Resonance Frequency Readout Circuit for a 900 MHz SAW Device

Area of Science:

• Electrical Engineering
• Sensor Technology
• Integrated Circuits

Background:

• Surface Acoustic Wave (SAW) sensors are crucial for various applications requiring precise frequency measurements.
• Existing readout circuits often face limitations in measurement speed and resolution.
• Developing monolithic, high-performance readout circuits is essential for advancing SAW sensor technology.

Purpose of the Study:

• To present a monolithic resonance frequency readout circuit for a 900 MHz RF SAW sensor.
• To achieve high resolution and significantly reduce measurement time.
• To demonstrate a novel frequency searching strategy for enhanced performance.

Main Methods:

• The readout circuit utilizes a fractional-N phase-locked loop (PLL) as the stimulus source.
• A phase-based resonance frequency detection circuit employing successive approximation (SAR) is implemented.
• A new resonance frequency searching strategy leverages the monotonic zero-crossing of the SAW device's phase-frequency response.
• An instant phase difference detecting circuit facilitates rapid SAR operation.

Main Results:

• The circuit was fabricated using 180 nm CMOS technology with a core area of 3.24 mm².
• Experimental results with a 900 MHz SAW resonator (Q=130) show a power consumption of 7 mW from a 1.6 V supply.
• Achieved frequency resolution of 733 Hz and relative accuracy of 0.82 ppm.
• Demonstrated a measurement time of 0.48 ms, the shortest reported in the literature.

Conclusions:

• The developed monolithic readout circuit offers high resolution and the shortest measurement time for 900 MHz RF SAW sensors.
• The novel resonance frequency searching strategy and dedicated phase detection circuit enable fast and accurate measurements.
• This work presents a significant advancement in SAW sensor readout technology, balancing measurement accuracy and speed.