Oscillators are used in many integrated RF circuits. Since their behavior is highly nonlinear, full system simulation can be expensive. Furthermore, the behavior of an oscillator can be (un)intendedly perturbed by that of other components and oscillators. We present a method to build nonlinear phase macromodels of voltage controlled oscillators and show how these can be used to predict the behavior of oscillators under perturbation. Model order reduction techniques are used to decrease simulation times. Numerical
results for realistic design illustrate the proposed approach.