New design techniques for SAW filters using slanted-finger IDTs

The authors describe two design techniques for slanted-finger interdigital transducers (IDTs) which could not be designed using conventional techniques. One technique provides a large stopband attenuation: individual fingers or groups of slanted-fingers are withdrawn from the selected portions of the IDT. The other technique realizes a nonlinear phase response: the center-to-center distance between transmitting and receiving ITDs is varied along the axis normal to the surface acoustic wave (SAW) propagation axis. Two examples of filters, fabricated on x-112y LiTaO₃, are shown with about a 30% fractional bandwidth and a 19 MHz center frequency. One filter has a 45-dB stop-band attenuation, a 1.10 shape factor, and ±0.05-dB passband ripples. The other filter has a nonlinear phase response which equalizes the phase response of an LC filter. This filter achieves a group delay distortion of less than ±10 ns