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This paper proposes a low-power remote frequency calibration method for passive UHF wireless transponders. The frequency of the local oscillator of passive UHF wireless transponders is adjusted to the desired values using an injection-locked phase-locked loop (IL-PLL). A new relaxation oscillator whose oscillation frequency is less sensitive to supply voltage fluctuation is proposed. The power consumption of the proposed IL-PLL is minimized by operating it the subthreshold. A detailed analysis of the nonharmonic injection locking of relaxation oscillators, including locking and pulling dynamics, is presented. A new integrating feedback is proposed to increase the lock range and hold the locked frequency in the absence of the injection signal. The proposed IL-PLL has been fabricated in TSMC 0.18- μm 1.8-V six-metal 1-poly CMOS technology. The performance of the IL-PLL is validated using both simulation and measurement results. The measured power consumption of the IL-PLL with a 10-mV (640-pW) 1-MHz injection signal is 960 nW. The lock range of the IL-PLL is 30 kHz without integrating feedback and 400 kHz with integrating feedback. The frequency of the locked oscillator drifts over time at a rate of 5 Hz/ms when the external injection signal is removed.