Constant Absolute Bandwidth Tunable Symmetric and Asymmetric Bandpass Responses Based on Reconfigurable Transmission Zeros and Bandwidth

Abstract

A two-pole reconfigurable and tunable bandpass filter (BPF) with constant absolute bandwidth (CABW) and tunable transmission zeros (TZs) is presented in this paper. The BPF consists of asymmetrical spiral resonators having open ends on one side and capacitor loaded on the other side. The varactor loaded coupled-feed scheme is used at input and output. The two coupling feed-lines are also coupled to each other. It is shown that the control over TZs using the coupling-capacitor loaded between the two resonators leads to two different symmetric frequency responses – first with one TZ on each side of the passband, second with no TZ on either side of the passband. Further, asymmetrical frequency responses having single TZ either on lower or upper side of the passband are achieved by varying the capacitors loaded to the end of coupling-line. CABW while tuning as well as tunable bandwidth (BW) are achieved in all type of responses by using the coupling-capacitor. While tuning, quality factor is maintained by varying the capacitor loaded at the end of the coupled-feed. For the demonstration of the concept, a two-pole tunable BPF is designed and fabricated. The fabricated BPF having size of 13.1 mm × 9.4 mm, shows four different responses based on the position of TZs. All four CABW responses of the BPF have a tuning range of 300 MHz. Also, a tunable bandwidth is achieved in every response of the BPF.