Resonance Frequency Readout Circuit for a 900 MHz SAW Device

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.

Equivalence of Open-Loop and Closed-Loop Operation of SAW Resonators and Delay Lines

“…1) Resonator: For a resonant sensor, the power spectral densities of the random phase fluctuations of the sensor and the amplifier are simply weighted by (19) and Eq. ( 15)).…”

“…Such systems not only allow for the detection of both amplitude and phase changes like with a vector network analyzer (VNA), but are also suited for the characterization of the frequency response of the sensor. However, especially due to the needed LO, these systems are often complex [18,19]. In the most common readout structure, the SAW sensor is inserted into the feedback loop of an amplifier, thus forming a closed-loop system in which the oscillating signal is frequency modulated when the phase response of the sensor changes [20,21,22].…”

Analysis of a semiconductor vibration and frequency sensor construction specifity

“…A brief overview of the resonance behavior and frequency tuning principles by varying either the mass or the stiffness of resonators is presented in [5]. A monolithic resonance frequency readout circuit with high resolution and short measurement time is reported in [6].…”

A Three-Dimensional Finite Element Analysis Model for SH-SAW Torque Sensors

“…Ever since the first generation of surface acoustic waves (SAWs) [1] was excited by interdigital transducers (IDTs) in 1965, SAW devices have been widely investigated and used NOW for several decades [2,3,4,5,6]. They are miniature, durable, highly sensitive, reliable, and cost-effective [7].…”