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A new technique is presented to obtain reduced-order models of microwave oscillators from experimental measurements. The models can then be applied to the analysis and design of single-oscillator or multioscillator configurations such as coupled-oscillator systems. The first-order model is given by the derivatives of the oscillator admittance function with respect to the amplitude and frequency. The experimental technique presented here is based on the extraction of these derivatives from the injection-locked response of the oscillator, synchronized to a small-signal source. The new approach to obtain the derivatives has been validated through comparison with the standard method using commercial harmonic-balance software. The experimental setup and all the key aspects related to calibration and equipment requirements are explained in detail. Finally, the proposed technique is applied to characterize a voltage-controlled oscillator at 4.97 GHz. Excellent agreement has been obtained between measurements and simulations.