Reflector-filter using an SAW waveguide directional coupler for IF applications in CDMA system

The reflector-filter design is an attractive technique to obtain steep skirt characteristics in a short chip length because the SAW propagation path is folded and the frequency response is synthesized by utilizing both IDT and reflector responses. In this paper, we propose a new reflector-filter structure using an SAW waveguide directional coupler to realize small-size CDMA IF filters. For the implementation of proposed reflector-filter, the key technology is the design of SAW waveguides. An Al grating, which has the stopband kept away from that of IDTs, is first employed to form a low-loss waveguide. The directional coupler is composed of two Al grating waveguides, and the coupling property is verified. Using the proposed reflector-filter structure, PCS-CDMA IF filters are fabricated on quartz substrates. The filter exhibits an insertion loss of 8.5 dB and fulfils the specified frequency response. The package size is 4.8×9.1 mm and a size half that of a conventional transversal filter is achieved