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A 4.2 GHz 0.24-μm CMOS VCO for a WLAN 802.11a image reject transceiver is demonstrated. The VCO provides quadrature output through injection locking of two LC-oscillators. Because the injection locking has an impact on the oscillation frequency and Q-factor, a quasi linear model is used to analyze this. Furthermore, as this quadrature VCO is prone to imperfections, an analysis of the static phase error is presented. The phase error is measured to 2.6-3.1° which can be traced back to the coupling from the inductors to the DC-supply connection. The phase noise is at most -109 dBc/Hz @ 1 MHz over the entire tuning range from 3.95-4.55 GHz while the power consumption is 21 mW.