A fractional-n frequency synthesizer can be constructed using two integer dividers, a divide-by-n and a divide-by-(n + 1) frequency divider. With a modulus controller, n is toggled between the two values so that the VCO alternates between one locked frequency and the other. The VCO stabilizes at a frequency that is the time average of the two locked frequencies. By varying the percentage of time the frequency divider spends at the two divider values, the frequency of the locked VCO can be selected with very fine granularity.
Read more about this topic: Frequency Divider
Other related articles:
... Delta-sigma fractional-n dividers overcome this problem by randomizing the selection of n and (n + 1), while maintaining the time-averaged ratios ...