Flat-Group-Delay RF Planar Filters With Transmission Zeros Using Transversal Circuits

• Published in: IEEE transactions on circuits and systems. I, Regular papers Vol. 70; no. 10; pp. 1 - 0
• Main Authors: Gomez-Garcia, Roberto, Yang, Li, Malki, Mohamed, Munoz-Ferreras, Jose-Maria
• Format: Journal Article
• Language: English
• Published: New York IEEE 01-10-2023; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Absorptive filter; Absorptivity; analog filter; Band-pass filters; bandpass filter (BPF); Bandpass filters; Bessel filter; Circuits; Delays; directional coupler; discrete-time-filter modeling; Filtering; Filtering theory; Group delay; hybrid ring; microstrip filter; microwave filter; planar filter; Radio frequency; reflectionless filter; reflective filter; Resonator filters; RF filter; Topology; Transfer functions; Transmission lines; transmission zero (TZ); transversal filter; Transversal filters
• ISSN: 1549-8328; 1558-0806
• DOI: 10.1109/TCSI.2023.3298219

Various families of flat-in-band-group-delay RF planar bandpass filters (BPFs) with multiple out-of-band transmission zeros (TZs) in their filtering transfer function are presented. As the first approach, a type of BPFs with theoretically-constant group-delay profile and perfect two-port-reflectionless behavior is proposed. The theoretical sufficient conditions to obtain a frequency-constant group delay in a generalized form of transfer function for these BPFs are derived. They are then particularized in transmission-line-based transversal-circuit realizations with flat-in-band-group-delay and quasi-reflectionless characteristics. Afterwards, a class of reflective-type BPFs based on directional power couplers that are arranged in transversal-mode topologies are addressed toward its practical demonstration for the first time. These BPFs, which exhibit flat in-band group delay, are compared with their classic Bessel-type BPF counterparts without TZs. Moreover, discrete-time models of such BPFs are derived, analyzed, and discussed. For experimental-validation purposes, proof-of-concept microstrip prototypes centered at 2.5-GHz of the engineered quasi-absorptive<inline-formula> <tex-math notation="LaTeX">\bm{/}</tex-math> </inline-formula>reflective-type flat-in-band-group-delay BPFs are designed, manufactured, and characterized.