Biography
Enrollment Date: 2013
Graduation Date：2016
Degree：M.S.
Defense Date：2016.05.31
Advisors：Hanjun Jiang
Department：Institute of Microelectronics，Tsinghua University
Title of Dissertation/Thesis：A High Performance Resonant Frequency Readout Circuit for SAW Sensor
Abstract:
Nowadays, the wireless sensor networks (WSN), composed of sensors, sensor readout circuits and wireless transceivers, are growing rapidly. The surface acoustic wave (SAW) sensor enjoys high sensitivity, low profile, and various sensing capacity among the sensors used in WSN, which makes it a promising novel sensor. The SAW sensor readout circuit is supposed to maintain high frequency resolution, low power, and fast measurement at the same time, as defined by the applications of WSN. The sensing mechanism of SAW sensor is to convert the change of a certain physical quantity to resonant frequency shift. Therefore, the readout circuit is nothing else than a resonant frequency measuring circuit. The time domain based architecture and frequency domain based architecture are most frequently used conventional architectures. However, the time domain based one has high resolution but takes too long to get the result and the frequency domain based one on the contrary. Neither of them can optimize frequency resolution and measuring time together. A novel architecture is proposed in this thesis to solve this problem. A 16-bit fractional-N PLL is used as a frequency generator. The critical part is the phase difference detecting circuits combined with the binary search algorithm. Then the PLL output frequency can be set to the resonant frequency quickly thanks to the fact that the phase difference is monotonic and crosses zero at the resonant frequency. Meanwhile, the proposed down mixing operation makes it possible to detect a tiny phase difference in a short time. In a word, the proposed architecture achieves high resolution and fast measurement at the same time and has superior overall performance compared with the state-of-art readout circuits. The readout circuit was implemented and fabricated in 180nm CMOS technology. The SAW was also fabricated and tested together with the CMOS chip. According to the experimental results, the power consumption is about 7mW, the relative frequency resolution is 0.816ppm and the measuring time is 480us. A FOM factor is defined in this thesis to evaluate the overall performance. The FOM of this work is 3pJ/GHz, which is better than the state-of-art readout circuits under the same FOM calculation. To sum up, the proposed readout circuit in this thesis have made a breakthrough in the overall performance of the SAW sensor readout circuits.