DIFFRACTION EFFECTS IN SMALL-APERTURE ACOUSTIC SURFACE WAVE FILTERS.

The formulation of the acoustic surface wave diffraction in terms of Huygens' principle is discussed and an experimental verification of the theory is given for an extreme example of an apodised acoustic surface wave filter with a 10 wavelength aperture. Computations show that diffraction effects are most severe when the receiving electrode occupies only a small part of the acoustic aperture, and particularly when the electrode is at the edge of the field. Frequency-dependence of diffraction effects is illustrated and errors are found to be greatest around the nominal null frequencies of the launching transducer. At the synchronism frequency, neither the number of electrodes in the launching transducer, nor the presence of acoustic attenuation, have an appreciable effect on diffraction.