Equivalent circuits for unidirectional SAW-IDTs based on the coupling-of-modes theory

A three-port distributed-parameter equivalent circuit representing the whole structure of a unidirectional interdigital transducer (IDT) is derived from generalized coupling-of-modes (COM) equations with complex parameters. The circuit has a simple form similar to that of a bidirectional IDT, but involves two additional impedance elements of opposite signs. It is shown that the two additional elements express well the behaviors of unidirectional IDTs and the directivity can easily be derived from the equivalent circuit. Some examples of calculations of device characteristics using the circuit are also presented.