Combination of SAW-transponder and RF-semiconductor devices offers new perspectives for passive radio sensors

SAW ID-tags can be employed as passive transponders for impedance sensors. An interdigital transducer (IDT) is used as a loadable reflector, other fixed reflectors are used as reference to compensate cross sensitivities. A varactor diode is applied as capacitive sensor to measure electric voltage with extremely high impedance with minor effect to the measurand. The voltage and consequently the load of the programmable reflector influences both, the phase and amplitude of the reflected acoustic wave and the response signal of the radio sensor. The principle is discussed, the tuning for an application to measure force with a piezoelectric cable is presented.