Fractional-N Synthesizers

The term fractional-N describes a family of synthesizers that allow the minimum frequency step to be a fraction of the reference frequency. Over the years, a number of methods have been proposed to realize fractional-N frequency synthesis that are based on the basic concepts of traditional integer-N synthesis. Among these methods, three techniques are best known in the industry. They are fractional dividerbased, current injection-based, and DS modulator-based fractional-N. This White Paper describes these methods and provides a general discussion of the use of fractional-N synthesizers in a low-cost, low-power radio.