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3.4. Phase Dithering
The extent to which you can reduce spur levels is dependent on many factors. The likelihood of repetition of derived phase values and resulting spurs, for a given angular precision, is closely linked to the ratio of the clock frequency to the desired output frequency. An integral ratio clearly results in high-level spurious frequencies, while an irrational relationship is less likely to result in highly correlated noise at harmonic frequencies.
The Altera NCO IP core allows you to finely tune the variance of the dither sequence for your chosen algorithm, specified precision, and clock frequency to output frequency ratio, and dynamically view the effects on the output spectrum graphically.