The stability analysis of nonlinear circuits is a central problem in the framework of microwave design, as applied to high-power amplifiers, oscillators, frequency dividers, and multipliers. However, reliable, user-friendly tools for the stability analysis of such components seem to be quite scarce, even within the very commercial suites for high-frequency circuit simulation. This contribution addresses the problem by generalizing Ohtomo's test - a popular tool born in the context of linear circuits - to the nonlinear case. Specifically, the new technique allows to analyze the characteristic zeros of a linearized large-signal solution resulting from a harmonic-balance (HB) simulation. As well known, the location of these zeros with respect to the imaginary axis of the complex plane is critical in relation to the qualitative nature of the solution, i.e., to its stability. To the best of the authors' knowledge, for the first time, a large-signal stability test based on the system determinant but not requiring direct access to the nonlinear elements is implemented completely within a commercial circuit simulator (i.e., without requiring an external mathematical environment for data postprocessing). Some limitations still present are discussed. The generalized Ohtomo test was validated against other methods and circuits from the literature.
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