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An ultra-low-power, low-voltage frequency synthesizer designed for implantable medical devices is presented. Several design techniques are adopted to address the issues in ultra-low voltage and current design. The charge pump (CP) in the synthesizer utilizes dynamic threshold-voltage and switch-coupled techniques to provide a high driving current with a low standby current. The synthesizer adopts a ring-based voltage controlled oscillator (VCO) that utilizes a dual resistor-varactor tuning technique to compensate for process-voltage-temperature (PVT) variations and the exponential voltage-to-current curve. Implemented in a 0.13-μm CMOS technology, the 0.5-V medical-band frequency synthesizer consumes 440 μW while exhibiting a phase noise of 91.5 dBc/Hz at 1-MHz frequency offset.