Innovative SPUDT based structures for mobile radio applications

Abstract

The IF filter is a key component in modern mobile radio receivers. Depending on the system design, the filter requirements goes from a very smooth prefiltering to a complete channel filtering. SPUDTs based structures are well suited for fractional bandwidths typically between 0.1% and 0.5%. Due to the reflectors presence inside the transducers, SPUDTs allows many different filters structures mostly using its reflective characteristic, its transduction characteristics or both. For base station applications, rejections in the 80 dB range are required. The first way to obtain as high a rejection is to use a cascade of 2 filters. We present a cascade of 2 filters at 201 MHz. The first is a classical SPUDT while the second is a new double track structure. A simple means to obtain as high a rejection in a single filter is to use a reflective array structure. We present a 71 MHz filter using this technique. The rejection is more than 75 dB for a 170 kHz 3 dB bandwidth and a 470 kHz 50 dB bandwidth. The simplest SPUDT structure consists of two SPUDTs on a single track. Due to its lack of symmetry, this structure is not well suited for balanced drive. A symmetric SPUDT structure was developed to fix this problem. Two balanced drive SPUDT filters are shown. The first is a 71 MHz GSM prefilter while the second is 85.4 MHz filter for CDMA. This filter exhibits a 0.85 MHz p0.2 dB bandwidth, a 1.7 MHz 40 dB bandwidth for a 14 dB insertion loss