*Question E4E12:
What causes interference received as a series of carriers at regular intervals across a wide frequency range?*  

*Correct Answer is  A: Switch-mode power supplies*  

### AI Response: (may contain errors)  

The correct answer to the question about interference received as a series of carriers at regular intervals across a wide frequency range is:

**A: Switch-mode power supplies**

### Explanation:

1. **Why the correct answer is correct:**
   - Switch-mode power supplies (SMPS) operate by rapidly switching the input voltage on and off to control the output voltage. This switching creates harmonics and can generate a series of carrier frequencies that can interfere with radio signals. The regular intervals of interference are due to the high-frequency switching cycles, which can produce noise across a wide frequency range.

2. **Why the other choices are incorrect:**
   - **B: Radar transmitters**: While radar systems can cause interference, they typically emit strong, pulsed signals at specific frequencies rather than a series of carriers across a wide range. The interference is more likely to be a single frequency or a narrow band rather than a series of carriers.
   - **C: Wireless security camera transmitters**: These devices usually operate on specific frequencies (like 2.4 GHz or 5.8 GHz) and do not generate a wide range of carrier frequencies. Their interference is more localized and not characterized by regular intervals across a broad spectrum.
   - **D: Electric fences**: Electric fences can create interference, but it is usually at a lower frequency and not characterized by a series of carriers. The interference is often more sporadic and not as regular as that produced by switch-mode power supplies.

### Useful Concepts:
- **Harmonics**: When a device like an SMPS operates, it generates harmonics of the fundamental frequency due to its switching action. These harmonics can spread across a wide frequency range.
- **Interference Patterns**: Regular intervals of interference suggest a periodic source, which aligns with the operation of switch-mode power supplies.

Understanding these concepts will help you recognize the sources of interference and their characteristics, which is crucial for the Extra Class exam.