Surface Acoustic Wave Devices

Abstract

A surface‐acoustic‐wave (SAW) device employs an acoustic wave guided along the surface of a piezoelectric crystal, in which the stresses and strains of the wave are coupled to electric fields. The wave has a low velocity, typically 3500 m/s, and propagates with low attenuation. These features enable compact low‐loss devices to be developed for electronics applications at frequencies up to several gigahertz. Arrays of metal electrodes are used to generate, detect, or reflect the waves. This technology has enormous versatility because standard lithographic techniques can be used to fabricate almost arbitrary geometries. This article summarizes the nature of surface waves and some relevant propagation effects, including diffraction. The interdigital transducer, used in all these devices for wave generation, is described. A survey of suitable materials is given, including quartz, lithium niobate, and lithium tantalate. Transversal bandpass filters are weighted using apodisation or withdrawal weighting. They are widely used in TV receivers, as well as in professional equipment which calls for very high‐performance devices. For radar, SAW pulse compression filters perform a correlation operation, increasing the range capability. Other devices perform correlation for spread‐spectrum communication waveforms; an example is the SAW nonlinear convolver, which is capable of instantaneous programmability. For mobile telephony there has been a strong demand for miniature low‐loss bandpass filters, and a wide variety of novel SAW devices have been developed. For example, impedance element filters give 1 dB loss at 900 MHz, with 30 MHz bandwidth and a package size of 2×2 mm 2 . For IF CDMA filters, a modified form of the transversal filter, called the R‐SPUDT (resonant single‐phase unidirectional transducer), enables both low loss and small size to be obtained. For GSM IF circuits, transverse‐coupled resonator filters have been developed to give a very narrow bandwidth of 200 kHz. The article concludes with a superficial survey of the theory of these devices.