A new technique is presented for the stability analysis of steady nonlinear regimes in microwave circuits, using commercial software. The Nyquist stability criterion is applied to the open-loop transfer functions of the circuit linearisation around the steady state. Through a simple nonlinear optimisation technique it is also possible to determine the predicted quasi-periodic or period-doubled steady solutions. The stability analysis of two varactor-based circuits has provided excellent results when compared with time domain simulations and measurements.