Development of an ultra-flat SAW filter module and its application to FASS: a frequency agile signal source

The development of a fast, agile signal in the frequency range of 10 MHz to 3 GHz based on direct digital synthesis of signals from 14 to 58 MHz is described. A high-performance SAW (surface acoustic wave) filter module was designed to provide IF (intermediate frequency) filtering after the baseband signal was converted to a center frequency of 304 MHz. The 44-MHz-wide filter module showed typical passband ripple less than 0.4 dB peak-to-peak. Triple transit, electromagnetic feedthrough, and other spurious responses were rejected by at least 50 dB. The circuit, the SAW filter, and its package were designed with careful attention to parasitics in order to achieve this performance. The filtering was accomplished by two identical SAW filters on 128° Y-rotated, X LiNbO₃ operated in cascade, with isolation amplifiers buffering each filter. The isolation amplifiers were designed to provide a low-impedance, balanced termination to each IDT (interdigital transducer), minimizing triple transit and EM feedthrough. The SAW filter was designed using the Remez exchange algorithm and optimized to compensate for and reduce second-order effects. An absorbing layer of polyimide was an important factor in assuring repeatably low spurious time-domain responses from the chip edges, while being compatible with 300°C package sealing. An on-wafer test hardware and software system was developed to predict packaged-device ripple performance and a precision test fixture with two-port calibration standards was developed to test feedthrough and spurious time-domain responses in the same environment as the final circuit