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A 900-MHz fully integrated VCO was fabricated in a 0.18-/spl mu/m foundry CMOS process. Under 1.5 V power supply, this VCO can be tuned from 667 MHz to 1156 MHz which corresponds to a 53.6% tuning range. The VCO has nearly constant phase noise over the whole tuning frequency, credit to the switched resonators used in this VCO. The phase noise at a 600 kHz offset is -123.1 dBc/Hz at 1125 MHz center frequency and -124.2 dBc/Hz at 667 MHz center frequency.