Drazen Jurisic and George S. Moschytz
In this paper we present a procedure for the design of low-sensitivity, active-RC filters that permits efficient functional tuning during the manufacturing process. Filters with finite zeros, such as elliptic (Chebyshev-Cauer) low-pass filters are primarily considered, although the method can be applied to the design of other filters, e.g. allpole filters, as well. We show how to partition a given ladder filter into two parts. The first is a ladder filter of reduced order compared to the original; the second is a second- or third-order active-RC filter section, the ‘tuning block’, which, alone is used to tune the overall filter. The ladder, the components of which are fixed, provides most of the selectivity, while the cascaded tuning block determines the band-edge characteristics, and can be tuned relatively easily. A detailed design procedure for the filter partitioning is given. By obtaining a doubly terminated ladder filter, which is cascaded with a tuning block, both the inherent low sensitivity of the ladder, and the tunability of the tuning block, are maintained. A Monte Carlo analysis of the partitioned filter demonstrates that the low sensitivity with respect to component tolerances, achievable by maintaining a doubly terminated ladder structure for the larger partitioned part of the filter, is preserved.