This paper presents an ultra-low-power, low-voltage frequency synthesizer designed for medical implantable devices. Several design techniques are adopted to address the issues in ultra-low voltage design. The charge pump (CP) in the synthesizer utilizes dynamic threshold-voltage and switch-coupled techniques to provide a high driving current while maintaining a low standby current. The synthesizer adopts a ring-based voltagecontrolled oscillator (VCO) that utilizes a dual resistor-varactor tuning technique for compensating process-voltage-temperature (PVT) variations and the exponential voltage-to-current curve. Implemented in a 130-nm CMOS technology, the 0.5-V medicalband frequency synthesizer consumes 440 µW with a phase noise of -91.5 dBc/Hz at 1-MHz frequency offset.

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