SAW Butterworth Contiguous Filters at UHF

Abstruct-The design, fabrication, and testing of a set of 16 dualchannel contiguous surface acoustic wave (SAW) Butterworth filters is described. Each filter has a narrow-band (10.15 MHz) and wide-band (20.3 MHz) channel, and center frequencies vary from 248.8 MHz to 401.2 MHz. Theory for determination of finger overlap functions yielding Butterworth frequency responses is given. The SAW substrate used is the minimal diffraction cut of LiTa03 which has excellent bandpass filter properties. In addition to having diffraction suppressed by a factor of 20 over isotropic materials, these properties include superior spurious mode rejection when compared to other popular SAW substrates, and a coupling constant or kZ of 0.0154. An accurate transducer circuit model including parasitic elements is determined to allow optimum choice of matching elements for addressing insertion loss, VSWR, and triple transit suppression design goals. Direct optical projection onto the SAW substrate from 1OX negative masters in combination with the photoresist stripping technique was used for fabrication. Agreement between theory and experiment is good and double detection frequency measurement using SAW Butterworth filters is demonstrated.