Page 52 - Phase-Locked Loops Design, Simulation, and Applications

MIXED-SIGNAL PLL BUILDING BLOCKS Ronald E. Best 37
It is obvious that a counter can scale down by an integer number only—for instance, by a
factor of 5 or 6. Certainly a counter cannot divide by say 5.3. Nevertheless, there exist so-
called fractional-N frequency synthesizers. These are synthesizers that are indeed capable of
creating, for example, an output frequency that is 5.3 times the reference frequency. Of course,
the down scalers used in such synthesizers are not able to divide immediately by 5.3.
Fractional-N ratios are realized by switching the divider ratio of a programmable counter from
one value to another in a consecutive reference cycle. This will be discussed in more detail in
Sec. 7.1.

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