E7H09: Direct digital synthesizer: What type of frequency synthesizer

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

The correct answer is A: A direct digital synthesizer. The correct answer is A: A direct digital synthesizer. A direct digital synthesizer (DDS) uses a phase accumulator, lookup table, digital-to-analog converter (DAC), and a low-pass anti-alias filter. This is the architecture of a DDS system. In a DDS, the phase accumulator generates a digital phase value that increments at each clock cycle. This phase value is used to address a lookup table (which contains amplitude values representing a sine wave). The lookup table outputs digital amplitude values that are converted to analog by the DAC. The low-pass filter removes aliasing artifacts and smooths the output. DDS systems provide precise frequency control and are commonly used in modern radio equipment for frequency synthesis. They can generate clean sine waves with excellent frequency resolution.

Updated: Dec 9, 2025 | Source: 2024-2028 Question Pool | Topic: E7H

E7H09E7H

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

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Deep Dive: E7H09

The correct answer is A: A direct digital synthesizer. A direct digital synthesizer (DDS) uses a phase accumulator, lookup table, digital-to-analog converter (DAC), and a low-pass anti-alias filter. This is the architecture of a DDS system. In a DDS, the phase accumulator generates a digital phase value that increments at each clock cycle. This phase value is used to address a lookup table (which contains amplitude values representing a sine wave). The lookup table outputs digital amplitude values that are converted to analog by the DAC. The low-pass filter removes aliasing artifacts and smooths the output. DDS systems provide precise frequency control and are commonly used in modern radio equipment for frequency synthesis. They can generate clean sine waves with excellent frequency resolution.

Why Other Answers Are Wrong

Option B: Incorrect. A hybrid synthesizer might combine different techniques, but the specific components listed (phase accumulator, lookup table, DAC, filter) are characteristic of a DDS, not a hybrid. Option C: Incorrect. A phase-locked loop synthesizer uses a phase detector, VCO, and reference oscillator, not a phase accumulator and lookup table. Option D: Incorrect. A direct conversion synthesizer is not a standard term. The components described are specific to DDS architecture.

Exam Tip

Phase accumulator + lookup table + DAC = DDS. Remember: A direct digital synthesizer uses a phase accumulator, lookup table, digital-to-analog converter, and low-pass filter.

Memory Aid

**D**DS = **D**igital **P**hase **A**ccumulator, **L**ookup, **D**AC, **F**ilter (think 'DDS = DPLDF')

Real-World Example

Your modern transceiver uses a DDS for frequency generation. The phase accumulator creates a digital phase ramp, the lookup table converts phase to amplitude values, the DAC converts these to analog, and the filter removes aliasing. This provides precise, stable frequency generation that can be controlled digitally with excellent resolution.

Source & Coverage

Question Pool: 2024-2028 Question Pool

Subelement: E7H

Reference: FCC Part 97.3

Key Concepts

Direct digital synthesizer DDS Phase accumulator Lookup table DAC

Verified Content

Question from the official FCC Extra Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the E7H topic.