Surface-acoustic-wave (SAW) filters are suitable for a wide range of commercial and military applications. Such lters can be made extremely small and durable, and they provide high rejection of unwanted signals. To learn more about them, a short but concise primer on SAW technology can be found on the Phonon Corp. web site at ( www.phonon.com). The tutorial article explains that there are two different types of SAW lters: transversal and resonator type SAW lters. The transversal type lter is modeled conceptually as a tapped delay line while the resonator type lter is modeled conceptually as an inductive-capacitive (LC) resonator network. The rst type of SAW lter provides nite-impulse-response (FIR) behavior with impulse signals while the second type of lter yields in nite-impulse-response (IIR) functionality for impulse signals. The passband phase response for the rst type of SAW lter is linear, while the passband phase response for a resonator-type SAW lter is nonlinear.
The short article offers that a basic SAW transversal lter is essentially two electromechanical transducers, which transmit and receive acoustic waves. Each transducer is composed of a planar set of periodic interdigital electrodes connected to two bus bars that are connected to an electric generator or load. Each electrode acts as an elementary acoustic source or detector with amplitude determined by electrode length and phase by electrode position. In addition to the electrode, a lter design may employ other electrodes for added impulse response length or to lower insertion loss. The article also details that a SAW resonator lter is essentially an interdigital transducer set between two grating re ectors. The re ectors form a resonant cavity and the transducer coupled the cavity to an external circuit, such as a receiver's electronics. SAW resonator lters are generally much narrower band than transversal lters, and have much lower insertion loss, but have nonlinear phase response and are more limited in shape factor.
By adjusting the length and position of the electrodes, a SAW lter designer can achieve almost any nite impulse response. For frequency-domain SAW lters, for example, FIR digital lter design techniques can be used to help optimize the performance for the shortest time response. In the time domain, the time-response sampling frequency may be used as a design parameter, often being chosen as equal to four times the center frequency. The width of the electrode is generally about one-half the period, or about oneeighth of an acoustic wavelength. Phonon's literature provides an iterative process for nding the optimum electrode length for a desired operating frequency. It also cautions to be aware of secondary effects, such as re ections inside the electrodes, and the in uences of the substrate and package materials. For speci ers considering SAW lters for a system-level design, the short literature serves as a useful introduction to SAW lter technology. Phonon Corp., 90 Wolcott Rd., P. O. Box 549, Simsbury, CT 06070; (860) 651-0211; Fax: (860) 651-8618; Internet: www.phonon.com.