A practical SPUDT design for SAW filters with Different-Width Split-Finger (DWSF) interdigital transducers has been developed. A newly developed DWSF-SPUDT has four electrode fingers per a wavelength. The adjacent electrode fingers have different widths respectively. The DWSF-SPUDT needs an adjustment of the electrode positions to obtain the /spl lambda//8 phase-difference. The optimum electrode arrangement is a function of the finger width ratio, which means the width of the wide finger divided by that of the narrow finger. It gives the symmetrical radiation conductance to the DWSF-SPUDT. As for this SPUDT, the electrical 1/Q factor and the directivity of SAW are analyzed as a function of the finger width ratio. The insertion loss of SPUDT depends on both factors. The electrical 1/Q factor and the directivity have a trade-off relation regarding the finger width ratio. The optimum finger width ratio has been obtained to achieve low insertion loss. The insertion loss is improved by 1 dB. Thus it is confirmed by simulation that the DWSF-SPUDT has good performance for an IF stage SAW filter for CDMA cellular phone applications.
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