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A fully integrated CMOS frequency synthesizer for UHF RFID reader is implemented in a 0.18-mum CMOS technology. Due to the large self-interference and the backscatter scheme of the passive tags, reader synthesizer's phase noise requirement is stringent to minimize the sensitivity degradation of the reader RX. The modified transformer feedback voltage-controlled oscillator (VCO) exhibits enhanced tank impedance and even harmonic noise filtering to achieve low phase noise. A third-order 2-bit single-loop SigmaDelta modulator is optimized for the proposed synthesizer in terms of phase noise and power. The synthesizer provides a frequency resolution of 25-kHz with a tuning range from 1.03 GHz to 1.4 GHz . Phase noise of -70 dBc/Hz inband, -104 dBc/Hz at 200-kHz offset and - 121 dBc/Hz at 1-MHz offset with a reference spur of -84 dBc are measured at a center frequency of 1.17 GHz and a loop bandwidth of 35 kHz. Power dissipation is 4.92 mW from a 0.8 V supply.