|Friday, 17 November 2006|
Surface acoustic wave (SAW) devices are an important class of piezoelectric devices, providing frequency control, frequency selection, and signal processing capabilities. Surface acoustic wave (SAW) filters use waves propagated on the surface of an elastic solid for electronic signal processing. A typical SAW filter uses a transducer to convert electromagnetic signal waves, which travel at the speed of light, to acoustic signal waves. A SAW device designed to handle complex signal processing functions can offer considerable cost and size advantages over competing technologies. SAW devices are typically implemented on a piezoelectric substrate and usually employ interdigital transducers (IDTs) located on the surface of the piezoelectric substrate to generate and detect acoustic waves. The basic structure of a SAW filter consists of one input and one output interdigital transducers (IDTs) deposited on a piezoelectric substrate. These interdigital transducers function as a transmitter and receiver for the surface acoustic waves. The interdigital transducer can be designed to give the SAW device various characteristics and functions, the filtering function being among the most important. An interdigital transducer includes electrode bus bars, and electrode fingers, extending from each electrode bus bar in an interdigitated configuration. The input and output transducers typically include interdigital electrodes formed on the top surface of the substrate. The shape and spacing of the electrodes determine the center frequency and the band shape of the acoustic waves produced by the input transducer. The amplitude of the surface acoustic waves at a particular frequency is determined by the constructive interference of the acoustic waves generated by the transducers. In operation, the surface acoustic wave filter converts an electrical signal applied to the input transducer into a mechanical surface wave which propogates on or in the substrate surface to the output transducer which then converts the surface wave back into an electrical signal with desired characteristics. The geometry of the interdigital transducers (beam width, pitch, number of fingers) on the piezoelectric substrate plays a significant role in the signal processing and frequency response characteristics of a SAW device. The choice and cut of piezoelectric substrate material from which the SAW filter is constructed and the electrode shape, spacing and location influence the characteristics of the SAW filter.
There’re various types of SAW filters. A transversal SAW filter has an IDT (interdigital transducer) disposed on a surface of a piezoelectric member for selecting a frequency component based on an interaction between a surface acoustic wave propagated through the surface of the piezoelectric member. Transversal filters utilizing a SAW device are generally modeled as an input signal that passes through a series of delays. Transversally coupled multi-mode resonator-type SAW filters, have been widely used as the intermediate-frequency (IF) filters of mobile communication terminals such as cell phone because they are small, low loss, have a narrow-band passband characteristics and also less out-of-band unwanted modes. When it is required to have a relatively broad bandwidth and flat phase characteristics in the pass-band as with IF filters for mobile telephones of code division multiple access (CDMA) system, SAW filters of the transversal type are being used. Reflective SAW filters generally have at least one input transducer, one output transducer and one reflector formed on a piezoelectric substrate. The reflector is typically a reflective grating including spaced apart grid lines defining gaps therebetween. The acoustic waves received by the reflector from the input transducer are reflected by the grid lines within the grating so that the reflected waves constructively and destructively interfere with each other and the wave path is folded. The constructively interfered waves are reflected back to the output transducer having a particular phase. Reflective SAW filters are, therefore, smaller in size and have high frequency selectivity, and thus are desirable for mobile phone communications systems. A SAW resonator filter has a resonator including an IDT provided on a piezoelectric substrate. In an edge reflection type SAW resonator filter, the resonator is provided between two opposite edges of the piezoelectric substrate, and the edges are used to reflect the shear horizontal (SH) waves. Since reflectors are not needed, a compact filter can be realized. SAW filters used in an RF (radio frequency) stage comprise two types: a longitudinally coupled type and a ladder type. The ladder SAW filter has lower loss than a vertical mode SAW filter. The ladder model is formed of a plurality of SAW resonators coupled to each other in a ladder form. The sophistication of the SAW filter depends on the sophistication of each one of the SAW resonators.
SAW filters play a key role in telecommunications. They have been widely used as bandpass and spectrum-shaping filters in mobile and wireless applications. SAW filters are particularly used in communication devices to provide selectivity at various stages of a receiver, such as at the front-end stage or at the IF stage of the receiver. The selectivity of a SAW filter is determined by its bandwidth which is defined as the frequency spectrum limited between the 3 dB points of the filter's frequency response. In recent years, small-size communication devices such as cellular phones have seen widespread use among many users. Such small-size communication devices employ many filters as frequency selecting devices. The surface acoustic wave filter is widely used as a filter in mobile communication apparatuses. SAW filters have been in use for mobile phones because they can contribute to providing compact devices requiring no tuning. SAW filters can be employed as intermediate frequency filters in the reception part of a mobile phone. Surface acoustic wave (SAW) filters for use in mobile phone communications systems are designed to be small in size, exhibit good out-of-bandwidth rejection, and provide narrow bandwidths with steep transition edges. These filters must meet various demands, including providing an adequately broad pass-band, a high edge steepness, and a best possible selection on the smallest possible chip area. SAW filters are also used in the radio-frequency (RF) sections of mobile phone sets, where they have played a significant role in the achievement of improved performance and small size. The type of SAW filter that has been mainly used is a ladder filter employing SAW resonators. A surface acoustic wave filter (SAW) in which a plurality of SAW resonator are arranged to define a ladder circuit is known and referred to as a ladder filter. In addition to its compact size, this type of filter offers the advantages of narrow bandwidth, low insertion loss in the passband, and high attenuation in the adjacent stopbands, and it does not require a matching circuit.