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This paper presents a low noise 900 MHz voltage controlled oscillator (VCO) implemented in 0.6 /spl mu/m CMOS technology. The VCO based on a 4-stage ring oscillator adopted a new differential delay cell to reduce the phase noise. The measured phase noise is -101 dBc/Hz at 100 kHz offset from the center frequency and comparable to that of LC based integrated oscillators. The measured value is smaller than any other previously reported ones in the ring oscillator VCO's. A prototype frequency synthesizer with this VCO shows phase noise of -117 dBc/Hz at 100 kHz offset. The VCO operates from 750 MHz to 1.2 GHz with a single 3 volt power supply.