*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)  

A direct digital synthesizer (DDS) is the correct answer because it uses the following components:

- **Phase Accumulator**: This component generates a digital phase signal by continuously adding a fixed value (the frequency control word) to its current value. This process creates a digital ramp that represents the phase of the output signal.
  
- **Lookup Table**: The phase accumulator's output is used to address a lookup table that contains the digital values of a sine wave. This converts the phase information into amplitude information.

- **Digital-to-Analog Converter (DAC)**: The digital sine wave values from the lookup table are converted into an analog signal by the DAC.

- **Low-Pass Anti-Alias Filter**: This filter smooths the output of the DAC to remove high-frequency components and produce a clean analog sine wave.

### Why the Correct Answer is Correct:
- **Direct Digital Synthesizer (DDS)**: This type of synthesizer uses all the components mentioned in the question: a phase accumulator, lookup table, DAC, and low-pass filter. It is known for its ability to generate precise and stable frequencies.

### Why the Other Choices are Incorrect:
- **Hybrid Synthesizer**: This is a general term that can refer to a combination of different synthesizer technologies, but it does not specifically use the components listed in the question.

- **Phase-Locked Loop (PLL) Synthesizer**: A PLL synthesizer uses a phase detector, voltage-controlled oscillator (VCO), and a loop filter. It does not use a phase accumulator or lookup table.

- **Direct Conversion Synthesizer**: This type of synthesizer directly converts a signal from one frequency to another without the use of a phase accumulator or lookup table.

### Key Points:
- A DDS is known for its high frequency resolution and fast switching capabilities.
- The phase accumulator and lookup table are unique to DDS, distinguishing it from other synthesizer types.