Invited paper The modeling of microwave transistors in the field of RF and Microwave circuits design is an inevitable task. The developed model must be capable of predicting the non-linear characteristics of the active transistor which has a significant role in the design of RFPA and oscillators. This necessitates the measurement of low-frequency (LF) noise sources in order to model them electrically and thereby optimises the impact of noise in the case of oscillator design. A dedicated low-frequency noise measurement setup [1] has been developed at XLIM laboratory which allows the measurement of low-noise spectral densities at the ports of the device under both DC and large-signal operation regime. Furthermore, the experimentally measured noise characteristics of InGaP-GaAs heterojunction bipolar transistors (HBT) has been successfully incorporated into the commercially available physics-based device simulator (TCAD Sentaurus) in order to compute GR and diffusion noise through numerical simulation [2]. This concurrent experimental characterization and physical simulation of HBT device allows to accomplish cyclostationary electrical modeling of the LF noise sources. Indeed, it also helps to improve the phase noise of oscillator circuits and to understand the physical origin of these noise sources. The importance of cyclostationary noise in the design of oscillator circuits will be discussed. The knowledge of noise location and its physical origin is significantly important for oscillator designers in order to accurately predict the associated phase noise spectrum. The harmonic balance and the related technique, namely the conversion matrix method is used to calculate the phase fluctuations caused by the noise sources. Nowadays, the simulation methods of phase noise in oscillators are well established, particularly recent modern software allows handling of both linear and cyclostationary noise sources. Some experimental voltage controlled oscillator designs over the microwave frequency range of 2 GHz to 24 GHz will be presented [3-6].