High selectivity SAW duplexer for W-CDMA Band VIII

Special requirements for Band VIII Wideband CDMA (W-CDMA) duplexers are the relatively large bandwidths of 3.9% for transmitting (TX) and 3.7% for receiving (RX) filters as well as the low duplex distance of 1.1%. Moreover, compared to systems at higher frequencies, for 1 GHz systems increased focus is put on miniaturization. In order to achieve good RX suppression within the TX band, the RX filter is comprised of one port resonators as well as a DMS track whereas the TX filter is a pure ladder type filter. The center frequencies of these building blocks show different dependencies with respect to metallization ratio and layer thickness. The consideration of this relationship within the design process was necessary to achieve a very low shift of the right skirt of the TX filter relative to the left skirt of the RX filter. Our duplexer is realized in a 3.0x2.5 mm² package on LTCC ceramic and offers a suppression of the W-CDMA RX-signal of 50 dB within the TX band. Typical values for attenuation of the W-CDMA signal in the transmission band are 1.7 dB for the TX path and 2.0 dB for the RX path. The comparison between simulation and measurement shows the applicability of the design concept which is the most appropriate one for duplexers with small duplex distance and good RX suppression within the TX band.