PERFORMANCE TRADE-OFFS IN LOW-LOSS SURFACE ACOUSTIC WAVE TRANSDUCERS

Abstract

The work presented in this dissertation, entitled "Perform ance Trade-offs in Low-Loss Surface Acoustic Wave Transducers" is concerned with the development of Uni-Directlonal Transducers (UDTs). These UDTs are used in fabricating Surface Acoustic Wave (SAW) filters which in turn are front end in VHF-UHF communica tion receivers. Out of the various UDTs, Group type Uni- Directional Transducers (GUDTs) and Three Phase Uni-Dlrectlonal Transducers (TPUDTs) are found to be most suitable as far as trade-off between Insertion Loss (IL), side-lobe levels and Band Width (BW) requirements are concerned. Cross-field equivalent circuit model is commonly used for the modelling of SAW devices due to its simplicity and less computational time required. The results obtained with this model are comparable with the experimental results. While using the cross-field equivalent circuit model for the UDTs for modell ing them, various parasitic losses like diffraction loss, beamstearlng loss, electrode resistance loss, air-loading loss mass-loading loss, propagation loss etc. have been neglected as they do not contribute more than 1 dB of losses combined togeth er. Various possible combinations of GUDTs and TPUDTs have been assumed to be fabricated on 128o rotated Y-cut X-propagatlng LiNb03. This material is selected because it has a higher coef ficient of electromechanical coupling than any other SAW material available in the market. This has an inherent property of sup pressing the undesirable side-lobe level to a greater extent and optimum Fractional Band Width(FBW). The only disadvantage with this material is that it has its temperature coefficient of delay on a little higher side. Frequency responses of these filters (with various possible combinations of GUDTs and TPUDTs, both at the transmitting as well as receiving end) have been computed. Quite interesting results have been obtained in terms of available Insertion loss, side-lobe suppression and required bandwidth by varying the number of electrodes, number of groups in Normal/Modified-type GUDTs and number of periodic sections in TPUDTs.

It is found that as the number of groups or the number of active fingers per group increases, the available bandwidth decreases as is expected generally. If the parameters taken into account for the analysis are appropriately chosen, theoretically, there will be no insertion loss. However, it is not always possl ble due to which very small losses are encountered in the re suits, baaed on the assumptions made earlier. Similarly, in the case of TPUDTs the bandwidth decreases an the number oi periodic sections are increased and vice-versa. However, it is observed (from the results obtained by the author as well as available in open literature) that the maximum side-lobe suppression has been possible by the use of eitlier of the following structures in the SAW filter transducers: (i) Different kinds of UDTs at the two ends(i.e.TPUDT, GUDT> (ii) Different kinds of TPUDTs at the two ends (i.e. the two TPUDTs at the two ends with different number of periodic sections) (iii) Different kinds of GUDTs at the two ends (i.e. Normal type GUDT and Modified-type GUDT) (iv) Using apodization in the input and/or output transducers. The main idea behind the use of these structures is that the side-lobes due to one transducer lie at a different frequency than the side lobes due to the other transducer. This results in a higher value of the insertion loss at a frequency where the side-lobes in the final frequency characteristics occur and hence a higher side-lobe suppression is possible. Maximum bandwidth is observed with TPUDTs at both ends with requisite number of periodic sections. It goes to a maximum of around 24%. In the case of GUDT filters the maximum available bandwidth is around 11% or so. In GUDT filters if number of active fingers become large and the number of groups is kept at a smaller value, the pass -band ripple becomes quite significant sometimes. State of art and technology for Single Phase Uni Directional Transducers (SPUDTs) has not yet come to the status that these transducers could be accepted for mass production But surely the future of low- loss SAW filters for applications in Signal processing lies in the development of SPUDTs as envisaged

by the pioneers in the field. Till that day we have to be con tented with the use of the UDTs discussed in this dissertation in detai 1. For the purpose of the analysis carried out by the author the two UDTs discussed here were modelled using a cross-field equivalent circuit model as indicated earlier and then a FORTRAN language program developed on DEC 2050. This program is capable of giving various frequency responses of GUDT, TPUDT and combi nation filters of these two UDTs. It can also be used for var - ious other purposes like obtaining the frequency response characteristics of comb filters etc. The results obtained are found to agree and are comparable with the theoretical results obtained using other models ( which are supposed to be quite accurate) as well as with the experi mental ones available in literature till-da.te. The quantitative results in terms of the tradeoffs between the three quantities (i.e. insertion loss, bandwidth, side-lobe suppression) and fre quency characteristics of various filters are presented in this dissertation. It is hoped that the results obtained will be useful to the ultrasonic community in the future low-loss high performance filter designs to be adopted In communication systems and like fields (in which these devices are required and are useful) for signal processing purposes.