Analysis and Design of Superharmonic Injection-Locked Multipath Ring Oscillators

Frequency dividers can be modeled as superharmonic injection-locked oscillators. We propose a general model for ring-oscillator based dividers. Theoretically, the maximum division frequency of a frequency divider can be decoupled from the number of phases at its output when multipath coupling of ring oscillators is used. Accurate expressions for the locking range are derived that are used to find the optimum structure to maximize the division frequency. The results of the model are incorporated into a design procedure that can rapidly explore various coupling and sizing of the multipath structure. A design of a divider chain for a 48-Gb/s serializing transmitter is used as a demonstration.