What information is contained in the lookup table of a direct digital synthesizer (DDS)?
The correct answer is B: Amplitude values that represent the desired waveform. The lookup table in a direct digital synthesizer (DDS) contains amplitude values that represent the desired waveform (typically a sine wave). These amplitude values are stored in memory and addressed by the phase accumulator output.
The phase accumulator generates phase values (0 to 2π or 0 to full scale). These phase values are used as addresses to look up corresponding amplitude values in the table. For a sine wave, the lookup table contains the sine function values - when phase is 0, amplitude is 0; when phase is π/2, amplitude is maximum; etc. The lookup table essentially stores a digital representation of one cycle of the waveform. This allows the DDS to generate the waveform by reading amplitude values from memory rather than calculating them in real-time.
Exam Tip
DDS lookup table = Amplitude values. Remember: The DDS lookup table contains amplitude values representing the desired waveform (like sine wave values). Phase addresses the table to get amplitude.
Memory Aid
"**D**DS **L**ookup = **A**mplitude **V**alues (think 'DL = AV' = Amplitude Values)"
Real-World Application
In a DDS generating a sine wave, the lookup table contains 256 or 1024 amplitude values representing one complete cycle of a sine wave. As the phase accumulator increments, it addresses different locations in the table, reading out the corresponding amplitude values. These digital amplitude values are then converted to analog by the DAC to create the output waveform.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. The phase relationship between reference and output is handled by the phase accumulator and frequency control, not stored in the lookup table.
Option C: Incorrect. The phase relationship between VCO and output is for PLL systems, not DDS. DDS doesn't use a VCO.
Option D: Incorrect. Frequently used frequencies aren't stored in the lookup table. The table stores waveform amplitude values, and frequency is controlled by how fast the phase accumulator increments.
题目解析
The correct answer is B: Amplitude values that represent the desired waveform. The lookup table in a direct digital synthesizer (DDS) contains amplitude values that represent the desired waveform (typically a sine wave). These amplitude values are stored in memory and addressed by the phase accumulator output. The phase accumulator generates phase values (0 to 2π or 0 to full scale). These phase values are used as addresses to look up corresponding amplitude values in the table. For a sine wave, the lookup table contains the sine function values - when phase is 0, amplitude is 0; when phase is π/2, amplitude is maximum; etc. The lookup table essentially stores a digital representation of one cycle of the waveform. This allows the DDS to generate the waveform by reading amplitude values from memory rather than calculating them in real-time.
考试技巧
DDS lookup table = Amplitude values. Remember: The DDS lookup table contains amplitude values representing the desired waveform (like sine wave values). Phase addresses the table to get amplitude.
记忆口诀
**D**DS **L**ookup = **A**mplitude **V**alues (think 'DL = AV' = Amplitude Values)
实际应用示例
In a DDS generating a sine wave, the lookup table contains 256 or 1024 amplitude values representing one complete cycle of a sine wave. As the phase accumulator increments, it addresses different locations in the table, reading out the corresponding amplitude values. These digital amplitude values are then converted to analog by the DAC to create the output waveform.
错误选项分析
Option A: Incorrect. The phase relationship between reference and output is handled by the phase accumulator and frequency control, not stored in the lookup table. Option C: Incorrect. The phase relationship between VCO and output is for PLL systems, not DDS. DDS doesn't use a VCO. Option D: Incorrect. Frequently used frequencies aren't stored in the lookup table. The table stores waveform amplitude values, and frequency is controlled by how fast the phase accumulator increments.
知识点
DDS lookup table, Amplitude values, Waveform generation, Direct digital synthesizer
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.