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A technique to improve VCOs phase noise performance is proposed, where an off-chip CPW open-stub with a length of around a quarter-wavelength is utilized. This technique is implemented in the design of a 20.9~22.0 GHz VCO. The VCO is manufactured in a 0.13 um CMOS technology. It is then mounted on an Al2O3 substrate by using Flip-chip technique. The CPW open-stub is fabricated on the substrate as well. The obtained VCO exhibits a low phase noise: -113 dBc/Hz at 1MHz offset. In comparison to the same VCO without the CPW open-stub, the most reduction in phase noise is about 5 dBc/Hz. This VCO consumes about 14 mW dc power and has a 5.1% tuning range.