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

Abstract

Any filter of order $N$ can be readily tuned for a constant absolute bandwidth (BW) using $2N+1$ 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 GHz 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.