Accurate computation of apodized SAW transducer capacitances

The passband slope of a SAW (surface acoustic wave) filter with large fractional bandwidth critically depends on the value of the 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 used. 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 nonconductive 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-LiNbO₃ were calculated and compared to measurements