A Low Area, Switched-Resistor Based Fractional-N Synthesizer Applied to a MEMS-Based Programmable Oscillator

MEMS-based oscillators have recently become a topic of interest as integrated alternatives are sought for quartz-based frequency references. When seeking a programmable solution, a key component of such systems is a low power, low area fractional-N synthesizer, which also provides a convenient path for compensating changes in the MEMS resonant frequency with temperature and process. We present several techniques enabling efficient implementation of this synthesizer, including a switched-resistor loop filter topology that avoids a charge pump and boosts effective resistance to save area, a high gain phase detector that lowers the impact of loop filter noise, and a switched capacitor frequency detector that provides initial frequency acquisition. The entire synthesizer with LC VCO occupies less than 0.36 sq. mm in 0.18 mu m CMOS. Chip power consumption is 3.7 mA at 3.3 V supply (20 MHz output, no load).