Design of wide band SAW coupled resonator filters on quartz

SAW transversely and longitudinally coupled resonator filters (TCRF and LCRF) on quartz normally use a cascade of two sections to obtain the desired number of poles for desirable performance. It is possible to design 3 or 4 pole filters in one section, resulting in lower insertion losses, smaller chip sizes and, most importantly, wider bandwidth. Single cascaded multi-pole coupled resonator filters (CRFs) are commonly implemented by increasing the number of acoustic modes used. In this paper, a new approach is proposed for designing 3 and 4 pole CRFs without cascading. This approach uses several parallel connected coupled resonators (CRs). For each CR only two main acoustic modes are unitized and the CRs all have different grating pitches. It is demonstrated that by choosing apertures and grating pitches of the CR elements properly, optimal filters with 3 or 4 pole responses can be achieved. It is a suitable method for both TCRF and LCRF. The results of investigations are illustrated by experimental responses of a TCRF and an LCRF.