Continuously tunable inductor and method to continuously tune an inductor

A circuit having a continuously variable inductance, comprising:

• an inductive-capacitive (LC) voltage controlled oscillator (VCO) having first and second nodes, a primary coil having first and second ends coupled directly to said first and second nodes, respectively, first and second cross-coupled NMOS transistors having first and second gates coupled directly to said first and second nodes, respectively, and first and second varactors having first and second electrodes coupled directly to said first and second nodes;
• a secondary coil magnetically coupled with the primary coil;
• first and second transistors in a closed loop with the secondary coil;
• a current source having an output amplitude variable in response to change in a control input to vary a magnitude of a mutual inductance between the primary and secondary coils;
• and
• first and second switches operable to directly couple gates of the first and second transistors (a) to the first and second nodes of the LC VCO, respectively, or (b) to the second and first nodes of the LC VCO, respectively, to control a sign of the mutual inductance relative to the inductance of the primary coil, such that the control input and the first and second switches are operable to continuously vary a combined inductance of the circuit over a substantial range, the combined inductance including a sum of an inductance of the LC VCO with the mutual inductance.