Electronic frequency compensation of AlN-on-Si MEMS reference oscillators

In this paper we report on the design of a frequency compensation system for AlN-on-Si MEMS reference oscillator to replace temperature compensated crystal oscillators (TCXOs) in cellular handsets. A 76.8 MHz, 105 ppm Temperature Stable, AlN-on-Si MEMS Oscillator is designed to demonstrate the frequency compensation scheme. Double layers of SiO2 are used for passive temperature compensation. The oscillator consumes 850 μA, with phase noise of −127 dBc/Hz at 1 kHz frequency offset. Temperature drift errors and initial frequency offset of ±8000 ppm are combined and further tackled electronically. A simple digital compensation circuitry generates a compensation word to a 21-bit MASH 1-1-1 ΔΣ modulator included in LTE fractional N-PLL for frequency compensation. Temperature is sensed using 4.6 μA, 11.5 bit temperature to digital converter TDC with resolution of 0.1 °C in 100 ms conversion time. The paper presents the first AlN-on-Si oscillator platform with ±0.5 ppm frequency stability over temperature ranges from −40 °C to 85 °C. The system runs on 1.8 V supply in 32 nm CMOS.