*Question E8C03:
Why should the phase of a PSK signal be changed at the zero crossing of the RF signal?*  

*Correct Answer is  A: To minimize bandwidth*  

### AI Response: (may contain errors)  

To understand why the phase of a PSK (Phase Shift Keying) signal should be changed at the zero crossing of the RF signal, let's break down the reasoning:

### Correct Answer: A - To minimize bandwidth

- **Minimizing Bandwidth**: Changing the phase at the zero crossing helps to ensure that the signal transitions occur at points where the amplitude of the RF signal is at its minimum (zero). This reduces the likelihood of spectral spreading, which can occur if phase changes happen when the signal is at a peak amplitude. By minimizing the bandwidth, the signal can be transmitted more efficiently and with less interference.

### Why the Other Choices are Incorrect:

- **B: To simplify modulation**: 
  - While changing the phase at zero crossings can help in maintaining signal integrity, it does not necessarily simplify the modulation process itself. The complexity of modulation schemes is determined by the number of phase states and the encoding method, not just the timing of phase changes.

- **C: To improve carrier suppression**: 
  - Carrier suppression refers to the ability to minimize the carrier signal's presence in the transmitted signal. Changing the phase at zero crossings does not directly improve carrier suppression; rather, it focuses on maintaining the integrity of the modulated signal. Carrier suppression is more related to the modulation technique and filtering methods used.

- **D: All these choices are correct**: 
  - Since only choice A is correct, this option is also incorrect. 

### Summary

In summary, the primary reason for changing the phase of a PSK signal at the zero crossing is to minimize bandwidth, which enhances the efficiency of the transmission. The other options do not accurately reflect the benefits of this timing choice in PSK modulation.

### Useful Rule of Thumb

- **Phase Shift Timing**: Always aim to change the phase of a PSK signal at points of minimal amplitude (zero crossings) to maintain signal integrity and minimize bandwidth.