Abstract
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 76.8MHz, 105ppm 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 850μA, with phase noise of −127dBc/Hz at 1kHz frequency offset. Temperature drift errors and initial frequency offset of ±8000ppm 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 4.6μA, 11.5 bit temperature to digital converter TDC with resolution of 0.1°C in 100ms conversion time. The paper presents the first AlN-on-Si oscillator platform with ±0.5ppm frequency stability over temperature ranges from −40°C to 85°C. The system runs on 1.8V supply in 32nm CMOS.
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