5 - Trade-Offs in Linear Phase SAW Filter Design

Abstract

This chapter highlights the effect of external circuit parameters on the overall response of a linear phase SAW filter network. Relationships have been derived for the maximum intrinsic fractional bandwidth of a SAW filter with bidirectional IDTs, as constrained by such external circuit loading. Second-order effects affect trade-offs in the design of linear phase SAW filters with bidirectional transducers. Though these second-order effects can never be totally eliminated, they can often be reduced to levels where good bandpass filter performance can be realized. The principal second-order effects that detract from SAW filter performance are predominantly due to external circuitry, finger reflections, bulk waves, diffraction, and triple-transit interference. A complete SAW filter network includes external circuit elements connected to input and output SAW transducers. The external elements may be entirely passive. Some designs may also incorporate transistor circuitry for changing impedance and/ or signal voltage levels. External circuit influences are usually referred to as circuit factor loading. Source and load circuit parameters affect the overall frequency response of a SAW filter network. The external circuitry also places a limitation on the maximum fractional bandwidth that a SAW filter can have before the network response becomes dominated by the frequency response characteristics of the input and output circuitry.