Network and drive considerations for SAW input IDT frequency tuning

Communications receivers are often faced with the problem of trying to receive a weak signal in the presence of one or more strong interfering signals. Whereas the receiver may have sufficient sensitivity to receive the weak signal against a quiet background, when the background includes one or more strong interfering signals, the receiver will be desensitized to a level determined by the strength of the largest interfering signal and the dynamic range of the receiver. In these situations, it is highly desirable to include some sort of front-end filtering to attenuate the interfering signals prior to the first gain stage. In this paper, we discuss the design of a selectable performance front-end filter using acoustic surface waves. Selectable performance will be achieved using a finite impulse response (FIR) transversal filter topology. This topology, while critical to obtaining selectable performance, is also inherently a limiting factor when implemented using acoustic surface waves. The filter will be constrained to a fixed set of delay times determined jointly by the wave velocity and the deposited metallization. The resulting performance limits on filter parameters such as center frequency, passband bandwidth, shape factor, etc. are discussed. Current plans are to implement the tap weighting scheme using silicon ASICs to do quantized weighting. Based on some preliminary modeling, a topology has been developed which should allow the passband frequency to be shifted by a programmed amount while also reducing the aliased side lobes of the transfer function