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] [2] [3]. For example, [1] reports a MEMS reference with 0.5 ppm stability but the power consumption (~100mW) and supply voltage (1.8V) are not suitable for low voltage/power radios. [2] reports a 32 kHz, 3 ppm 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 1.1mW thin-Film Bulk Acoustic Resonator (FBAR) frequency reference operating from a 0.75V supply suitable for low voltage / low power radio applications. The reported FBAR reference achieves a stability of +/-3 ppm from 0 to 90C. We achieve this by using an electronic temperature compensation scheme to improve the intrinsic +/-50 ppm stability of an FBAR oscillator down to +/-3ppm (figure 1). The core of the temperature compensation scheme is a 4.6 pJK 2 temp sensor (3X better than the state of the art FOM for sub 1V temp sensor [4]) that achieves a 1.75 mK resolution at a 100 mS sampling time. Figure 1 shows the block diagram of the FBAR frequency reference. Transistors MP1 and MN1 form a Pierce oscillator. The oscillator in this work operates at 750 MHz, but the frequency can be changed to any frequency (>500MHz & < 10 GHz) at design time by using an appropriate FBAR. The oscillator operates from a 650mV regulated supply voltage generated using a low voltage bandgap reference and an LDO. A 6 bit thermometric MIM cap array is used to stabilize the oscillator frequency to the targeted value. The cap array has a tuning range of 300ppm; sufficient to correct the frequency for temperature and process variations. The raw tuning resolution of the cap array is ~ 5 ppm, and a 2 nd order sigma delta modulator runs from a divided-by-4 clock and dithers the cap …