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 (402, 422, 442) 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 (402, 422, 442), characterized in that the second tunable resonator (402, 422, 442) 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 (402, 422, 442) 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 (402, 422, 442), characterized in that the second tunable resonator (402, 422, 442) 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 (402, 422, 442) 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 (402, 422, 442), characterized in that the second tunable resonator (402, 422, 442) 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).