Implementation and Investigation of the Operation of a SAW Sensor in the Feedback Loop of Sinusoidal Oscillators

SAW sensors find widespread use across various measured quantities, especially in biological or chemical applications. Measurements with SAW usually happen through the open-loop approach, which requires a vector network analyzer to measure the phase and magnitude of the output signal. These devices do not allow automation and integration in multi-sensor systems. An alternative approach called closed-loop offers reduced cost and complexity. In this method, only the frequency is to be observed. Closed-loop circuits implement SAW sensors in the feedback loop of sinusoidal oscillators. Doing so is still a challenge due to the lack of reliable oscillator topologies that resist perturbations in the sensor's surface. In this paper, we design a closed-loop SAW measurement circuit that effectively maintains oscillation when the SAW sensor is included in the closed loop. We compare the inclusion of SAW equivalent models in the feedback loop of our oscillator. We discuss their differences to investigate which SAW model that most accurately represents the response of the real sensor. Our results confirm the effectiveness of our oscillator as part of a sensor, shifting > 100 kHz when the SAW device is perturbed with a drop of water.