Two-Layer-Magic-T-Based Bandpass, Quasi-Bandstop, and Dual-Passband Balanced Filters With Differential-/Common-Mode Reflectionless Behavior

• Published in: IEEE transactions on microwave theory and techniques Vol. 72; no. 4; pp. 2267 - 2282
• Main Authors: Zhao, Xi-Bei, Wei, Feng, Yang, Li, Gomez-Garcia, Roberto
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.04.2024; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Absorption; Absorptive filter; Absorptivity; balanced filter; Band-pass filters; bandpass filter (BPF); Bandpass filters; Bandstop filters; Broadband; broadband filter; common mode (CM); complementary diplexer; differential filter; differential mode (DM); Diplexers; dual-band filter; Filtering; Filtering theory; magic-T; microstrip filter; Microstrip filters; microstrip-to-slotline transition; Microwave filters; multilayer filter; planar filter; Prototypes; quasi-bandstop filter (BSF); Radio frequency; reflectionless filter; Resistors; wideband circuit
• ISSN: 0018-9480; 1557-9670
• DOI: 10.1109/TMTT.2023.3308390

An approach for the realization of radiofrequency (RF)/microwave balanced filters with simultaneous differential-mode (DM) and common-mode (CM) two-port-reflectionless behavior for bandpass, quasi-bandstop, and dual-passband applications is reported. These RF filtering components exploit as basic constituent circuit block a type of two-layer broadband magic-T, in which its two-side/coplanar arms are designed as the balanced feeding ports at the top layer. Specifically, two replicas of this wideband magic-T on a microstrip-to-slotline transition are back-to-back connected between their <inline-formula> <tex-math notation="LaTeX"> {\Delta } </tex-math></inline-formula> terminals by means of a double-complementary-diplexer-based single-ended low-reflection filtering unit inserted at the bottom layer. In addition, their <inline-formula> <tex-math notation="LaTeX"> {\Sigma } </tex-math></inline-formula> accesses are loaded with a resistor to perform the CM RF-power-absorption functionality. Thus, balanced symmetrical-absorptive RF filtering devices for both the DM and CM operations are developed. Such distinctive feature makes this family of microwave filters unique since most of related prior-art balanced filters mainly focused on obtaining the RF-power-absorption property only for the DM or the CM. The theoretical operational foundations and RF design principles of the engineered class of balanced filters with double DM and CM symmetrical-absorptive capabilities are described. Furthermore, their two-layer implementations exemplified in second- and fourth-order bandpass filters (BPFs), a second-order quasi-bandstop filter (BSF), and a second-order dual-band BPF are presented. For experimental-validation purposes, three different prototypes centered around 2 GHz corresponding to single-band BPF and quasi-BSF devices, as well as a dual-band BPF circuit with passbands located at 1.75 and 2.25 GHz, are manufactured and tested. For all these prototypes, 10-dB-referred DM and CM two-port-RF-power-absorption relative spectral ranges larger than 3:1 are measured. Their expected RF filtering functionalities and highly suppressed CM transmission at all frequencies are also verified.