*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 frequency synthesizer used in radio and signal processing applications. Let's break down why the correct answer is correct and why the other choices are incorrect:

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

   - **Explanation**: A DDS generates a variable output frequency by using a fixed-frequency reference clock, typically derived from a crystal oscillator. This means the output frequency can be changed easily while maintaining the high stability and accuracy of the crystal oscillator. This is a key characteristic of DDS technology, making it highly desirable for applications requiring precise frequency control.

2. **Incorrect Choices**:

   - **A: Extremely narrow tuning range**
     - **Explanation**: DDS systems are known for their wide tuning range, not a narrow one. They can generate a broad range of frequencies by adjusting the digital input values, which is contrary to this choice.

   - **B: Relatively high-power output**
     - **Explanation**: DDS devices typically produce low-power signals that often require amplification for practical use. They are not characterized by high-power output, making this choice incorrect.

   - **C: Pure sine wave output**
     - **Explanation**: While DDS can produce sine waves, they are not inherently "pure." The output may contain spurious signals or harmonics due to the digital-to-analog conversion process. Additional filtering is often needed to achieve a pure sine wave, so this choice is misleading.

In summary, the defining feature of a DDS is its ability to provide a variable output frequency with the stability of a crystal oscillator, making option D the correct choice.