This paper describes an original method to estimate the locking ranges of injection-locked frequency dividers (ILFDs) and their sensitivity to variations in parameter values. The synchronization capability of a given oscillator architecture with respect to possible injection points is explored by applying small-amplitude signals and adopting a phase-domain macromodel based on the paradigm of the perturbation projection vector. It is shown that the method provides synthesis information and guidelines that help to improve the ILFD design process and the selection of a proper injection strategy. © 2006 IEEE