Wireless passive SAW sensors using single-electrode-type IDT structures as programmable reflectors

Sensor information can be read out wirelessly by an antenna if the surface acoustic wave (SAW) delay line is connected to an external classical sensor. This is a passive device also called impedance-loaded SAW sensor. In the last decade, many researchers have made great efforts researching such devices, in which the double-electrode-type interdigital transducer (IDT) has been used as programmable reflector because its short-circuit reflection coefficient is very small. In this paper, the single-electrode-type IDT structure is proposed to be used as programmable reflector. The metallic periodic structure deposited on 128°YX LiNbO3 shows an extraordinary characteristic: for a certain ratio of the film thickness to the periodicity, the reflection of the short-circuited single-electrode-type IDT almost disappears. This characteristic enables great amplitude modulation when the impedance-loaded SAW sensor employs the single-electrode-type IDT as programmable reflector. The fundamental operating frequency of SAW devices using the single-electrode-type IDT structure is twice that of SAW devices using the double-electrode-type IDT structure with the same critical dimension. This means higher operating frequencies for such devices can be reached with the achievable linewidth in current manufacturing technologies.