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A 40 GHz differential CMOS push-push VCO is proposed for high-frequency applications. It is shown analytically that the phase noise of the VCO output at the full-rate frequency is close to 6 dB higher than that of the half-rate frequency. The result of the phase noise analysis is verified by simulations and measurements. A phase noise of - 101 dBc/Hz was achieved at 1 MHz offset frequency. The proposed push-push VCO design enables higher VCO frequency operation with differential output, which is suitable for millimeter wave frequency synthesizers.