Surface Acoustic Wave Transponder
Surface acoustic wave (SAW) devices are based upon the piezoelectric effect and on the surfacerelated dispersion of elastic (= acoustic) waves at low speed. If an (ionic) crystal is elastically deformed in a certain direction, surface charges occur, giving rise to electrical voltages in the crystal (application: piezo lighter). Conversely, the application of a surface charge to a crystal leads to an elastic deformation in the crystal grid (application: piezo buzzer). Surface acoustic wave devices are operated at microwave frequencies, normally in the ISM range 2.45 GHz.

Electroacoustic transducers (interdigital transducers) and reflectors can be created using planar electrode structures on piezoelectric substrates. The normal substrate used for this application is lithium niobate or lithium tantalate. The electrode structure is created by a photolithographic procedure, similar to the procedure used in microelectronics for the manufacture of integrated circuits.

Figure 3.30 illustrates the basic layout of a surface wave transponder. A finger-shaped electrode structure – the interdigital transducer – is positioned at the end of a long piezoelectric substrate, and a suitable dipole antenna for the operating frequency is attached to its busbar. The interdigital transducer is used to convert between electrical signals and acoustic surface waves. An electrical impulse applied to the busbar causes a mechanical deformation to the surface of the substrate due to the piezoelectric effect between the electrodes (fingers), which disperses in both directions in the form of a surface wave (Rayleigh wave). For a normal substrate the dispersion speed lies between 3000 and 4000 m/s. Similarly, a surface wave entering the converter creates an electrical impulse at the busbar of the interdigital transducer due to the piezoelectric effect.

Individual electrodes are positioned along the remaining length of the surface wave transponder. The edges of the electrodes form a reflective strip and reflect a small proportion of the incoming surface waves. Reflector strips are normally made of aluminium; however some reflector strips are also in the form of etched grooves (Meinke, 1992).

A high-frequency scanning pulse generated by a reader is supplied from the dipole antenna of the transponder into the interdigital transducer and is thus converted into an acoustic surface wave, which flows through the substrate in the longitudinal direction. The frequency of the surface wave corresponds to the carrier frequency of the sampling pulse (e.g. 2.45 GHz, Figure 3.31). The carrier frequency of the reflected and returned pulse sequence thus corresponds with the transmission frequency of the sampling pulse. Part of the surface wave is reflected off each of the reflective strips that are distributed across the substrate, while the remaining part of the surface wave continues to travel to the end of the substrate and is absorbed there.

The reflected parts of the wave travel back to the interdigital transducer, where they are converted into a high-frequency pulse sequence and are emitted by the dipole antenna. This pulse sequence can be received by the reader. The number of pulses received corresponds to the number of reflective strips on the substrate. Likewise, the delay between the individual pulses is proportional to the spatial distance between the reflector strips on the substrate, and so the spatial layout of the reflector strips can represent a binary sequence of digits.

Due to the slow dispersion speed of the surface waves on the substrate the first response pulse is only received by the reader after a dead time of around 1.5 ms after the transmission of the scanning pulse. This gives decisive advantages for the reception of the pulse.

Reflections of the scanning pulse on the metal surfaces of the environment travel back to the antenna of the reader at the speed of light. A reflection over a distance of 100 m to the reader would arrive at the reader 0.6 ms after emission from the reader’s antenna (travel time there and back, the signal is damped by >160 dB). Therefore, when the transponder signal returns after 1.5 ms all reflections from the environment of the reader have long since died away, so they cannot lead to errors in the pulse sequence (Dziggel, 1997).

The data storage capacity and data transfer speed of a surface wave transponder depend upon the size of the substrate and the realisable minimum distance between the reflector strips on the substrate. In practice, around 16–32 bits are transferred at a data transfer rate of 500kbit/s (Siemens, n.d.).

The range of a surface wave system depends mainly upon the transmission power of the scanning pulse and can be estimated using the radar equation. At the permissible transmission power in the 2.45GHz ISM frequency range a range of 1–2m can be expected.