The phrase "phase truncation" often comes up in the context of DDS. What is it, and why should I care?
In a DDS, phase truncation occurs at the interface between the phase accumulator and the angle-to-amplitude converter. Phase truncation refers to the fact that only a subset of the bits at the output of the phase accumulator (i.e., some of the MSBs) appear at the input of the angle-to-amplitude converter. That is, the digital phase word at the output of the accumulator is "truncated" before being delivered to the input of the angle-to-amplitude converter. For example, a DDS might have a 32-bit accumulator, but only 15 bits (MSBs) get passed along to the angle-to-amplitude converter.
The reason for phase truncation is to reduce the power consumption and complexity of the angle-to-amplitude converter. It has no impact whatsoever on the frequency resolution of a DDS, as this relates to the number of bits in the phase accumulator regardless of any truncation.
Phase truncation can, however, impact spurious performance. Fortunately, ADI DDSs are designed so that phase truncation spurious is minimal. In fact, spurious free dynamic range (SFDR) of ADI DDSs are limited by DAC harmonics rather than by phase truncation spurious.
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