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A 57-GHz CMOS voltage-controlled oscillator (VCO) using tunable LC source-degeneration for operation frequency and tuning-range enhancement is demonstrated. The tunable LC source-degeneration is made by adding two tunable LC tanks, where varactors are used as the needed capacitors, to the source terminals of the cross-coupled transistor pair of the VCO. Compared with the traditional cross-coupled transistor pair, the proposed one significantly decreases the equivalent parallel capacitance (CEQ). This in turn results in the increase of both the operation frequency and the tuning-range of the VCO. The measurement result shows the tunable LC source-degeneration achieved a 185.3% enhancement (from 1.36 GHz (53.36-54.72 GHz) to 3.88 GHz (53.36-57.24 GHz)) in tuning-range by tuning the varactors of the proposed cross-coupled transistor pair.