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An approach to adjusting and optimising the phase-noise performance of CMOS LC oscillators is presented. The technique is based on controlling the bulk-source voltage of the cross-coupled transistors in the oscillator core. It is shown that the impedance in the path of the current through the oscillator core to ground at the second harmonic of oscillation can be maximised through changing the value of the parasitic capacitances. The technique is validated by measured results from a proof-of-concept LC oscillator fabricated in a 90 nm CMOS process.