An improved tunable filter topology for HF preselection

In the field of military electronics, there remains a need for tunable HF preselectors featuring: small size, low power consumption, and reliable operation through environmental extremes. The maturation of SAW and RF component technologies over the past decade have led to a renewed interest in the feasibility of tunable SAW filters for this role. The filter topologies reported in the literature were found to suffer limitations in center frequency tuning range, bandwidth tuning range, and aliasing of the passband during tuning. We have developed a new filter topology that addresses these limitations. The topology is based upon both input and output IDT frequency tuning in combination with differing IDT synchronous frequencies. In theory, the additional degrees of freedom of this topology allow for an efficient answer to the previous limitations. Broadband center frequency tuning is obtained since the center frequency is no longer limited to a range defined by a fixed input IDT. Improved bandwidth control is obtained trough selective overlap of the input and output IDT transfer functions. Aliasing effect are minimized by proper selection of the nominal input and output IDT center frequencies. In practice, of course, there are still limitations to the performance that can be achieved. In this paper we discuss both the theoretical advantages and practical limitations of the new filter topology.