Surface acoustic wave (SAW) radio transponders make it possible to read identification codes or measurement values from a remote location. The decisive advantage of these SAW transponders lies in their passive operation (i.e., no power-supply), and in the possibility of wireless installation at particularly inaccessible locations. The passive SAW transponders are maintenance free. Identification marks respond to an interrogation signal with their nonchanging identification pattern. In wireless SAW sensors the physical or chemical properties to be detected change the propagation characteristics of the SAW. SAW radio transponders are advantageously placed on moving or rotating parts and in hazardous environments such as contaminated or high voltage areas. They also can be used for contactless measurements in high vacuum process chambers, under concrete, extreme heat, or strong radioactive radiation, where the use of conventional sensors is complicated, dangerous, or expensive. In this paper we discuss the principles of wireless passive SAW transponders and present a radio frequency interrogation unit and several passive radio SAW sensors developed for noncontact measurements of temperatures, pressures, torques, and currents.

Theory and application of passive SAW radio transponders as sensors

The evolution of SAW oscillator technology over the past 17 years is described and a review of the current state of the art for high-performance SAW oscillators is presented. This review draws heavily upon the authors' own experience and efforts, which have focused upon the development of a wide variety of SAW oscillators in response to numerous high-performance military system requirements. >

Precision surface-acoustic-wave (SAW) oscillators

Surface acoustic wave (SAW) devices can be turned into novel identification and sensor elements (transponders) that do not need any power supply and may be interrogated wirelessly. Such a transponder picks up an electromagnetic request signal and stores it until all echoes caused by multipath propagation have died away. Then, a characteristic response signal is beamed back to the interrogator unit. In radio-link sensors, a physical or chemical quantity influences the propagation properties of the SAW and consequently changes the response pattern of the device. This contribution surveys the operating principle of such sensors and their state-of-the-art performance. The discussion is supported by illustrative examples such as temperature sensors and sensors for mechatronic applications.

State of the art in wireless sensing with surface acoustic waves

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-based radio sensor systems

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.

SAW devices as wireless passive sensors

An electronic device package is made by first making in a stencil member (11) an opening (12) in the shape of a closed loop that surrounds an inner portion (13) of the stencil member. The closed loop is a continuous open ing except for a plurality of web members (15), each of which extends across the opening to secure the inner stencil portion to the remainder of the stencil member. A glass slurry (7) is forced through the opening of the stencil member onto a first substrate (18) so as to form on the substrate a substantially, closed loop of glass slurry, which is thereafter glazed to form a glass loop (32) bonded to the first substrate. The first substrate is used as cover plate and placed over a second substrate (24) containing an electronic device (23) such that the glass loop surrounds the electronic device and contacts the second substrate along its entire length. The glass is heated sufficiently to soften it and cause it to bond to the second substrate as well as the first substrate. Cooling of the glass loop develops continuous hermetic seals to both first and second substrate, thereby to isolate the electronic device from the external environment.

http://ieeexplore.ieee.org/iel5/10319/32752/01537833.pdf

A New All Quartz Package for SAW Devices

A high performance, miniature SAW resonator oscillator has been designed and fabricated based upon a combination of hybrid electronic circuitry and the novel all quartz packaging technique.111 The entire 401.65 MHz SAW resonator oscillator is contained within a volume of only 0.25 cubic inches and yet state-of-the-art shortand long-term stability performance is possible. Typically the single side band ( S S B ) phase noise to carrier level, &(f), is in the range of -65 to -75 dBc/Hz at f=10 H z carrier offset, while the fractional frequency stability, af/f, is in the range of 8 to ~ O X ~ O ~ ~ for 0.01<r<l seconds. In addition to focusing on minimum oscillator size, technical goals also included minimum oscillator electronic dc power consumption ( ~ 2 0 milliwatts) for a circulating loop power of 0 dBm, or greater. In order to minimize frequency drift over a fifteen minute observation interval, the miniature oscillator’s temperature was stabilized using a thin film heating element in direct contact with the all quartz packaged SAW resonator and the miniature oscillator was then placed within an insulated enclosure.

A Miniature Hybrid Circuit SAW Oscillator Using an All Quartz Packaged Resonator

One of the major advantages of SAW controlled oscillators is their very low level FR4 noise sidebands. However, in many applications where this low noise level is important, the oscillator must operate in an environment with high vibration levels. Evaluating the degradation of noise sidebands due to vibration is very difficult at this time since very little information exists in the literature. Therefore, a number of measurements on rotated Y-cut quartz SAW devices were made to determine the level of noise sidebands in a vibration environment. A number of different substrate-mounting techniques have been evaluated for both delay lines and resonators. The devices were vibrated in each of three perpendicular directions with both random and sinusoidal vibration used. The observed sensitivity to vibration varied by two orders of magnitude for different mounting geometries, with the lowest sensitivities being comparable to that of a well-designed AT-cut bulk wave resonator.

http://ieeexplore.ieee.org/iel5/10281/32714/01534534.pdf

J. Callerame

Papers

A New All Quartz Package for SAW Devices

A Miniature Hybrid Circuit SAW Oscillator Using an All Quartz Packaged Resonator

Sensitivity of SAW Delay Lines and Resonators to Vibration