An Efficient Technique for Tuning and Design of Filters and Diplexers

This article introduces a new technique for filter design and tuning using the phase of the input impedance (PII) as the design parameter. This novel method is applicable to both narrow and wideband filters. Compared with conventional methods, this approach requires less computation time and provides a clear step-by-step procedure for identifying the proper interresonator coupling and the resonant frequencies of the resonators. In practice, the physical realization of the filter always has a nonideal I/O port, which can introduce an unexpected unknown transmission line between the physical reference plane and the port of the corresponding inverter in the circuit model. In this article, the PII response is used to determine the equivalent electrical length of this unknown transmission line. The technique is demonstrated through the design of a wideband planar filter with a fractional bandwidth of 72%, and the tuning of a 4-2 combline filter with two transmission zeros. Finally, a wideband diplexer is developed using the proposed technique.