*Question E7H09:
What type of frequency synthesizer circuit uses a phase accumulator, lookup table, digital-to-analog converter, and a low-pass anti-alias filter?*  

*Correct Answer is  A: A direct digital synthesizer*  

### AI Response: (may contain errors)  

To understand why the correct answer is a direct digital synthesizer (DDS), let's break down the components mentioned in the question:

1. **Correct Answer: A direct digital synthesizer (DDS)**
   - A DDS uses a **phase accumulator** to generate a phase value that corresponds to a desired frequency. 
   - The **lookup table** is used to convert the phase value into a digital representation of the waveform (like sine or square waves).
   - The **digital-to-analog converter (DAC)** then converts this digital waveform into an analog signal.
   - Finally, the **low-pass anti-alias filter** smooths the output signal, removing high-frequency components that could distort the desired waveform.
   - This combination allows for precise frequency generation and fine-tuning.

2. **Why the other choices are incorrect:**
   - **B: A hybrid synthesizer**
     - Hybrid synthesizers typically combine analog and digital techniques but do not specifically use a phase accumulator and lookup table in the same way as a DDS.
   - **C: A phase-locked loop synthesizer (PLL)**
     - A PLL uses feedback to lock the output frequency to a reference frequency. It does not primarily rely on a phase accumulator or lookup table for waveform generation.
   - **D: A direct conversion synthesizer**
     - Direct conversion synthesizers convert RF signals directly to baseband without intermediate frequency stages. They do not utilize the components listed in the question.

3. **Useful concepts:**
   - **Phase Accumulator**: A digital counter that accumulates phase information, which is crucial for generating precise frequencies.
   - **Lookup Table**: A precomputed table that allows for quick access to waveform values based on the phase input.
   - **DAC**: Converts digital signals to analog, essential for producing the final output waveform.
   - **Low-pass Filter**: Eliminates unwanted high-frequency noise, ensuring a clean output signal.

In summary, the direct digital synthesizer is characterized by its use of a phase accumulator, lookup table, DAC, and low-pass filter, making it the correct answer for this question.