*Question G7C05:
Which of the following is characteristic of a direct digital synthesizer (DDS)?*  

*Correct Answer is  D: Variable output frequency with the stability of a crystal oscillator*  

### AI Response: (may contain errors)  

A direct digital synthesizer (DDS) is a type of electronic oscillator that generates waveforms digitally. Let's break down the correct answer and the other choices.

### Correct Answer: 
**D: Variable output frequency with the stability of a crystal oscillator**

- **Explanation**: 
  - DDS allows for precise control of output frequency, which can be changed rapidly and accurately. 
  - The stability of a crystal oscillator is a key feature of DDS, ensuring that the output frequency remains stable over time and temperature variations. 
  - This makes DDS ideal for applications requiring high frequency accuracy and stability.

### Why the Other Choices are Incorrect:

- **A: Extremely narrow tuning range**  
  - **Explanation**: DDS actually offers a wide tuning range, allowing for continuous frequency adjustment across a broad spectrum. It is not limited to a narrow range.

- **B: Relatively high-power output**  
  - **Explanation**: DDS circuits typically produce low power output. They are often used in conjunction with amplifiers to achieve higher power levels, but the DDS itself does not inherently provide high power.

- **C: Pure sine wave output**  
  - **Explanation**: While DDS can generate sine waves, it can also produce other waveforms (like square, triangular, etc.) depending on the digital processing. Therefore, it is not limited to pure sine wave output.

### Summary:
- DDS is characterized by its ability to provide variable output frequency with the stability of a crystal oscillator, making it versatile for various applications in communications and signal generation. 

This understanding will help you recognize the unique features of DDS in the context of the Extra Class exam.