Balanced low-loss narrowband 3-IDT double mode SAW filters with improved selectivity

This paper presents the new balanced low-loss narrowband DMS filters with improved selectivity on 42° YX LiTaO₃. The filters are realized as 3-IDT scheme. A narrow passband of the filters is formed by choosing a definite distance between the central IDT and side IDTs. Also the long central IDT and long reflectors are used. A shape of the frequency response of the filters is determinated by the weighting function of the central IDT, length of the reflectors, relationship between the electrode periods of the central IDT, side IDTs and reflectors. An optimization of the mentioned parameters with a computer simulation using an equivalent circuit model allows to obtain a specified selectivity of the filters with a fractional bandwidth of less than 1% on retention of the low loss and in the absence of the matching elements. To increase the selectivity of the filters the cascaded connection and phase weighting are employed. The 300 MHz samples of the balanced SAW filters have shown an insertion loss of 3 dB, 2-dB bandwidth of 2.9 MHz, stopband attenuation of 60 dB at ±12 MHz offset from a center frequency in a 50-Ω balanced system. The filters are not require the matching elements and housed in the 5×5×1.35 mm SMD package.