Bernd Steiner

Two-finger (TF) SPUDT cells [Correspondence]

SPUDT cells including two fingers only are hitherto known for so-called NSPUDT directions. In this 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. Moreover, SPUDT cells composed of two fingers suited to complete imperfect NSPUDT behavior or to compensate that are to be found. TF cells for pure mode directions on substrates like 128degYX LiNbO3, YZ LiNbO3 and STX quartz were found by means of an optimization procedure. The forward direction of a TF-cell SPUDT on 128degYX LiNbO3 was determined experimentally. The properties of the new cells are compared with those of conventional SPUDT cells. The reflectivity of TF cells on 128degYX LiNbO3 turns out to be two up to three times larger than that of DART and Hanma-Hunsinger cells at the same metal layer thickness. Furthermore, properties of TF cells on STX and SST quartz are reported. Examples for completing and compensating imperfect NSPUDT behavior are given.

SAW filters including one-focus slanted finger interdigital transducers

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.

Design of SPUDT filters on quartz considering waveguide modes

This paper presents an innovative method to achieve maximum performance in SPUDT filters on quartz under particular consideration of waveguide modes. Waveguide modes are known to have a possible strong impact on pass band and transition band performances of SPUDT filters if more than one waveguide mode exists. Consequently, a standard approach is to create a single mode design to prevent it. In some cases, however, it may be advantageous to design a SPUDT filter using several waveguide modes. A patch for the COM theory to analyse the waveguide modes is presented. This method is then used to evaluate possible filter performance degradations caused by parasitic waveguide modes. By means of simulated and experimental results of some filter examples, filter performance is shown which may be achieved by considering or using waveguide modes.

Two-finger (TF) spudt cells

SPUDT cells including two fingers only are hitherto known for so-called NSPUDT directions. In this 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. Moreover, SPUDT cells composed of two fingers suited to complete imperfect NSPUDT behavior or to compensate that are to be found. TF cells for pure mode directions on substrates like 128degYX LiNbO3, YZ LiNbO3 and STX quartz were found by means of an optimization procedure. The forward direction of a TF-cell SPUDT on 128degYX LiNbO3 was determined experimentally. The properties of the new cells are compared with those of conventional SPUDT cells. The reflectivity of TF cells on 128degYX LiNbO3 turns out to be two up to three times larger than that of DART and Hanma-Hunsinger cells at the same metal layer thickness. Furthermore, properties of TF cells on STX and SST quartz are reported. Examples for completing and compensating imperfect NSPUDT behavior are given.

Optimising slanted-finger interdigital transducer (SFIT) filters

This paper presents an innovative design method for achieving a maximum performance in slanted-finger interdigital transducer (SFIT) filters by optimising certain parameters. This will lead to an improved performance as for example the mechanical reflections will not be only utilised to make the IDTs of the SFIT filter unidirectional but also to improve the filter response. For instance, passband ripple problems caused by triple transit echo may be prevented. Moreover, the skirt steepness may be increased without increasing the number of finger groups. In addition, the chip size can be reduced. This method will be compared with conventional design methods. Simulated and experimental results of some filter examples help to show the performance that can be achieved by this optimisation method. A fast analysis, which is necessary for optimising, has been developed. The method, which is based on a scaling principle to determine all P-matrix elements, is also described.

New SPUDT cells including fingers and gaps wider than a quarter wavelength

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 YZLiNbO3 and 128°YXLiNbO3 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 YZLiNbO3 confirms the existence of a strong unidirectional effect pointing really to a big reflection factor per wavelength.

Second-harmonic suppression in DART-SFIT devices

Important SPUDT cell types (DART, EWC) yield large 2nd harmonic levels. The goal of this work is to reduce the level of 2nd harmonic in SFIT devices using DART cells by design tools. One-track and two-track solutions of DART-SFIT filters are presented. For suppressing 2nd harmonic of one-track SFITs these are divided into partial SFITs. The gaps between them are widened by half a DART cell length and opposite finger polarities of the partial SFITs surrounding the considered gap are adjusted. The influence of this method on the 2nd harmonic level is studied for partial SFITs connected serially and in parallel, resp. Two-track solutions of SFITs are composed of two sub-SFITs arranged in acoustically parallel tracks. For suppressing 2nd harmonic the sub-SFITs are shifted to each other by half a minimum DART cell length and are additionally sloped. Some combinations of widened gap numbers of 1st and 2nd SFIT were analyzed in the case of one-track SFIT filters. Test structures were implemented and analyzed. As an experimental result, the combination 2/3 widened gaps in 1st/2nd SFIT yields the lowest level of 2nd harmonic. We obtain approximately 10 dB additional 2nd harmonic suppression in comparison to the case of no widened gaps. The analysis of comparable two-track solutions according to the channel model yields lower 2nd harmonic levels compared to the one-track solutions.