Compensation for Second-Order Effects in SAW Filter课件.ppt

Chapter 6 Compensation for Second-Order Effects in SAW Filter Scope of this chapter This chapter examines some second-order effects that can degrade Rayleigh-wave filter design, together with some design trade-offs affecting their performance (可適用於leaky-SAW filter). a) IDT finger reflections b) bulk wave interference c) diffraction d) beam-steering e) acoustic attenuation f) TTI g) electromagnetic feedthrough Bulk Waves the Multistrip Coupler All excited IDTs that generate surface waves will also generate bulk waves to some extent. Because bulk waves (with transverse- and longitudinally polarized components) have higher velocities than the surface waves, interference will be most pronouned at the high-frequency end of the filter passband. a) passband distortion b) reduction of out-of-band suppression c) increased insertion loss Bulk Waves the Multistrip Coupler Various fabrication techniques can be employed to reduce the severity of the bulk wave interference. The use of very-thin piezoelectric substrates (0.05cm thick) to reduce the total volume available to bulk waves. Rougher the bottom surface of the substrate to scatter the emitted bulk waves away from the receiver. The use of adhesive materials to the bottom surface to absorb impinging bulk waves. A slot milled into the bottom surface between input output IDTs. Bulk Waves the Multistrip Coupler 但是因為bulk waves can radiate in all directions, none of these techniques will serve to combat those components that travel along the surface with the SAW. 壓電材料產生bulk waves radiation的程度不一 ST-quartz 較 YZ-lithium niobate 較 128°-rotated YX-lithium niobate容易產生。 No hard fast rule exists as to which substrate to use.(此部份需與其他因素考慮) Bulk Waves the Multistrip Coupler As an added complication in some wideband IDT design, it is found that with YZ-lithium niobate the amount of radiated bulk wave power relative to SAW power increase drastically as the number of finger pairs is reduced below about Np