*Question E8D09:
What is considered an acceptable maximum IMD level for an idling PSK signal?*  

*Correct Answer is  D: -30 dB*  

### AI Response: (may contain errors)  

To determine the acceptable maximum Intermodulation Distortion (IMD) level for an idling Phase Shift Keying (PSK) signal, we need to understand what IMD represents and its implications for signal quality.

### Correct Answer: -30 dB

1. **Why the correct answer is correct:**
   - An IMD level of -30 dB indicates that the distortion products are 30 dB below the level of the fundamental signal. This is considered acceptable because it means that the signal remains clean and the distortion is low enough not to interfere with the quality of the transmission. In digital communications, maintaining a low IMD is crucial for ensuring reliable data transmission.

2. **Why the other choices are incorrect:**
   - **A: +5 dB**: This level indicates that the distortion is only 5 dB above the fundamental signal, which is too high. It suggests significant distortion that could degrade the signal quality and lead to errors in data transmission.
   - **B: +10 dB**: Similar to +5 dB, an IMD level of +10 dB is also too high. It indicates that the distortion is becoming more pronounced, which can negatively impact the performance of the PSK signal.
   - **C: +15 dB**: This level is even worse, with distortion 15 dB above the fundamental signal. Such a high level of distortion would likely result in poor signal integrity and increased error rates in data communication.

### Useful Rules of Thumb:
- A lower IMD level is always preferable in communication systems, especially for digital modes like PSK.
- Generally, an IMD level of -30 dB or lower is a good target for maintaining signal quality in digital communications.

In summary, an IMD level of -30 dB is acceptable for an idling PSK signal because it ensures minimal distortion, while the other options indicate levels of distortion that could compromise the integrity of the signal.