Developments in Low Loss, Low Ripple SAW Filters

Abstract

SAW filters are often required by systems to possess low loss, low passband ripple and high out-of-band rejection properties simultaneously. This paper describes two recent developments in the design of such low loss filters. In the first approach, three bidirectional transducers are arranged colinearly to eliminate half of the 6 dB bidirectional loss. The unique feature of this design lies in the use of centrosymmetric transducers for broadband triple transit suppression. Two design examples are reported. The filter fractional bandwidths range from 1.8% to 10.5%. For the second approach, a new type of connecting and matching the group type undirectional transducer (UDT) is described for narrowband filters. In this method, one of the two groups in the UDT is inductively matched to serve as acoustic reflector. Triple transit level of better than -50 dB is achieved for a device of 8 dB loss. The in-band peak-to-peak (p-p) amplitude ripple is less than 0.05 dB.