25.9 A ±3ppm 1.1mW FBAR frequency reference with 750MHz output and 750mV supply

Multiple emerging wireless applications (body-worn devices and IoT, for example) will demand previously impossible thin-film form factors and low system cost. One key enabling technology for this paradigm is a new class of radios that offer cost/size approaching RFID while still maintaining peer-to-peer connectivity like more complex radios. These radios need to be cheap and thin, which means they should be fabricated using wafer-scale semiconductor processing. The existing paradigm (quartz crystals used as a frequency reference in radios) is a huge bottleneck in reducing cost and size of these devices. MEMS frequency references have replaced quartz crystals in some applications [1-3]. For example, [1] reports a MEMS reference with 0.5ppm stability but the power consumption (~100mW) and supply voltage (1.8V) are not suitable for low-voltage/low-power radios. [2] reports a 32kHz, 3ppm reference for mobile time-keeping applications, but is unsuitable for radio frequency synthesis due to its low output frequency. In this paper, we report a thin-Film Bulk-Acoustic-Resonator (FBAR) frequency reference suitable for low-voltage/low-power radio applications. The reported FBAR reference achieves a stability of +/- 3ppm from 0 to 90C. We achieve this by using an electronic temperature compensation scheme to improve the intrinsic +/-50ppm stability of an FBAR oscillator down to +/- 3ppm (Fig. 25.9.1). The core of the temperature compensation scheme is a temperature sensor that achieves a 1.75mK resolution at a 100mS sampling time.