US Patent 7498856 Fractional-N frequency synthesizer

A circuit, with applications to phase-locked loops and frequency synthesis, where a divider circuit shuffles between dividing the output of a voltage-controlled oscillator by N or N+1, where N is an integer, and where a phase frequency detector provides three logic signals to a charge pump so that one of three values of current may be sourced to a loop filter, with the result that the circuit behaves as a conventional phase-lockup loop fictitious divider circuit that is capable of dividing the output of the voltage-controlled oscillator by a non-integral value.