A Direct Digital Frequency Synthesizer Based on Optimized Grouping Strategy of Multiple Parallel Excess-Four Rotations

The direct frequency synthesizer (DDFS) used in navigation or software-defined radio is usually implemented with read-only memory (ROM) or CORDIC modules. To reduce ROM usage and the pipeline stage of CORDIC, the excess-four rotation structure was previously adopted as a hybrid structure containing both ROM and CORDIC to perform coarse rotation and fine rotation respectively, where the grouping method of the sub-rotations in the fine rotation module may influence the overall performance and circuit area. In this paper, the grouping strategy of the sub-rotation modules from excess-four and excess-two structures is discussed to provide a more flexible phase-bit width and SFDR. Among the discussed grouping manners, the 4 ROT2s structure is chosen for evaluation with 102 dBc of SFDR and 2090 of gates usage, and the evaluation structure obtains the least SFDR of 102.0 dBc and the highest SFDR of 102.8 dBc. Synthesis results suggest the evaluation structure reduces circuit area to 0.0155 \(\mathrm {mm^2}\) from 0.019 \(\mathrm {mm^2}\) while maintaining the same SFDR compared with previous works, where this reduction is at the cost of power consumption and operating data rate.