A low-area switched-resistor loop-filter technique for fractional-N synthesizers applied to a MEMS-based programmable oscillator

A fractional-N synthesizer is used in a programmable 1 to 115 MHz MEMS oscillator. A high gain phase detector lowers the impact of loop filter noise, and a switched-resistor loop filter avoids a charge pump and boosts effective resistance to save area. The entire synthesizer with LC-VCO occupies 0.31 mm
²
in 0.18 ¿m CMOS. Chip consumption is 3.7 mA drawn from a 3.3 V supply for a 20 MHz output with no load.