Multiple in-line reflecting arrays: main properties and application prospects

摘要

Utilizing large numbers of closely coupled cavities separated by reflecting elements creates new degrees of flexibility in SAW filter design. Instead of wide reflection and narrow transmission bands as in dual in-line resonators, multiple narrow reflection bands separate wider transmission bands. The proposed structure consists of two weighted transducers and a rejection element between them. The rejection element is formed out groups of reflecting strips separated by proportional to half wavelength. This new element presents rejection properties useful for correction of filter response because it possesses a relatively flat passband and two main rejection bands symmetrical relative to the central frequency. Transparent and compact analytical formula is derived for dual periodic reflecting arrays and withdrawal weighting of reflecting strips is successfully utilized to reduce the passband ripple level. Design approaches are examined. Examples of filters with demanding specifications (similar to CDMA specifications) are modeled and an application strategy is discussed. Issues connected to phase nonlinearity are briefly addressed. This novel approach appears to be useful in filters with 0.1-1% bandwidth with very steep skirts of concave shape on ST-quartz. Depending on the reflection strength and specifications, the filter length may be reduced by up 30-40% in comparison to ordinary transversal designs. Otherwise additional improvement of already ultimate designs looks feasible because the accumulation of similar second order effects may be reduced.