E8C03: PSK: Why should the phase of a PSK signal be

Q: 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 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.

Updated: Dec 9, 2025 | Source: 2024-2028 Question Pool | Topic: E8C

E8C03E8C

Why should the phase of a PSK signal be changed at the zero crossing of the RF signal?

Previous Next

Deep Dive: E8C03

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.

Why Other Answers 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.

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 Example

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.

Source & Coverage

Question Pool: 2024-2028 Question Pool

Subelement: E8C

Reference: FCC Part 97.3

Key Concepts

PSK Phase shift keying Zero crossing Bandwidth minimization

Verified Content

Question from the official FCC Extra Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the E8C topic.