A Dual Bandpass Filter Design Using Strong Coupling, Evanescent Mode and Modular Concept

Abstract

This paper presents a new design concept of a dual bandpass filter. Based on the strong coupling between two resonators, a dual 1-pole band-pass filter is designed and is used as the basic building block. By providing appropriate weak coupling between these building blocks, a higher-order dual bandpass filter can be realized. In addition, these building blocks can be stacked vertically and/or horizontally to construct a compact filter. In this way, by using 3D full wave EM and circuit cosimulation, the simulation time required in the design stage can be reduced. In addition, it also provides a way to post-tune each building block individually and further reduces the time required in prototype post tuning process. However, using this design approach, the band separation ratio is limited by the physically achievable coupling. For demonstration, an L-band dual 4-pole bandpass filter is designed with passband frequencies of 1.2 GHz ∼ 1.255 GHz and 1.55 GHz ∼ 1.6 GHz. To reduce the size of the filter and obtain a wide stopband bandwidth, a suitable evanescent mode cavity is used to realize the resonant structure. The measurement result shows that the insertion losses of the low passband and high passband are 1.03 dB ∼ 2.00 dB and 1.02 dB ∼ 1.75 dB, respectively; the return loss of both passbands is better than 15 dB. Furthermore, up to 5 GHz, the stopband rejection level is better than 80 dB.