Measurement of the equivalent circuit parameters of transversely coupled SAW resonator filters

Transversely coupled SAW resonator filters (TCFs) are usually described by their transmission characteristics (S21) at a given termination impedance. The element values of their equivalent electrical circuit diagram can not be derived directly from these data. They are, however, important for the applications of TCF´s in voltage controlled SAW oscillators (VCSO). Because of the fundamental similarity of two-pole TCF´s to bulk wave monolithic two-pole crystal filters (MCF´s), the equivalent circuit of a MCF can be used for describing the TCF. The circuit parameters can be determined from the measurement of the complex impedance at the symmetric and the antisymmetric frequency of the SAW device. This paper describes the theoretical background and the measurement procedure, and presents experimental results showing a good correlation between the observed filter performance and the results computed from the equivalent circuit parameters obtained with this method