  In this paper, we discuss the definition of Q factor for nonlinear
oscillators. While available definitions of Q are often limited to linear
resonators or oscillators with specific topologies, our definition is
applicable to any oscillator as a figure of merit for its amplitude stability.
It can be formulated rigorously and computed numerically from oscillator
equations. With this definition, we calculate and analyze the Q factors of
several oscillators of different types. The results confirm that the proposed Q
formulation is a useful addition to the characterization techniques for
oscillators.
