Question
Why should the phase of a PSK signal be changed at the zero crossing of the RF signal?
Answer Options
- A) To minimize bandwidth
- B) To simplify modulation
- C) To improve carrier suppression
- D) All these choices are correct
Correct Answer: A
Explanation
Phase-Shift Keying (\text{PSK}) encodes digital data by shifting the phase of the \text{RF} carrier (e.g., by 180^\circ for a binary shift). If this phase shift is applied instantaneously at the peak of the \text{RF} cycle, the sharp discontinuity in the \text{RF} envelope causes massive sidebands to be generated.
To prevent this wideband splatter, the phase of a \text{PSK} signal should be changed only when the \text{RF} signal crosses zero volts. Changing phase at the zero crossing ensures a smooth transition between symbols, effectively rounding the wave and minimizing the steepness of the transient change. This technique is essential to minimize bandwidth and keep the transmitted signal clean and narrow for efficient spectrum use.
This topic was automatically created to facilitate community discussion about this exam question. Feel free to share study tips, memory tricks, or additional explanations!