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

Abstract

MEMS resonators have recently emerged as an alternative structure to crystal resonators in providing frequency references which achieve better than 50 ppm accuracy over the industrial temperature range. As illustrated in Fig. 13.1.1, a programmable oscillator utilizing a MEMS resonator is achieved by wire bonding a MEMS resonator die to a CMOS die that contains an oscillator sustaining circuit, temperature sensor, fractional-N synthesizer, and various digital blocks. The output of the sustaining circuit provides a 5 MHz reference frequency to the fractional-N synthesizer, which outputs a higher frequency that can be digitally adjusted with sub-ppm resolution over a ≫10% tuning range. By then sending the fractional-N synthesizer output into a programmable frequency divider (i.e., divide-by-N circuit), any frequency in the range of 1 to 115 MHz can be achieved by proper combination of the fractional-N synthesizer and programmable divider settings.