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In this paper, we present a flexible measurement setup for the characterization of the phase noise (PN) and frequency stability of microwave electron devices operating under nonlinear oscillating conditions. The setup embeds the device-under-test (DUT) within a feedback loop, and forces it into a large signal (LS) oscillating regime, by controlling the loop amplitude and phase. The DUT input and output terminations are also controlled, by exploiting loadand source-pull capabilities. By exploiting this setup, we demonstrate that the PN performance of microwave oscillators can be strongly affected by the device nonlinear RF working regime. As well known, the oscillator PN is mainly related' to the frequency stability of the circuit and the active device low-frequency noise up-conversion to RF mechanisms. By using measurements performed with the proposed setup, it is shown that these aspects depend also on the device LS operating conditions; hence, the dynamic LS load line and the amplifying class of operation have a significant role for the oscillator PN. The experimental results described in the paper, along with the analyses proposed by using simulations, are in accordance with recently published nonlinear noise modeling approaches based on cyclostationary noise sources. The investigation performed by means of the setup can provide information for both the design of low-phase-noise (LPN) oscillators and the parameter extraction and validation of nonlinear noise models of electron devices. Furthermore, this setup is also proposed as a tool for the evaluation of power amplifier PN when a dedicated residual phase noise measurement system is not available. [ABSTRACT FROM AUTHOR]