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The annular interdigital transducer (AIDT) is an IDT whose electrodes are shaped to follow the wave surface for surface acoustic waves propagating on an anisotropic piezoelectric material. At the resonance frequency, waves emitted in all directions are in phase at the center of the device and create an intense acoustic spot. We use a phase sensitive heterodyne interferometer to characterize the formation of the focal spot at the center of the AIDT as a function of frequency. It is found that the displacement distribution is characterized by an angular spectrum restricted in Fourier space to the slowness curve for the SAW on the free surface. The transduction of waves is dictated by an emission factor that depends both on frequency and emission angle. The shift of the focus with frequency - which is predicted by the model because of a combination of anisotropy and dispersion in the electrode grating - is analyzed and verified experimentally.