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Push-push voltage-controlled oscillators (VCOs) achieve high oscillation frequencies by relying on the boosted second harmonic component of the oscillator. These VCOs however, typically require a high power to deliver a reasonable output swing. In this paper, we first derive an analytical expression that relates the amplitude of the second harmonic of an LC VCO to the C-V characteristics of its varactor. Then based on the results of the analysis, we present the design of a low- power and compact 27-GHz push-push VCO in 65 nm CMOS that exhibits a 28.5% tuning range while consuming 21 mW (including buffers) from a 1 V supply. At 27-GHz output, the VCO achieves a phase noise of -101 dBc/Hz at 1-MHz offset.