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A 4.2 GHz sub-harmonic low-phase-noise oscillator is presented. Optimisation of phase noise has been achieved by a combination of a 2 GHz bulk acoustic wave (BAW) resonator and a differential Colpitts topology circuit adopting STMicroelectronics 0.25 μm SiGe:C BiCMOS technology. The measured phase noise is -132 dBc/Hz at 100 kHz offset while the power consumption of the core oscillator is 21 mW with a figure of merit of -211 dBc/Hz. To the authors knowledge, this sub-harmonic oscillator achieves the best phase-noise level for a BAW based oscillator at 4 GHz band.