Ballato electronic circuit models for programmable SAW filters

In recent years novel microelectronic technologies such as silicon-on-insulator and GaAs HBTs have advanced to near-maturity. Given the foundry availability of these technologies, it is prudent to re-evaluate the prospect of programmable SAW filters whereby IDT finger tap weights can be controlled electronically in order to achieve, for example, an adaptive filter module. To facilitate the design of these programmable SAW filters it is advantageous to use an electronic circuit model for the IDT electrodes. To this end, a SPICE-based circuit model for SAW IDTs is presented which employs the two-transformer model first presented by Ballato in 1972 for Bulk Acoustic Wave devices. The element factor for the IDT electrodes is incorporated via a frequency-dependent transformer turns ratio. Acoustic transmission line velocities and characteristic impedances can be calculated separately by such methods as the Datta-Hunsinger models for scattering coefficients from electrodes. The energy storage term can be incorporated as a velocity perturbation in the acoustic transmission line. Results of the Ballato electronic circuit model are corroborated by analytical calculations for simple IDT structures on 128°-rotated Y-cut X-propagating LiNbO₃.