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Quartz MEMS resonators are attractive for use in oscillators for stable frequency control applications in navigation and wireless communication systems by virtue of their reduced cost, size, weight, and power (CSWaP), combined with the potential for integration with electronics. Low phase noise oscillators enable rapid GPS signal acquisition and efficient use of wireless communication frequency spectrum. We present a UHF fundamental-mode oscillator based on a miniature thickness shear mode piezoelectric quartz resonating element fabricated via wafer scale MEMS fabrication. This stable low phase noise oscillator was used to acquire and track the GPS signal in a commercial avionics system, the first demonstration of its kind for MEMS oscillators. An oscillator sustaining ASIC capable of operating in tandem with the quartz MEMS resonator is also described. High-performance UHF oscillators in most navigation and communication systems are typically fabricated using HF or VHF resonators with frequency multiplying sustaining circuits. Consequently, they operate with high power consumption (usually >100 mW) and have sizable volume (usually >100 mm3) [1] because of use of larger, low-frequency crystals. Quartz MEMS resonator technology has been developed at HRL Laboratories for UHF oscillators with reduced CSWaP. The MEMS fabrication process enables mass production of UHF resonators on a single wafer. The resonators can then be integrated with their sustaining electronics by either direct wafer bonding or hybrid packaging. Wafer-scale fabrication of quartz MEMS resonators was described in a previous publication [2] and patent [3]. Using this fabrication method, we produced miniature resonators that are shown in Fig. 1. HRL also designed and fabricated voltage controlled oscillator (VCO) printed circuit boards to allow rapid prototyping of oscillators. The ability to pull oscillator frequency with a 100 ppm adjustment range gave us the flexibility to perform temperature compensation functions to stabilize the oscillator frequency in the -40 to 85 deg. C temperature range, satisfying most commercial and military specifications. Fig. 2 and 3 show the photograph of a 663 MHz VCO with a frequency pulling varactor and its phase noise performance. This oscillator was tested as a precision frequency source in a Rockwell Collins GPS-4000 satellite receiver. The oscillator was able to help lock and track GPS signals, which we believe is the first demonstration of its kind for MEMS oscillators. We also designed and fabricated through the IBM/GlobalFoundries 7WL 0.18 micron process a SiGe BiCMOS Pierce oscillator sustaining ASIC to be integrated with the resonator. The singulated ASIC and the packaged MEMS resonator were die attached to a printed circuit board and wire bonded together for ease of testing (Fig. 4). This integrated device has a volume of <-140 dBc/Hz.

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Journal: TechConnect Briefs
Volume: 4, Informatics, Electronics and Microsystems: TechConnect Briefs 2017
Published: May 14, 2017
Pages: 68 - 71
Industry sector: Sensors, MEMS, Electronics
Topic: MEMS & NEMS Devices, Modeling & Applications
ISBN: 978-0-9988782-1-8