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We present an RF-powered transponder with temperature sensor for environmental monitoring. The transponder gathers power from the ISM (860-960 MHz) band RF signal. A temperature-compensated ring oscillator and an oversampling synchronizer are proposed for low power and robust system clock generation. The simple structure of the temperature sensor is achieved by sharing major reference signals with the ring oscillator. The generated clock frequency has a variation of less than 5% for 1-V supply voltage and 90°C temperature changes. The temperature sensor has a resolution under 1°C in the range from -10°C to 80°C. The transponder dissipates only 8-μW during the active state and occupies 0.3 mm2 with a 0.25-μm CMOS process.