The Design of Low Phase Noise MEMS Oscillators for Frequency References

As modern complementary metal oxide semiconductor (CMOS) processes scale, communications devices continue to become more powerful and more portable. Technology is progressing to single chip transceivers with more and more components integrated on chip [I, 2]. We are reaching the point at which the physical space in a communications device is completely dominated by those components that cannot be integrated on chip. The relatively modern field of MEMS (micro electro mechanical systems) has the potential to provide solutions for a number of these challenges. MEMS devices are created using processing techniques similar to those used for CMOS and there is significant research towards combining MEMS and CMOS in the same process [3, 4, 51. Once MEMS and CMOS can be manufactured together, components such as filters and oscillators could be integrated on the same chip as the transceiver. Even if the components can be integrated, they must still meet the stringent specifications of modern communications architectures. This research investigates the requirements for a MEMS oscillator to replace the crystal oscillator used as the low frequency reference for a frequency synthesizer in a cellular phone. This frequency synthesizer produces the carrier frequency used by the transceiver and is subject to stringent phase noise requirements to avoid interference with other users and meet signal to noise requirements of the overall system. The goal of this research is to demonstrate that high quality oscillators can be designed using MEMS resonators and to gauge how far technology must progress for these MEMS oscillators to replace the crystals used in modern communications systems.
This thesis focuses on the design issues invoIved in creating MEMS based oscillators that can be used as the frequency reference in a GSM system. Chapter 2 introduces frequency synthesizers and their phase noise requirements. Chapter 3 describes the requirements of the reference oscillator and introduces traditional crystal oscillators as a benchmark. Chapter 4 describes MEMS resonators and the key parameters for oscillator design. Chapter 5 evaluates the differences between various MEMS oscillator topologies. Chapter 6 describes the MEMS oscillators that were designed and their experimental results. Chapter 7 concludes the thesis with recommendations on improving the oscillator phase noise as well as future work that will be required before MEMS can replace crystal oscillators.
Publication date: 
May 31, 2004
Publication type: 
Master's Thesis
Seth, M. (2004). The Design of Low Phase Noise MEMS Oscillators for Frequency References: Research Project. United States: University of California, Berkeley.

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