what is it, why and how would I use it?
A standard DDS relies on a single accumulator, referred to as the phase accumulator. The frequency relationship between the sample rate and the output clock in this configuration has a set denominator (equal to 2^x, where x is the bit width of that accumulator). The numerator is user controlled variable that we at ADI refer to as the frequency tuning word. While this provides very fine tuning accuracy, it is not possible to hit some output frequencies precisely - for example using a 250 MHz sampling clock, you cannot get a 25 MHz output precisely, the closest you can come is 25.00000002328 MHz. That may seem trivally off, but there are applications, primarily instrumentation applications, where precision frequency generation is critical.
See application note AN-953, which gives a detailed explanation of programmable modulus as it relates to the AD9913.
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