Resonance Frequency Readout Circuit for a 900 MHz SAW Device.

Heng Liu,Chun Zhang,Zhaoyang Weng,Zhihua Wang,Yanshu Guo

doi:10.3390/s17092131

Abstract

A monolithic resonance frequency readout circuit with high resolution and short measurement time is presented for a 900 MHz RF surface acoustic wave (SAW) sensor. The readout circuit is composed of a fractional-N phase-locked loop (PLL) as the stimulus source to the SAW device and a phase-based resonance frequency detecting circuit using successive approximation (SAR). A new resonance frequency searching strategy has been proposed based on the fact that the SAW device phase-frequency response crosses zero monotonically around the resonance frequency. A dedicated instant phase difference detecting circuit is adopted to facilitate the fast SAR operation for resonance frequency searching. The readout circuit has been implemented in 180 nm CMOS technology with a core area of 3.24 mm2. In the experiment, it works with a 900 MHz SAW resonator with a quality factor of Q = 130. Experimental results show that the readout circuit consumes 7 mW power from 1.6 V supply. The frequency resolution is 733 Hz, and the relative accuracy is 0.82 ppm, and it takes 0.48 ms to complete one measurement. Compared to the previous results in the literature, this work has achieved the shortest measurement time with a trade-off between measurement accuracy and measurement time.

Highlights

The surface acoustic wave (SAW) sensor is a promising multi-functional sensor for pressure [1], mass [2] and temperature [3] measurement in Internet-of-everything (IoE), labs-on-a-chip, biomedical applications [4,5,6,7,8,9]
The one-port SAW device in the experiment was fabricated on a LiNbO3 /SiO2 /Si substrate based on the ion implantation and the wafer bonding
The proposed chip and the SAW device are on the test printed circuit board (PCB), and the PCB is controlled by a MCU board which is further connected to PC for data collection

Summary

Introduction

The surface acoustic wave (SAW) sensor is a promising multi-functional sensor for pressure [1], mass [2] and temperature [3] measurement in Internet-of-everything (IoE), labs-on-a-chip, biomedical applications [4,5,6,7,8,9]. A SAW device is composed of a piezoelectric substrate and inter-digital transducers (IDT) deposited on the substrate [10]. A resonance frequency readout circuit is required to convert the sensor resonance frequency, which is correlated to the specific physical parameter of the SAW device under measurement, into digital numbers for further processing. MEMS resonance sensors operate with frequencies in the MHz range [11,12,13,14,15,16], while some others work at a few GHz [17,18]. The resonance frequency readout circuit has been designed for a 900 MHz one-port SAW device presented in [19] which has a trade-off between the power consumption and the sensitivity consistency. The one-port SAW device in the experiment was fabricated on a LiNbO3 /SiO2 /Si substrate based on the ion implantation and the wafer bonding

Methods

Results

Conclusion