Two-Port Network Model of Fluid-Loaded SAW Delay-Line Sensors

Surface acoustic wave (SAW) devices usually come in the form of inter digital transducers (IDTs) deposited on a substrate. In addition to being used as filters and delay lines in communication systems, their use nowadays expands to various sensor and actuator applications [1,2]. There are two different ways to employ a SAW device as a sensor. The first is the propagative signal type and the second is the reflective signal type. The output response of a propagative signal type can be predicted by an electro-acoustical equivalent circuit, which is comprised of input/output IDT admittances and the substrate delay line admittance. In a propagative signal type, the main objective is to sense the change in load impedance that is attached to the output IDT [3]. The reflective type is mainly used in bio/chemical applications, and measures the reflected signal due to the electrical impedance or admittance change by fluid loading at the delay line of the substrate [4,5]. Up to now, studies on reflective signal types have heavily focused on the experimental side, and little theoretical work has been done. In this paper, we consider a delay-line type SAW sensor used in fluid loading conditions, and propose an equivalent circuit model that can predict the overall response of the SAW sensor.© 2011 ASME