Modeling of feedthrough and ground loops in SAW filters

Feedthrough and ground loops contribute to time spurious at t=0 in SAW filters. These effects not only limit the rejection level that can be achieved with the filter, but also the amplitude and phase characteristics in the passband. Thus, they must be accurately medalled when designing SAW filters requiring low amplitude and phase ripples and high close-in rejection. Feedthrough is caused mainly by capacitive coupling between the input and output transducers and is dependant on the substrate thickness and method of mounting as well as the physical separation of the transducers. Ground loops are more difficult to predict since they depend on the printed circuit board layout used. It is found that this can be modeled accurately with a common mode inductance to ground. A method will be presented which can be used to predict the feedthrough capacitance between two transducers and a filter design will be presented which can be used which incorporates this effect in the design resulting in improved filter performance. The ground loops equivalent circuit will also be presented along with 521 measurement of a filter and the corresponding computer analysis with the ground loop model