Fast-Lock Hybrid PLL Combining Fractional-$N$ and Integer-$N$ Modes of Differing Bandwidths

Abstract

We introduce a single-loop PLL that operates in a narrower-bandwidth, integer-N mode during phase lock and in a wider-bandwidth, fractional-N mode during transient. This hybrid PLL, as a generalization of the conventional variable-bandwidth PLL that shifts only its bandwidth, simultaneously achieves the fast-locking advantage of the fractional-N PLL and design simplicity of the integer-N PLL, and as such, brings benefits in certain important PLL applications. In addition, the frequency division mode switching, unique in the hybrid PLL, enables a new, more digital protocol to execute bandwidth switching. A CMOS IC prototype attests to the validity of the proposed approach.