RF MEMS reference oscillator platform with ±0.5ppm frequency stability for wireless handsets

This paper reports on the design of a complete RF MEMS reference oscillator platform to replace bulky quartz in wireless handsets targeting LTE specifications. 106 fs jitter, 76.8 MHz AlN-on-Si reference oscillator is designed with SiO₂ layers for passive temperature compensation achieving frequency stability of 105 ppm across temperature ranges of -40°C to 85°C. The oscillator consumes 850 μA, with figure-of-merit FOM of 216 dB. Initial frequency offset of ±8000 ppm and temperature drift errors are combined and further addressed electronically. A novel simple digital compensation circuitry is presented in the paper. The circuit generates a compensation word as an input to 21 bit MASH 1-1-1 sigma delta modulator incorporated in RF LTE fractional N-PLL for frequency compensation. Temperature is sensed using low power band-gap front end circuitry with 12 bits temperature to digital converter TDC characterized by a resolution of 0.075°C, consuming only 4.6 μA. The achieved output frequency stability is ±0.5 ppm over temperature ranges from -40°C to 85°C for the most stringent 700 MHz LTE band.