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Frequently Asked Question

What are phase truncation spurs?

Phase truncation is a part of the DDS architecture. For a DDS with a 32-bit phase accumulator to directly convert 32 bits of phase to a corresponding amplitude would require a lookup table with 4,294,967,296 entries. If each entry is stored with 8-bit accuracy, then 4-gigabytes of lookup table memory would be required. Clearly, it would be impractical to implement such a design. The solution is to use a fraction of the most significant bits of the accumulator output to provide phase information. For example, in a 32-bit DDS design, only the uppermost 12 bits might be used for phase information. The lower 20 bits would be ignored (truncated) in this case. The phase errors introduced by this truncation result in errors in amplitude during the phase-to-amplitude conversion by the DDS. Since the amplitude errors are periodic in the time domain, they appear as line spectra (spurs) in the frequency domain and are what is known as phase truncation spurs. Other spurs are introduced by the internal DAC. Most DDS systems are designed so that the phase truncation spurs are less than the DAC spurs.