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This brief presents a digitally tunable frequency generator for crystal-less low-power radios in size-constrained wireless-body-area-network applications. A switched-capacitor frequency-to-voltage converter is used to lock a ring oscillator (RO) to an on-chip reference voltage. The proposed frequency lock loop (FLL) significantly reduces the supply and temperature dependences of the RO frequency by 400x and 13x, respectively, to 0.23%/V and 126 ppm/°C compared to a RO without the FLL. The output frequency can be digitally tuned from 325 to 420 MHz with a resolution of 80 ppm to allow channelization or calibration.