Design of Wideband Quasi-Reflectionless Filter with High Selectivity and Flat Passband

• Published in: IEEE transactions on circuits and systems. II, Express briefs Vol. 70; no. 11; p. 1
• Main Authors: Zhang, Siran, Liu, Hongmei, Wang, Zhongbao, Fang, Shaojun
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.11.2023; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Absorptivity; Band-pass filters; Bandpass filter; Bandpass filters; Bandwidths; Broadband; flat passband; high selectivity; Impedance; Passband; Reflection; reflectionless; Rejection; Resonator filters; Transversal filters; Wideband
• ISSN: 1549-7747; 1558-3791
• DOI: 10.1109/TCSII.2023.3284420

In the paper, the design of a wideband quasireflectionless filter (BPF) with high selectivity and flat passband is presented. It is consisted of a bandpass section (two-stage coupled line loaded by an open-circuited stepped impedance stub) and two absorptive branches (two types of L-shaped microstrip structure separated by a lossy resistor). Theoretical analysis of the proposed BPF is provided and the parametric studies are also given. During the investigation, the following features are obtained which includes (1) The transmission zeros (TZs) can be controlled by the impedance ratio of the open-circuited stepped impedance stub. (2) The passband transmission poles (TPs) are related to the coupling of the coupled line. (3) By adding the absorptive branches, reflectionless over the test band are realized with enhanced frequency selectivity and out-of-band rejection. For validation, a prototype operating at 2 GHz was designed, fabricated, and measured. Measurement results show that the 3-dB passband fractional bandwidth reaches 98.5 % with flat passband response and high frequency selectivity. Besides, the BPF exhibits a 10-dB reflection bandwidth of 200 % and an out-of-band rejection of more than 30 dB.