Design and Evaluation of a Low-Cost SAW Resonator Read-Out System at 2.4 GHz

The functionality of using surface acoustic wave (SAW) resonators in wireless sensing applications has been demonstrated in many publications. When used, their resonance frequency has to be determined with a precision of up to 10 ppm with measurement periods in the microsecond range. For this purpose, the so-called instantaneous frequency measurement (IFM) can be used, which uses a six-port receiver to measure a frequency-dependent phase delay. This work presents a system that incorporates an alternative low-cost circuitry that multiplexes the four six-port outputs on only one detection circuitry with the help of a radio frequency (RF) switch. This reduces the system cost and eliminates the measurement errors otherwise caused by differing gain and group delay values in the baseband filters. Based on this idea, a complete SAW readout system is presented and experimentally evaluated, which also can be used as a sensor node as it incorporates a Bluetooth interface. The shown measurement results demonstrate that the system can achieve a precision of 7.9 ppm concerning frequency measurement at an update rate of 41.7 Hz, demonstrating the feasibility of the alternative detection circuit.