FAQ: What does the programmable modulus feature provide?

Document created by DSB Employee on Apr 9, 2012Last modified by DSB Employee on Apr 9, 2012
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What does the programmable modulus feature provide?



In a conventional DDS, the denominator in the output frequency equation (Fout = FTW / 2 ^32 * system clock) for 32-bit phase accumulator as an example is fixed. In programmable modulus mode, the denominator becomes programmable. See attached application note AN-953 for more details. Having this flexible provides for much more output frequency selections. In addition, the DDS becomes arguably the highest integer N divider available using programmable modulus mode. For example, in a conventional DDS only binary divisons of the system clock provide a integer sub multiple at the output. With programmable modulus mode, one can program the DDS to output any integer relationship between N = 2 to 2^32. So, now integer relationships like 1/3, 1/5, 1/6, 1/7 can be tuned or outputted. This a great feature for reducing period jitter since now harmonic fold back spurs are harmonically related to the fundamental output frequency.