用多次孔径、相位校正法补偿声表面波衍射的研究

Studies of SAW Diffraction Compensation Using Iterative Aperture and Phase Correction

由于切趾加权叉指换能器中小孔径电极的存在,表面波的衍射成了一个不可忽略的二阶效应,它严重影响到实验滤波器在通带附近的旁瓣抑制,讨论了抛物线近似的衍射理论及考虑衍射后的叉指换能器频率响应和时间响应,并利用中心频率法与有效电极法分别对叉指换能器的电极孔径、相位作了多次叠代校正,补偿了器件中的衍射效应。还对校正和未校正的多种情况下的叉指换能器实验频率响应做了对比、分析。所用的方法也适合于声表面波其它二阶效应的修正。

A surface acoustic wave (SAW) bandpass filter using one apodized and one unapodized interdigital transducer(IDT) can, in theory, be easily designed to have very low sidelobes (i. e., high attenuation) in the stopband if second-order effects are neglected. However, because of the existence of short apertures in apodized IDT which damage the attenuation of the sidelobe near the passband, SAW diffraction can not be neglected. Recently published experimental diffraction patterns obtained for surface-wave comb structures, which have been misaligned with respect to a pure-mde axis are, compared with diffraction fields calculated through the use of an angular spectrum of plane waves originally developed for ultrasonic bulk waves in anisotropic crystals. The application to diffraction calculations of parabolic approximations for the phase-velocity surface near pure-mode axis is discussed, and a theory model is presented here using the parabolic-anisotropy approximations to account for diffraction. We calculate the effects of diffraction of the SAW filter in both frequency and time domains. Two methods called the center frequency correction method and the effective tap method are iteratively used to correct the aperture and phase deviations so that the diffracted frequency response is close to the desired response. Finally we contrast and analyze the corrected and uncorrected IDT experimental results in detail. The technique presented to compensate for diffraction can also be used to compensate for other kinds of second-order effects.