Gerd Scholl

SAW-based radio sensor systems

Surface acoustic wave (SAW) devices can be used as identification and sensor elements (SAW transponders) for mea- suring physical quantities such as temperature, pressure, torque, acceleration, humidity, etc., that do not need any power supply and may be accessed wirelessly. The complete wireless sensor system consists of such a SAW transponder and a local radar transceiver. An RF burst transmitted by the radar transceiver is received by the antenna of the SAW transponder. The passive transponder responds with an RF signal—like a radar echo—which can be received by the front-end of the local transceiver. Amplitude, frequency, phase and time of arrival of this RF response signal carry information about the SAW reflection and propagation mechanisms which in many cases can be directly attributed to the sensor effect for a certain measurand. Usually no intersymbol interference (ISI) due to environmental echoes occur, due to the high delay time of the SAW transponder in the order of some s. The present work reviews the operating principles of such sensor systems and their state-of-the-art performance by way of some examples which include the wireless measurement of temperature, pressure, torque, acceleration, tire-road friction, magnetic field, and water content of soil. Index Terms—Acceleration, local radar transceiver, magnetic field, passive SAW transponder for sensing, pressure, temper- ature, tire-road friction, torque, water content of soil, wireless measurement.

SAW devices as wireless passive sensors

Surface acoustic wave (SAW) radio sensors make it possible to read measurement values from a remote location. The decisive advantage of these SAW sensors lies in their passive operation with no need for a separate power supply, and in the possibility of wireless installation at particularly inaccessible locations. The passive SAW sensors are maintenance free. The physical or chemical properties that shall be detected change the propagation characteristics of the SAW. In this paper we compare the sensitivity of different types of SAW sensors. These are resonators, reflective delay lines, and dispersive structures. Examples for several applications are presented.

Wireless passive SAW sensor systems for industrial and domestic applications

The authors discussed the principle of wireless sensing with passive SAW devices and investigated binary SAW sensors and other SAW radio sensors for the non-contact measurement of temperatures, pressures and torques. An RF identification system for the Munich subway, a SAW sensor system for the diagnosis and control of electric motors and a temperature monitoring system for high voltage surge arresters are the applications which were presented. Finally a low-cost radar unit for domestic applications was described.

Efficient analysis tool for coupled-SAW-resonator filters

The advantages of the coupled-mode (COM) formalism and the transmission-matrix approach are combined in order to create exact and computationally efficient analysis and synthesis tools for the design of coupled surface acoustic wave resonator filters. The models for the filter components, in particular gratings, interdigital transducers (IDTs) and multistrip couplers (MSCs), are based on the COM approach that delivers closed-form expressions. To determine the pertinent COM parameters, the COM differential equations are solved and the solution is compared with analytically derived expressions from the transmission-matrix approach and the Greens function method. The most important second-order effects, such as energy storage, propagation loss, and mechanical and electrical loading, are fully taken into account. As an example, a two-pole, acoustically coupled resonator filter at 914.5 MHz on AT quartz is investigated. Excellent agreement between theory and measurement is found.<<ETX>>

New resonator filter with high sidelobe suppression

A new resonator filter for IF applications which is superior to conventional approaches is reported. The design places acoustically coupled resonator cascades between two weighted single-phase unidirectional transducers (SPUDTs). This allows narrow bandwidth and small chip size, high sidelobe suppression, moderate group delay ripple and low loss at the same time. For full custom design sophisticated CAD tools are necessary. A filter was built at 225.0 MHz with a chip size smaller than 10 mm in a surface mounted device (SMD) package. The fractional bandwidth is 0.14%, sidelobes are suppressed more than 40 dB, the group delay ripple is smaller than 0.5 mu s, and the insertion loss is 4.5 dB.<<ETX>>

Novel concepts for GaAs/LiNbO/sub 3/ layered systems and their device applications

Thin semiconductor quantum well structures fused onto LiNbO/sub 3/ substrates using the epitaxial lift-off (ELO) technology offer the possibility of controlling the surface acoustic wave (SAW) velocity via field effect. The tunability of the conductivity in the InGaAs quantum well results in a great change in SAW velocity, in general, accompanied by an attenuation. We show that an additional lateral modulation of the sheet conductivity reduces the SAW attenuation significantly, enhancing device performance. At high SAW intensity the bunching of electrons in the SAW potential also leads to a strong reduction of attenuation. These effects open new possibilities for voltage-controlled SAW devices. We demonstrate a novel, wireless, passive voltage sensor, which can be read out from a remote location.

Efficient design tool for SAW-resonator filters

The advantages of the coupling-of-modes (COM) formalism and the transmission-matrix approach are combined to create exact and computationally efficient analysis and synthesis CAD tools for the design of SAW-resonator filters. The models for the filter components, especially gratings, interdigital transducers (IDTs). and multistrip couplers (MSCs), are based on the COM approach, which delivers closed-form expressions. In order to determine the relevant COM parameters, the integrated COM differential equations are compared with analytically derived expressions from the transmission-matrix approach. The most important second-order effects such as energy storage, propagation loss and mechanical and electrical loading are fully taken into account. As an example, the authors investigate a two-pole, acoustically coupled resonator filter at 914.5 MHz on AT quartz. Excellent agreement between theory and measurement is found.<<ETX>>

A new SAW-resonator with electronically switchable resonance frequency

A new two-port SAW-resonator is presented. Its transmission frequency can be switched between two discrete resonances by changing the electrical termination of additional transducers within the resonators cavity. This can be done by mechanical switches or by FETs which allow for electronical switching with low power consumption. The device can be applied to provide a simple change of a transmitters carrier frequency. It may be also used for low-cost, low-bit-rate frequency shift keying transmission systems. Extending the principle to more than two resonances a switchable oscillator for channel selection can be realized. In this case an advantageously low oscillators phase noise is achieved. This is due to the high quality factor of SAW-resonators.

Feedback-controlled compensation of parameter fluctuations of mobile radio SAW IF channel filters

Surface Acoustic Wave (SAW) filters with high sidelobe suppression, steep passband skirts and small chip sizes are key components for channel selection in modern mobile communication systems. These filters are fabricated on highly stable quartz substrates with precise photolithographic techniques. However, unavoidable fabrication tolerances, temperature drift and aging require some design reserve for achieving a specific filter characteristic. We report on a new receiver design which overcomes these drawbacks and does not demand a design reserve, thus improving the filter performance drastically. As an essential part of this receiver we developed a new SAW device.<<ETX>>