Measure-predict tuning of hybrid thin-film filters

Abstract

To achieve highly accurate filter characteristics in hybrid integrated active filter circuits, a new tuning procedure for Sallen-andKey-type hybrid filters has been developed and implemented. The proposed tuning procedure takes into account the loss factor of thin-film capacitors and nonideal amplifiers. An example that describes the tuning procedure in detail is presented for a second-order low-pass filter. It is also applicable in the same manner to all second-order functions. The new tuning procedure replaces a previous approach that was iterative and therefore time consuming when high precision was required. The new approach guarantees the pole frequency to within 0.2 percent in one tuning step, and the pole <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</tex> to within I percent in an additional step. The tuning procedure requires that the capacitors be measured to within 0.1 percent, the dissipation factor to within 10 percent, and the resistors to within 0.02 percent. With the aid of a computer, the procedure has been completely automated; it is therefore well suited for processing in manufacture. It is being used in on-line production of thin-film active filters.