The Use of Frequency-Selective Multistrip Coupling in Surface Acoustic Wave Transversal Filters

Abstract

This paper describes how frequency-selective multistrip couplers can be used to maximum practical advantage in surface acoustic wave transversal filters. It is shown how multistrip couplers incorporating a geometric jog in one track may be designed to introduce notches into a filter characteristic or to suppress unwanted harmonic responses without imposing design restrictions on the tr;lnsducPr geometry. rt is also shown that part of the filter selectivity can be obtained from a weighted bandpass multistrip coupler; this eases the selectivity requirements on the interdigital transducers, which allows minimisation of the trans -ducer length and reduces the number of small sources. A technique for the incorporation of harmonic suppression into a weighted bandpass multi -strip coupler is described in detail. Methods for the synthesis and analysis of bandpass multistrip couplers are discussed. Experimental measurements on filters incorporating third harmonic suppressed bandpass multistrip couplers are reported and are shown to be in good agreement with theoretical predictions.