Deep Dive: E7H10
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.
Why Other Answers 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.
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 Example
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.
Source & Coverage
Question Pool: 2024-2028 Question Pool
Subelement: E7H
Reference: FCC Part 97.3
Key Concepts
Verified Content
Question from the official FCC Extra Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the E7H topic.