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An LC VCO with a new varactorless frequency tuning technique is presented, which is suitable for microwave and millimetre-wave applications. The continuous frequency tuning with a small output power variation is achieved using tunable negative inductance and negative capacitance cells. Fabricated in a 0.35-μm SiGe BiCMOS process, the varactorless VCOs achieves a continuous tuning range of 15% from 1.57 GHz to 1.83 GHz, with an output power variation of 0.2 dB and a constant power consumption. The measured phase noise is -117.3 dBc/Hz and -116.7 dBc/Hz at 1 MHz offset from a 1.57 GHz and a 1.83 GHz carrier frequency respectively.