The CAD simulation of solid-state oscillators is considered in this article. A negative resistance concept is frequently utilized to design series feedback microwave transistor oscillator. The article discusses using nonlinear design tools for negative resistance oscillator simulation. It describes the main reasons of simulation failure for microwave oscillator design: the position of the oscillator probe in the circuit, the values of the oscillator probe parameters. As illustrated by a number of examples of oscillator models, small-signal negative resistance simulation results can be used for correct placing of oscillator probe in the circuit. These results can be used for enhancing the validity of steady-state oscillation detection using CAD simulation of solid-state oscillators.