Equivalent Circuit Analysis of Double Mode Surface Acoustic Wave Filters with Series- or Parallel-Connected One-Port Resonator.

This paper describes the derivation of equivalent circuit model and its parameters for the diagnosis of double mode surface acoustic wave (DMS) filters when a one-port surface acoustic wave resonator (SAWR) is connected in series or parallel for making the passband corner steep. The derivation procedure is as follows: (a) Static capacitance elements are estimated from the admittance matrix [Y] at frequencies much lower than the passband, (b) modified Butterworth-Van Dyke model is applied to SAWR, and its parameters are determined by the numerical elimination of a notch appearing in the response after the connection. A bump is also used for the series-connected case. And (c) the circuit parameters for the DMS portion are estimated using the scattering parameters after eliminating influences of SAWR. The accuracy of this procedure is confirmed by comparing the parameters extracted after the connection with those of the original DMS and SAWR. For the validation of this elimination process, DMS and SAWR are connected numerically using their scattering parameters. An automatic fitting software is developed and applied for these fitting processes. It is shown that the equivalent circuit parameters of the original DMS can be recovered successfully in high accuracy.