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An RF-powered transponder with temperature and photo sensors is designed and fabricated for environmental monitoring. The transponder gathers power from external ISM (860-960MHz) band RF signal and senses ambient temperature and light. It contains a supply voltage generator, a temperature-compensated ring oscillator, an oversampling synchronizer, a PTAT temperature sensor and an abrupt transition buffered photo sensor. Its internal clock frequency has variation less than 7% for 1.5-V supply voltage and 90°C temperature change. The transponder occupies 0.4mm2 with 0.25-μm CMOS process and dissipates only 5.14-μW during active state.