Fractional and integer pll architectures

Abstract

A digital fractional PLL introduces an accumulated phase offset before the digital VCO to achieve the fractional part of the division ratio. To provide this phase offset, the digital accumulator can integrate a fractional component Δn. By forcing Δn to zero, the PLL becomes an integer-N PLL. A de-skew timing configuration can be used to remove any time mismatch between integer and fractional counters of the PLL. A digital PLL can merge the function of frequency generation (DVCO) and that of fractional frequency counting into the same circuit block by reusing various phases of the frequency output to generate a fractional frequency count. A digital integer PLL can include a comparator, wherein the feedback loop of this PLL forces the phase difference between the reference clock and feedback signals to approach zero. By changing the duty cycle of feedback signal, the frequency tracking behavior of the loop can be varied.