Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 147

x

z

y

Figure 4.91 The section through a crystal shows the surface distortions of a surface wave
propagating in the z -direction (reproduced by permission of Siemens AG, ZT KM, Munich)

Finger period p

Section A-B

A B

Piezoelectric substrate

Electric period q

(a) (b)

Figure 4.92 Principal structure of an interdigital transducer. Left, arrangement of the fin-
ger-shaped electrodes of an interdigital transducer; right, the creation of an electric field between
electrodes of different polarity (reproduced by permission of Siemens AG, ZT KM, Munich)

Interdigital electrode structures in the form of interleaved fingers make effective
electroacoustic transducers. Each pair of such interleaved fingers (Figure 4.91) form a
so-called interdigital transducer (Latin digitus = finger, inter = between). A δ -shaped
electrical pulse applied to the busbar of an interdigital transducer results in a mechan-
ical deformation at the surface of the substrate between fingers of different polarity
due to the piezoelectric effect. This deformation is proportional to the electrical field
and propagates as a surface wave in both directions at velocity v (see Figure 4.92).
Conversely, a surface wave entering the converter generates a signal proportional to