Design Methodology of a High-Q Tunable Coaxial Filter and Diplexer

摘要

Any filter of order N can be readily tuned for a constant absolute bandwidth (BW) using 2N + I independent tuning elements. This significantly increases the size and the cost, and also adds to the complexity of the tuning control mechanism. This article demonstrates the feasibility of realizing a high-Q tunable coaxial filter, which is tuned by a single rotational tuning element irrespective of the filter order. The proposed filter has low variations in the absolute BW and insertion loss (IL) over a relatively wide tuning range. A prototype four-pole filter is developed at 2.5 CHz with a fractional BW of 4 to verify the concept. The measured tuning range of the filter is 20, within which the BW variation is better than 10 and IL variation is better than 0.05 dB. The proposed concept is easily expandable to filters with higher order. Furthermore, the concept is adopted to design a tunable diplexer which can be tuned using a single tuning mechanism. The proposed high-Q tunable filter is promising for use in the frequency-agile communication architecture at the cellular base-station and aerospace applications.