A Compact High-Performance EWC/SPUDT SAW Channelizer

A family of Electrode-Width-Controlled (EWC) SPUDT filters for a miniature SAW channelizer was presented in the 1993 IEEE Ultrasonics Symposium. The effort reported was expanded, and led in 1996 to the successful development of a fullfledged compact high-performance SAW Channelizer module. To commemorate the passing of Clinton S. Hartmann, this paper presents previously unpublished techniques for the SAW channelizer filters. The first area concerns the implementation of the transduction and reflection tap weights of the SPUDT design. Here a highly accurate yet practical scheme involving a modified EWC structure with three equal-width electrodes was devised. Desired reflectivity is achieved in part by varying the separation between the two transduction electrodes, whereas the distance from the centerline of these two to the center of the nominally "wide" reflecting electrode remains lambda/2. This leads to a wide range of net reflectivity, and enables a fabrication insensitive electrode pattern. The second concerns the design of a frequency selective multi-strip coupler (MSC). The MSC was conceived from the perspective of basic short transducer sections with lambda/8 wide electrodes placed on lambda/4 centers. The sections are electrically interconnected, with each section odd multiple of lambda/4 long. Without incurring excessive loss, the MSC suppresses the strong second harmonic response of the EWC/SPUDT effectively. The third area concerns various techniques for spurious signal reduction in both time and frequency domains. These are by way of judicious SAW die layout, wire bonding scheme, and processing of both the top and bottom surfaces of the SAW chip. The channelizer module has 7 unshielded SAW filters in a single multi-chip module with no electrical matching for the SAW transducers. The filters are centered 20 MHz apart from 267.5 to 387.5 MHz. Results of the channelizer performance will be presented to illustrate the success enabled in part by the techniques discussed.