Quasi-static analysis of floating-electrode unidirectional SAW transducers (FEUDT´s)

Single-phase unidirectional transducers are commonly used to control multiple-transit ripples in low-loss SAW filters. One method of obtaining the necessary directivity is to incorporate floating electrodes, as done in the Floating Electrode Unidirectional Transducer (FEUDT). Unlike the DART transducer, FEUDT´s are known to be effective with strongly-piezoelectric substrates such as lithium niobate. However, the analysis of FEUDT´s can be very complex, as shown by previous algebraic and numerical work. This paper describes a Green´s function analysis for FEUDT´s with regular electrodes. The substrate excitation is described by electrostatic and SAW Green´s functions, and the quasi-static approximation is made by assuming that electrode voltages and charges can be deduced from the electrostatic function alone. This gives a simple and effective method for transducer analysis, particularly when the electrodes are regular. The quasi-static method is suitable when electrode reflections are negligible, which is the case for FEUDT´s because they have the electrode pitch less than λ/2. Here, the quasi-static theory is extended to allow for floating electrodes, giving algebraic results for a FEUDT in agreement with earlier analysis. The locations of the transduction and reflection centres are easily deduced, and the method can be applied to a wide variety of FEUDT´s.