C.C.W. Ruppel

Mechanical sensors based on surface acoustic waves

Abstract A survey is given of SAW sensor systems. Three categories of sensors are defined as transit-time, resonator and chirp-type devices and each category is subdivided into active and passive signal-processing systems. In these, the mechanical change of the SAW device is evaluated by difference measurements of the transit time, phase or resonance frequency, which are described quantitatively. To show the high resolution of active sensor systems, an electronic spirit level (based on SAW resonators) is described. New SAW chirp sensors allow the dispersive interdigital transducer or reflector geometry to be adjusted to the sensor system resolution and measurement range. In passive systems the sensor device is connected to an antenna. The SAW is excited by an interrogating radio impulse. After a SAW transit time, the sensor radiates back to the interrogator an impulse containing the sensors information. The time and frequency response of all sensor categories and the possibilities of temperature compensation are discussed.

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.

Review of models for low-loss filter design and applications

The most frequently used models for surface acoustic wave (SAW) devices are the impulse model, the equivalent circuit models, the Coupling-of-Modes model, and the matrix models. While the impulse-model is only a first order model the other models include second order effects, e.g. reflections, dispersion, and charge distribution effects. The influence of diffraction and refraction on the transfer function of a SAW filter can be described by the angular spectrum of straight-crested waves model. A survey of these different models will be given. The simulation of low-loss filters requires flexible analysis tools, which can cope with different geometries and substrates. Operating with a parameter set, which depends only on the substrate crystal and not on the specific geometry of the SAW filter, is advantageous. Due to the high insertion attenuation of conventional transversal filters the requirements on the accuracy of the analysis are focused on S21, whereas for low-loss filters all elements of the S-matrix are important. The comparison of simulations with a P-matrix model, which fulfills the above mentioned prerequisites, and measurements of different types of low-loss filters, e.g. SPUDT, DMS, and transverse-mode coupled resonator filters are presented

Spread spectrum communications using chirp signals

We report on the use of broadband chirp signals for spread spectrum systems in indoor applications. The presented system concepts make use of chirp transmission and pulse compression. Different modulation schemes for the chirp signals resulting in different system performance and complexity are compared in terms of bit error rate for the AWGN channel and for frequency selective indoor radio channels. We present simulations and measurement results from demonstrator systems which use surface acoustic wave (SAW) devices for the generation and matched altering of the chirp signals. The RF and IF frequency and transmission bandwidth of the presented systems are 2.4 GHz, 348.8 MHz, and 80 MHz, respectively. Due to the processing gain of 16 dB-made possible by the use of SAW devices-and the large transmission bandwidth the system is insensitive against frequency selective fading, CW interference and noise.

Universal Method for Compensation of SAW Diffraction and Other Second Order Effects

All known methods for diffraction compensation replace the unapodized transducer by a single tap. T his assumption neglects variations of the SAW beam profile over the length of the unapodized transducer and leads to unsatisfying results of the compensation procedure in many cases of interest. Our compensation algorithm computes corrections to the taps of the apodized transducer from the deviations of the analysed frequency r esponse (including second order effects) from the desired response, using a quasi-Newton-opt imization method which takes the unapodized transducer into consideration. This method was successfully applied to remove diffraction e ffects from analysed and measured frequency r esponses, however, the same method can be applied to other second order effects as well.

SAW-based radio sensor systems for short-range applications

Wireless autonomous surface acoustic wave (SAW) sensors offer highest flexibility for modern sensor systems. Because no battery or wiring is required for power-supply and communication tasks they can advantageously be employed for nearly all kinds of short-range identification and measurement applications, where the use of conventional sensors is comfortable, impossible or dangerous, e.g. on moving or rotating parts or in industrial process chambers. In the present work the basics of SAW-based radio sensor systems are reviewed and different examples out of a manifold of possible applications will be given.

Optimum Design of SAW-Filters by Linear Programming

A method for designing the geometry of SAW bandpass filters with arbitrary passband and group delay response is presented. The 6-function model describes the relation between filter geometry and\ frequency response. (No second order effects are included in the design procedure). The design problem is defined so that the method of linear programming can be used. The geometry is determined by optimizing the stop band rejection under the restriction of a fixed filter length. This method allows the design of filters with smooth amplitude and phase response in the passband.

Properties of radio frequency Rayleigh waves on langasite at elevated temperatures

The properties of Rayleigh waves operating both at radio frequency (RF) and at high temperatures (HT) on langasite crystal cuts with Euler angles of (0/spl deg/, 138.5/spl deg/, 26.6/spl deg/) and (0/spl deg/, 30.1/spl deg/, 26.6/spl deg/) are investigated. By evaluating the frequency response of surface acoustic wave (SAW) test devices the SAW velocity, the coupling factor, and the propagation attenuation have been determined experimentally in a temperature range from 20/spl deg/C to 500/spl deg/C. The SAW devices could be operated up to frequencies of 3 GHz. However, considering a future sensor application a limiting factor is a strong increase of the acoustic losses in conjunction with frequency and temperature.

Passive radio requestable SAW water content sensor

Using a SAW reflective delay line, a new passive sensor for a remote measurement of water content in sandy soil was built. The system operates in the European 434 MHz ISM-band and consists of a request unit and a passive sensor connected by a radio link to the request unit. The SAW device is fabricated on LiNbO3, YZ-cut, and mounted and sealed in a standard DIL 16 package. The device includes four electroacoustic transducers (IDTs), three of them being used as reflectors. One transducer is connected to an external antenna to pick up the RF request signal of the request unit and to radiate back the RF response. Two reflectors are used for reference purposes. The bus bars of the median reflector are connected to an electrical transmission line with two rods added to their end. The rods are put into sandy soil. Together, the transmission line and the two rods with the sand-water-mixture between them have a characteristic impedance Zload, which terminates the second transducer and affects the amplitude and phase of the acoustic reflection. A change of the water content of the soil changes Zload due to the high permittivity of free water. The amplitude and phase of the acoustic reflection at the second transducer changes due to a variation of the terminating Zload, which appears in a different attenuation and phase of the corresponding peak in the time domain. Thus the RF response of the sensor carries the information of the water content between the rods, which therefore can be detected and evaluated in the request unit.

Spectrum shaping SAW filters for high-bit-rate digital radio

The structure of a QAM radio system is explained. Some novel designs that demonstrate the current status of spectral shaping SAW (surface acoustic wave) filters are presented. Compensation of second-order effects is discussed. Experimental results for a filter for a 140-Mb/s 16 QAM cochannel system with a roll-off factor of 0.19 are shown. Two other examples of filters for a 140-Mb/s, 64 QAM system with a 35% relative bandwidth are also presented.<<ETX>>