Why should the phase of a PSK signal be changed at the zero crossing of the RF signal?
The correct answer is A: To minimize bandwidth. The phase of a PSK signal should be changed at the zero crossing of the RF signal to minimize bandwidth. Changing phase when the signal amplitude is zero (at the zero crossing) creates the smoothest transition and minimizes spectral spreading.
If phase changes occur when the signal amplitude is non-zero, it creates abrupt transitions that generate wide spectral sidebands. By changing phase at zero crossings, the transition is smoother and the signal occupies less bandwidth. This is important for bandwidth-efficient digital modes. PSK31 and other narrow-band PSK modes use this technique to keep bandwidth minimal while maintaining reliable communication.
Exam Tip
PSK phase change = At zero crossing to minimize bandwidth. Remember: PSK phase should change at the zero crossing of the RF signal to minimize bandwidth. This creates smooth transitions and reduces spectral spreading.
Memory Aid
"**P**SK **Z**ero **C**rossing = **M**inimize **B**andwidth (think 'PZC = MB')"
Real-World Application
You're using PSK31, which changes phase at zero crossings. When the RF signal crosses zero, the phase shifts to the new symbol value. This creates smooth transitions that minimize bandwidth. If phase changed when the signal was at peak amplitude, it would create abrupt transitions and wider bandwidth.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option B: Incorrect. Changing phase at zero crossing doesn't necessarily simplify modulation. The benefit is bandwidth reduction, not simplification.
Option C: Incorrect. Zero-crossing phase changes don't specifically improve carrier suppression. Carrier suppression is achieved through balanced modulation, not timing of phase changes.
Option D: Incorrect. Not all options are correct. Only bandwidth minimization is the reason for zero-crossing phase changes.
题目解析
The correct answer is A: To minimize bandwidth. The phase of a PSK signal should be changed at the zero crossing of the RF signal to minimize bandwidth. Changing phase when the signal amplitude is zero (at the zero crossing) creates the smoothest transition and minimizes spectral spreading. If phase changes occur when the signal amplitude is non-zero, it creates abrupt transitions that generate wide spectral sidebands. By changing phase at zero crossings, the transition is smoother and the signal occupies less bandwidth. This is important for bandwidth-efficient digital modes. PSK31 and other narrow-band PSK modes use this technique to keep bandwidth minimal while maintaining reliable communication.
考试技巧
PSK phase change = At zero crossing to minimize bandwidth. Remember: PSK phase should change at the zero crossing of the RF signal to minimize bandwidth. This creates smooth transitions and reduces spectral spreading.
记忆口诀
**P**SK **Z**ero **C**rossing = **M**inimize **B**andwidth (think 'PZC = MB')
实际应用示例
You're using PSK31, which changes phase at zero crossings. When the RF signal crosses zero, the phase shifts to the new symbol value. This creates smooth transitions that minimize bandwidth. If phase changed when the signal was at peak amplitude, it would create abrupt transitions and wider bandwidth.
错误选项分析
Option B: Incorrect. Changing phase at zero crossing doesn't necessarily simplify modulation. The benefit is bandwidth reduction, not simplification. Option C: Incorrect. Zero-crossing phase changes don't specifically improve carrier suppression. Carrier suppression is achieved through balanced modulation, not timing of phase changes. Option D: Incorrect. Not all options are correct. Only bandwidth minimization is the reason for zero-crossing phase changes.
知识点
PSK, Phase shift keying, Zero crossing, Bandwidth minimization
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.