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Usually fractional dividers are implemented using an integer divider and then switching its "divide-by" setting between two adjacent settings in a systematic manner. For example, to get 4.5, you would keep the divider programmed to 4 for 50% of the time and to 5 the other 50%. For other fractions, you use other percentage of the two adjacent settings. A better way of doing this (while preserving the same idea) is to use a sigma-delta modulator to toggle between the two settings (you can search for this on the web, lots of material is available). There are other ways to do fractional division, but the above is the best performing/most commong I am aware of.
to:eternal_nan
can you give some papers about sigma-delta modulator using in fractional frequency dividers?i want to know how to specify the sigma-delta modulators' structure,parameters.
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