Accurate computation of apodized SAW transducer capacitances

Abstract

The passband slope of a SAW filter with lar e fractional bandwidth critically depends on the value of t f e operating capacitances of the input and output interdigital transducer (IDT). To reduce the number of design cycles required to achieve a flat passband response, a program for the computation of all relevant contributions to the IDT capacitance was developed. For the analysis of the apodized transducer, a track division approach is employed. The capacitance contributions from the different tracks are computed by means of an interfinger nodal capacitance matrix. As this matrix is calculated from the Green's function of the boundary value problem, the presence of the grounded back-plane and any non-conductive adhesive is automatically taken into account. The operating capacitances of two different SAW filters with center frequencies of 70 MHz and 140 MHz fabricated on YZ -LiNbOs were caltulated and compared to measurements. The dependency of the capacitances on the thickness of a non-conductive adhesive is also shown. Even small variations in adhesive thickness cause noticeable changes in the transducer capacitances.

A method for analyzing periodic strips with apodization

“…The evaluated charge distributions on the strips are then applied to obtain total charge on the transducer fingers by the simple rule that it is proportional to the finger length (this is the infinite strip approximation [4]). There are also numerical approaches to evaluation of transducers' capacitance [5].…”