This paper describes useful design techniques for wide-band linear phase SAW filters using slanted finger interdigital transducers (SFITs): to increase stop band attenuation, to flatten the pass band response, and to draw SFIT patterns for photo-masks. In order to increase stop band attenuation, a withdrawal-weighted SFIT is employed. A combination of a withdrawal-weighted SFIT and an unweighted SFIT provides good stop band responses, even though the number of finger pairs is small. In order to achieve a desired amplitude response in the pass band, an aperture-weighting technique for divided channels of SFITs is presented. A design method for drawing SFIT patterns for photo-masks is also described. A linear-phase SAW filter using SFITs with a 50% fractional bandwidth is designed and fabricated on 128/spl deg/ y-x LiNbO/sub 3/. A sidelobe suppression of 40 dB and a 3 dB to 40 dB shape factor of 1.12 were realized. Good agreement between theoretical and experimental results is obtained.

Design techniques for SAW filters using slanted finger interdigital transducers

Withdrawal weighted (WW) fan-shaped interdigital transducers (FIDT) are used to design surface acoustic wave (SAW) filters with a good shape factor, a flat passband, and an out-of-band rejection of 50 dB and more. Utilization of "stepped" actions as a weighting action for FIDT's ensures a simple filter structure synthesis procedure that does not take a long time on a personal computer. Three modifications of SAW filters with 2% bandwidth, in-band ripple of 0.1 dB, and out-of-band rejection of 50-55 dB have been fabricated on ST-quartz. >

Withdrawal weighted fan-shaped SAW transducers

This paper presents a study on using the coupling‐of‐model (COM) model to analyze wide‐band SAW filters with slanted finger interdigital transducers (SFIT). In contrast to other design methods, the COM model is more convenient and more simple to use for designing different types of electrodes by just changing the elements of a P‐matrix. In this paper, we give a complete description of how to use the COM approach to analyze SFIT SAW filters and show a comparison of simulated and experimental frequency responses of the SFIT SAW filter. Two SFIT SAW filters with 30% bandwidth, 60 MHz center frequency, and different numbers of SFIT pairs were designed. On the experimental side, SFIT SAW filters using YZ‐LiNbO3 as the substrate material were fabricated and the on‐wafer frequency responses were measured. Comparisons made between the simulated and measured frequency responses showed good agreement.

Analysis of sfit saw filters using coupling of modes model

/pdf/a-review-controlling-the-propagation-of-surface-acoustic-3ier2d1031.pdf

A review: controlling the propagation of surface acoustic waves via waveguides for potential use in acoustofluidics

For analysis, slanted-finger interdigital transducers (SFITs) are usually divided into many channels in parallel to the propagation direction. Every channel is considered to be a subfilter. This procedure neglects the diffraction completely. This paper introduces the diffraction in the SFIT analysis by use of the angular spectrum of plane waves (ASPW) approach. The analysis is based on a one-component bulk-wave model. The integrals of an ASPW are carried out by summing up M wave components with various wave vectors. Two wave modes propagating toward both forward and backward directions within SFIT are assumed to take the coupling effect into account. The amplitudes of both wave modes are expressed by M-dimensional vectors. By means of the boundary conditions at every finger edge, the transfer matrix and transduction vector are found. As a result, coupling-of-modes-like equations that link wave components at adjacent finger and gap regions are obtained. By matrix-matrix and matrix-vector multiplication, the complex amplitudes of all the wave components in all the finger and gap regions are calculated. These amplitudes yield the total wave field including reflection at all fingers. The wave field yields the piezoelectric part of the input and output transducer currents as functions of the filter input and output voltages representing the piezoelectric part of the Y matrix. The model is used for analyzing a unidirectional SFIT filter, the transducers of which are composed of SPUDT cells on 37/spl deg/ YX quartz. The simulated transmission behavior is compared with experimental results. Good agreement is found. Especially for the case of a large slanted angle, the profound deformation appearing within the passband and the high-frequency transition band can be clearly identified as the most important influence of the diffraction on the transmission behavior.

Diffraction analysis of slanted-finger interdigital transducers

Results of the investigations of 50% wideband surface acoustic wave (SAW) filters with 70-MHz central frequency and multichannel filters with weighted tapered interdigital transducers (TIDTs) are presented. Computer results are given for different constructions of SAW filters with TIDTs to which variable metallization ratio, apodization, capacitance, or withdrawal weighting techniques are applied. In tested filters stopband rejection from 31 to 42 dB is obtained with relative accuracy of 1 dB within passband. Frequency responses of reflected signals and SAW amplitude profiles launched by TIDTs as a function of frequency and distance are given. Computer and experimental results of a 10-channel filter are presented. >

J. Lamperski

Papers

SAW filters with weighted tapered interdigital transducers