input word to mash 1-1-1 in fraction-N PLL

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ali kotb

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Hello All
I know that the accumulator size of SDM is important for PLL resolution , the fraction of feedback divider = K/2^f. this fraction ranges from 0-->1
where K = input word ranges from 1 to (2^f)-1
f = number of accumulator bits
2^f = accumulator size

My Question : what defines the input word? how it is changed to get the 0->1 range fraction, or is fixed on the best spur level (highest value possible ?)
how it is entered to the Sigma delta ?
 

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