Selectable frequency bandpass filter

Claim 1

A selectable frequency filter comprising: substrate means defining at least a substrate surface layer of piezoelectric material capable of propagating acoustic surface waves, a plurality of electrical filters disposed on the piezoelectric surface of said substrate means, each of said electrical filters comprising input and output acoustic surface wave transducers so constructed and arranged to provide a frequency response including at least one relatively narrow passband centered at a frequency chosen from a pre-determined set of discrete frequencies and effectively zero transmission elsewhere, the frequency responses of the respective input and output acoustic surface wave transducers comprising the individual electrical filters differing from each other, at least one set of input and output acoustic surface wave transducers providing a frequency response including a plurality of relatively narrow passbands centered at a plurality of frequencies chosen from the pre-determined set of discrete frequencies wherein at least one of said plurality of relatively narrow passbands coincides with a relatively narrow passband included in a frequency response of another set of input and output acoustic surface wave transducers, switching means operably associated with each of said plurality of electrical filters and being selectively closed to connect particular electrical filters in parallel between the input and output of the selectable frequency filter, the particular electrical filters connected in parallel including at least two filters having coinciding relatively narrow passbands included in the frequency responses thereof such that reinforcement of duplicated relatively narrow passbands occurs to provide a resultant frequency response having a dominant relatively narrow passband at one frequency of the pre-determined set of discrete frequencies at which the selectable frequency filter passes energy more readily than at any other frequency.