A new generalized two track RSPUDT structure for SAW filter size reduction: design and performance

High volume communication receivers continue to add features such as multimode operation, voice recognition, and Internet access with shrinking size. These evolutions reduce the available space for the RF and IF filtering functions. For systems such as CDMA and PCS the SAW IF filter is one of the largest components. This trend has lead to the development of DART filters, followed by the RSPUDT and recently by the two track RSPUDT. For the CDMA IF SAW filter the length of the SAW die has shrunk from 22 mm for a classical filter to 7×5 mm² for a two track RSPUDT design with simultaneous filter performance improvements. This paper will discuss the more common two track architectures with parallel and series connected transducer connections and basic principles of operation using the concept of symmetric and antisymmetric modes. We will then present a new generalized two track structure using the RSPUDT. This new structure works by designing each track to be in phase in the passband and out of phase in the rejection bands. This approach leads to steeper transitions and smaller die size. Implementation on the mask of the connections of the two different tracks will then be discussed. Experimental results will be included for both a CDMA (21× 8 mm²=>9×5 mm² package) and PCS (13×6.5 mm²=>5×5 mm² package) filter