SPUDT cells including two fingers are only known thus far for so-called NSPUDT directions. In that case, usual solid-finger cells are used. The purpose of the present paper is to find SPUDT cell types consisting of two fingers only for pure mode directions. Two-finger (TF) cells for pure mode directions on substrates like 128°YX LiNbO(3) and YZ LiNbO(3) were found by means of an optimization procedure. The forward direction of a TF-cell SPUDT on 128°YX LiNbO(3) was determined experimentally. The properties of the new cells are compared with those of conventional SPUDT cells. The reflectivity of TF cells on 128°YX LiNbO(3) turns out to be two to three times larger than that of distributed acoustic reflection transducer (DART) and Hanma-Hunsinger cells at the same metal layer thickness.
The present paper describes a new filter type, slanted finger interdigital transducer (SFIT) that allows fast analysis as a precondition for fast optimization. Therefore, filter structure and analysis are especially suitable for optimizing unidirectional SFIT (USFIT) filters. For analysis, a SFIT filter is usually divided into many narrow channels. Every channel is considered to be a subfilter and is analyzed separately. Therefore, the total filter analysis is very time consuming. For reducing computing time, we suggest a one-focus structure. In contrast to conventional SFIT filters, not only the finger and gap widths but also the spaces between transducers of different channels differ by one and the same scaling factor. As a consequence, all prolonged finger edges of both transducers intersect in one point called focus. As a result, the parameters of all subfilters can be calculated from the parameters of only one subfilter by simple frequency scaling. Consequently, the total time for analysis is essentially reduced. However, one-focus SFIT filters with continuous finger edges show a deep minimum within the passband. This problem can be overcome by using stepped one-focus structures.
The basic types of SPUDT cells like DART, EWC, Hanma-Hunsinger, FEUDT and TF include quite narrow fingers and gaps. For application in GHz SAW devices these fingers are too narrow to implement the electrode structures in the required quality by means of the available technologies. Solutions of SPUDT cells with sufficiently wide fingers and gaps on YZLiNbO 3 and 128ºYXLiNbO 3 with 3, 4, 5 fingers and with lengths of 2, 3, 3 wavelengths (in the same order) are presented. The cells have been found by means of an optimization procedure. Due to bulk wave generation there is a relatively big loss. This can be a problem if the SPUDTs are too long. In order to achieve a sufficient unidirectional effect in spite of the small number of fingers big reflection factors per wavelength are needed. The experimental study of a cell type with five fingers on YZLiNbO 3 confirms the existence of a strong unidirectional effect pointing really to a big reflection factor per wavelength.
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