Abstract
In this paper the different classes of surface acoustic wave devices are briefly reviewed and examples of the state of the art are given. The fabrication techniques and the application of SAW components are also discussed.
INTRODUCTION
Elastic surface waves were first theoretically predicted by Lord Rayleigh at the end of the last century and they have been studied by geophysicists since then. It is only about 15 years ago that electronic engineers became interested in the application of Surface Acoustic Waves (SAW) to electronic signal processing devices. During those 15 years SAW technology has experienced a tremendous growth. As will be discussed in this paper, this can be attributed to a number of unique features of SAW devices, but the single most important factor has probably been the planar character of SAW technology. In this age of microelectronics, this compatibility with planar integrated circuit fabrication methods, has made possible the very rapid advances in the performances of SAW devices. It is the aim of this paper to give an overview of the state of the art in the various parts of the SAW technology. First the basic principles of SAW propagation and transduction will be briefly reviewed. Next transversal filters, perhaps the best known SAW components, will be discussed. The coded time domain devices, widely used in communications equipment and electronic warfare, are described in the following section. One of the latest and most promising developments in SAW components, namely the resonators, are analysed in Section 5, while the fabrication techniques for SAW devices are discussed in the last part of this paper. 
SAW TRANSVERSAL FILTERS
A transversal filter consists of a delay line containing taps at regular intervals T. Those taps are summed through a set of weighting networks a n to form the output of the transversal filter ( Figure 4 FIGURE 6 Transversal filter with multistrip coupler.
operation, simplicity of fabrication, and reliability due to the fact that it is a passive structure. In SAW transversal filters, weighted taps are usually realised by finger pairs with varying finger overlaps a n ( Figure 5 ). 14 The pulse compression filter for chirp radar has been, and probably still is the foremost system application for SAW components. Time-bandwidth products of the order of 1000 can be realised by means of dispersive interdigital transducers. Even higher time-bandwidth products, up to 5000, can be obtained with reflective array compressors. 16 As shown in Figure 9 the dispersive elements consist of Quartz crystal resonators, using acoustic bulk waves, are very well known components in the electronic industry. Those resonators are based on the total reflection of bulk waves on the two parallel surfaces of the crystal and are limited for the fundamental mode of operation to frequencies below 50 MHz. In order to make SAW resonators it is necessary to have perfect reflectors for surface waves. It is not possible to reflect completely a surface wave on a sharp discontinuity since in that case, mode conversion into bulk waves will always take place. A solution to that problem lies in the use of distributed reflectors. A grating consisting of small surface discontinuities such as metal strips or grooves and with a periodicity of half a wavelength acts as a perfect mirror for surface waves without any mode conversion into bulk waves. 8 A resonating cavity for surface waves is therefore formed by depositing two distributed reflectors, spaced by the right distance, on the surface of the crystal. 9 Coupling with this SAW resonator is achieved by putting an IDT in the cavity spacing (Figure 10a ). The impedance characteristic of the classic equivalent circuit (Figure 10b) 

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