A new low-loss SAW filter technique

For mobile radio applications, IF filters need to satisfy requirements of low loss, narrow bandwidth and small size. Using conventional SAW technology, the narrow bandwidth requirement leads to designs which are too long to satisfy the size requirement. We report here a new filter architecture giving substantial length reduction for a given bandwidth. Unidirectional SPUDT transducers are used to obtain low conversion loss and low SAW reflections when matched. The device uses a multi-track arrangement in which phase changes ate introduced such that the first-transit signals are cancelled while the triple-transit signals are reinforced. Consequently, the device response involves two reflections of the waves by the SPUDT's, giving reduced bandwidth and also-suppressing the stop-band sidelobes. Results are shown for an experimental 71 MHz filter on 37° Y-cut quartz, with a 3 dB bandwidth of 380 kHz. A 40 dB sidelobe rejection was obtained without the need for weighting. The chip length was 10.1 mm., small enough for mounting in a miniature SMD package