PERFORMANCE OF SAW INTER DIGITAL TRANSDUCERS USING SCATTERING MATRIX THEORY

Abstract

In this dissertation conventional Inter-Digital Transducer (IDT), Quadrature Phase Unidirectional Trans ducer (QPUDI), Three Phase Uni-Directional Transducer (TPUDT), unweighted Group-type Uni-Directional Trans ducer (GUDT) and symmetrically weighted GUIS' have been analyzed with the help of Scattering matrix theory. Scattering matrix analysis is more comprehensive than the one obtained by cross-field model. An attempt has been made to answer the question, Why use Scattering Matrix theory?'. The results obtained by utilising it have been quoted and it is concluded that other models either are not in a position to yield such information or are not as accurate. Very frequently it has been reported in litera ture that at synchronous frequency theoretical minimum insertion loss (IL) for a delay line made up of (i) Bi- Directional Transducers (BDTs) is 6 dB, and (ii) Uni- Directional Transducers (UDTs) is 0 dB (the directivity being infinite in this case. In literature it has already been mathematically established through scattering para meter characterization of UDTs that any UDT realised by two conventional BDTs displaced from one another, cannot obtain an IL of less than 0.97 dB, and hence any delay line con sisting of two such UDTs cannot have an IL of less than 1.94 dB. It is also well established that it is not possible to achieve perfect matching and perfect unidirectionality in such a UDT at the same time, but has probably been overlooked by the group of people working in this field.

It has been found that theoretical minimum IL in case of BDTs increases with number of active fingers, of GPUDTs increases with number of active fingers per phase, of TPUDTs also increases with number of periodic sections, of GUDTs it depends on number of groups and the number of active fingers per group. If in a GUDT, the number of active fingers are kept constant, minimum IL decreases, as number of groups increases. If, on the other hand, the number of groups is kept constant, minimum IL increases with number of active fingers per group. These results obtained using scattering matrix theory are in conformity with the results obtained earlier by other theories and are found to be more accurate. A very simple technique has been developed to analyze symme trically weighted GUDTs. Reduction in loss and increase in available bandwidth through harmonic operation of GUDTs and TPUDTs has been discussed at length. Width of meander line in a GUDT can be modified in such a manner that it becomes unidirectional at higher harmonics. GUDT1 s geometry can be tailored accordingly. Other ideas helpful in lowering of loss and increase of bandwidth are discussed in different chapters and everything has been collected in last chapter on recaptulation and conclusions. Scattering matrix elements are used for studying performance of symmetrically weighted GUDT, as a special case.