Electrostatically variable saw delay lines - Theory and experiment

A theoretical and experimental investigation has been made of surface acoustic wave (SAW) timedelay change produced by the application of a dc electric field. Measurements were made on the three high-coupling cuts of lithium niobate for the electric field applied in the plane of the surface with a novel electrode configuration and normal-tothe- surface with a standard "convolver" plate. A theory has been developed for the normal field configuration, where the electric field is uniform, and for the in-plane case, where the field is nonuniform. The dominant contribution for the latterc omes from the center of the electrodes. For the 38 X-cut of lithium niobate, the normal field sensitivity (fractional timedelay change per applied electric field γ). is 141x10^-12m/V, while γ for the in-plane field is 6x10^-12m/V. The in-plane configuration has the advantage of having lower insertion loss and dispersion as the SAW does not have to propagate through a metal electrode. For 16.5° doubly rotated lithium niobate, a normal field device with a substrate thickness of 0.16 mm yielded a fractional timedelay change of 0.9x10^-6/V. To our knowledge this is the largest time-delay change per applied voltage yet reported. The time-delay change is 10^-4very linear with applied voltage; the second derivative is typically multiplied by slope. Both electrode configurations yield monolithic devices that dissipate no dc power.