how much the phase changes in response to the modulating signal, which is independent of the carrier frequency. Whether you're phase modulating at 1 MHz or 100 MHz, if the phase deviation and modulating frequency are the same, the modulation index is the same. This is different from some other modulation parameters that do vary with carrier frequency.
Exam Tip
PM modulation index = Independent of carrier frequency. Remember: Phase modulation index does not depend on RF carrier frequency. It's determined by phase deviation and modulating frequency only.
Memory Aid
"**P**M **M**odulation **I**ndex = **I**ndependent of **C**arrier (think 'PMI = IC')"
Real-World Application
You're phase modulating a signal. Whether the carrier is at 14 MHz or 28 MHz, if you apply the same modulating signal and get the same phase deviation, the modulation index is the same. The carrier frequency doesn't affect the modulation index - only the phase deviation and modulating frequency matter.
FCC Part 97.307Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. PM modulation index doesn't increase with carrier frequency. It's independent of carrier frequency.
Option B: Incorrect. PM modulation index doesn't decrease with carrier frequency. It's independent of carrier frequency.
Option C: Incorrect. PM modulation index doesn't vary with the square root of carrier frequency. It's completely independent of carrier frequency.
题目解析
The correct answer is D: It does not depend on the RF carrier frequency. The modulation index of a phase-modulated emission does not depend on the RF carrier frequency. The modulation index is determined by the phase deviation and modulating frequency, not the carrier frequency. In phase modulation (PM), the modulation index is the peak phase deviation in radians. This is determined by how much the phase changes in response to the modulating signal, which is independent of the carrier frequency. Whether you're phase modulating at 1 MHz or 100 MHz, if the phase deviation and modulating frequency are the same, the modulation index is the same. This is different from some other modulation parameters that do vary with carrier frequency.
考试技巧
PM modulation index = Independent of carrier frequency. Remember: Phase modulation index does not depend on RF carrier frequency. It's determined by phase deviation and modulating frequency only.
记忆口诀
**P**M **M**odulation **I**ndex = **I**ndependent of **C**arrier (think 'PMI = IC')
实际应用示例
You're phase modulating a signal. Whether the carrier is at 14 MHz or 28 MHz, if you apply the same modulating signal and get the same phase deviation, the modulation index is the same. The carrier frequency doesn't affect the modulation index - only the phase deviation and modulating frequency matter.
错误选项分析
Option A: Incorrect. PM modulation index doesn't increase with carrier frequency. It's independent of carrier frequency. Option B: Incorrect. PM modulation index doesn't decrease with carrier frequency. It's independent of carrier frequency. Option C: Incorrect. PM modulation index doesn't vary with the square root of carrier frequency. It's completely independent of carrier frequency.
知识点
Phase modulation, Modulation index, Carrier frequency, Phase deviation
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.