*Question E7H11:
What are the major spectral impurity components of direct digital synthesizers?*  

*Correct Answer is  C: Spurious signals at discrete frequencies*  

### AI Response: (may contain errors)  

Direct digital synthesizers (DDS) are used to generate precise frequencies by digitally creating a waveform and then converting it to an analog signal. Understanding the spectral impurities of DDS is important for ensuring signal quality.

### Correct Answer: C: Spurious signals at discrete frequencies

1. **Why the correct answer is correct:**
   - **Spurious signals at discrete frequencies** are unwanted signals that appear at specific frequencies in the output of a DDS. These are typically caused by imperfections in the digital-to-analog conversion process and the finite resolution of the digital components. They manifest as distinct, non-harmonic frequencies that can interfere with the desired signal.

### Why the other choices are incorrect:

2. **A: Broadband noise**
   - **Broadband noise** refers to noise that is spread across a wide range of frequencies. While DDS systems can have some broadband noise due to the digital components, it is not the major spectral impurity. The primary concern with DDS is the discrete spurious signals rather than a continuous noise spectrum.

3. **B: Digital conversion noise**
   - **Digital conversion noise** is a general term that could refer to various types of noise introduced during the digital-to-analog conversion process. However, the specific issue with DDS is the presence of spurious signals at discrete frequencies, not just general conversion noise.

4. **D: Harmonics of the local oscillator**
   - **Harmonics of the local oscillator** are multiples of the fundamental frequency of an oscillator. In DDS, the concern is not harmonics of a local oscillator but rather the spurious signals that arise from the digital synthesis process itself.

### Additional Notes:

- **Spurious-Free Dynamic Range (SFDR):** This is a key performance metric for DDS, representing the ratio between the amplitude of the desired signal and the largest spurious signal. A higher SFDR indicates better performance with fewer spurious signals.

Understanding these concepts helps in identifying and mitigating issues related to spectral purity in DDS systems, which is crucial for maintaining signal integrity in communication systems.