Investigation of parasitic effects in multi-transducer SAW RF filters

This paper discusses the relative importance of the different parasitic effects in multi-transducer SAW filters at UHF frequencies. This is accomplished by using a lumped-element circuit model which links the circuit elements directly to the filter geometry and enables one to identify the sources of unwanted features in the filter frequency characteristics. Further, the model complexity may be reduced by retaining only those circuit elements that lead to the most severe effects. It is shown how Ohmic losses in connection pads and bond wires, coupling capacitances between pads, and inductive effects due to the bond wires are modeled and how they contribute to the filter characteristics. An experimental interdigitated IDT (IIDT) filter consisting of 15 transducers and operating at 1 GHz serves as an example. Its passband distortions could easily be traced back to stray capacitances between pads or IDTs while the stopband characteristics are mainly determined by inductive coupling effects between the 17 bonds wires. The latter result indicates that in multi-transducer devices like IIDT filters or image-impedance connected twin-track filters parasitic effects negligible in simpler structures are of paramount importance.