An integrated circuit (400; 420; 440; 460) comprising: a semiconductor substrate; a voltage controlled oscillator disposed on the semiconductor substrate, the voltage controlled oscillator comprising - a voltage controlled oscillator core including a cross-coupled transistor pair (M 1 , M 2 ), a first tunable resonator coupled to the cross-coupled transistor pair (M 1 , M 2 ), - a filtered current source including a current source coupled to the cross-coupled transistor pair, (M 1 , M 2 ) and a second tunable resonator coupled between the cross-coupled transistor pair (M 1 , M 2 ) and the current source ; and - a tuning circuit comprising a tuning input, a first tuning output coupled to a first tuning input of the first tunable resonator , and a second tuning output coupled to a second tuning input of the second tunable resonator , characterized in that the second tunable resonator comprises: an LC tank including a second varactor (C 2 ) coupled in parallel with an inductor (407; L 2 ) ; and a transformer comprising a first winding being the inductor of the LC tank or being coupled in series to the LC tank and a second winding having a first end coupled to the current source and a second end coupled to the cross-coupled transistor pair (M 1 , M 2 ). a semiconductor substrate; a voltage controlled oscillator disposed on the semiconductor substrate, the voltage controlled oscillator comprising - a voltage controlled oscillator core including a cross-coupled transistor pair (M 1 , M 2 ), a first tunable resonator coupled to the cross-coupled transistor pair (M 1 , M 2 ), - a filtered current source including a current source coupled to the cross-coupled transistor pair, (M 1 , M 2 ) and a second tunable resonator coupled between the cross-coupled transistor pair (M 1 , M 2 ) and the current source ; and - a tuning circuit comprising a tuning input, a first tuning output coupled to a first tuning input of the first tunable resonator , and a second tuning output coupled to a second tuning input of the second tunable resonator , characterized in that the second tunable resonator comprises: an LC tank including a second varactor (C 2 ) coupled in parallel with an inductor (407; L 2 ) ; and a transformer comprising a first winding being the inductor of the LC tank or being coupled in series to the LC tank and a second winding having a first end coupled to the current source and a second end coupled to the cross-coupled transistor pair (M 1 , M 2 ). - a voltage controlled oscillator core including a cross-coupled transistor pair (M 1 , M 2 ), a first tunable resonator coupled to the cross-coupled transistor pair (M 1 , M 2 ), - a filtered current source including a current source coupled to the cross-coupled transistor pair, (M 1 , M 2 ) and a second tunable resonator coupled between the cross-coupled transistor pair (M 1 , M 2 ) and the current source ; and - a tuning circuit comprising a tuning input, a first tuning output coupled to a first tuning input of the first tunable resonator , and a second tuning output coupled to a second tuning input of the second tunable resonator , characterized in that the second tunable resonator comprises: an LC tank including a second varactor (C 2 ) coupled in parallel with an inductor (407; L 2 ) ; and a transformer comprising a first winding being the inductor of the LC tank or being coupled in series to the LC tank and a second winding having a first end coupled to the current source and a second end coupled to the cross-coupled transistor pair (M 1 , M 2 ). an LC tank including a second varactor (C 2 ) coupled in parallel with an inductor (407; L 2 ) ; and a transformer comprising a first winding being the inductor of the LC tank or being coupled in series to the LC tank and a second winding having a first end coupled to the current source and a second end coupled to the cross-coupled transistor pair (M 1 , M 2 ). <EOS>