The dependencies of SAW-transducer equivalent-circuit-model parameters on transducer geometry

A new equivalent-circuit (EC) model for SAW transducers has been developed which incorporates the measured dependencies of model parameters on the number of transducer electrode pairs, N, and the acoustic aperture. W. Three EC parameters that relate the transducer geometry to the circuit-model elements are the electrode-pair capacitance per unit length, C-e, the capacitance ratio, r, and the resonant velocity, V-r, which is proportional to the velocity of the SAW. For conventional transducer EC models these parameters are independent of N and W. Parameter values were extracted from RF waveguide-probe measurements of an array of SAW transducers with resonant frequencies near 900 MHz fabricated on 64 degrees Y-cut X-propagating lithium niobate. The transducers had numbers of electrode pairs ranging from 50 to 150, acoustic apertures ranging from 5 to 20 wavelengths, and bus-bar heights ranging from 5 to 15 wavelengths. Even with a fixed electrode width-to-pitch ratio and thickness-to-wavelength ratio, results showed approximately 25% variation in C-e, 50% variation in r, and 0.4% variation in V-r due to the variations in N and W. The parameters varied little with bus-bar height. The measured and computer-simulated responses of a SAW ladder filter designed for a GSM receiver application at 947 MHz were compared. The new model which accounts for geometrical variations in the EC parameters shows an improvement in accuracy over previous models.