A NEW WIDE-STOPBAND LOW-PASS FILTER WITH GENERALIZED COUPLED-LINE CIRCUIT AND ANALYTICAL THEORY

Abstract

A generalized coupled-line circuit is introduced to construct a wide-stopband low-pass fllter in this paper. This circuit conflguration includes two-section coupled lines and a connected transmission-line stub. Due to the symmetry of this proposed structure, closed-form equations for scattering parameters are investigated. The transmission zeros and poles locations for difierent circuit parameters are discussed, and the corresponding design curves are given. For theoretical veriflcations, four typical numerical examples are designed, calculated and illustrated. Furthermore, two single-stage low-pass fllters (LPF 1 and LPF 2) with the 1-dB cutofi frequencies of 0.72GHz and 1.45GHz are fabricated and measured. The implemented LPF 2 has 19-dB stopband rejection in the range of 2.05 to 6.36GHz. Finally, two LPF cells (LPF 1 and LPF 2) and an additional connected transmission line are used to construct a new two-stage low-pass fllter (LPF 3) with extended stopband. The measured 16-dB stopband of LPF 3 is up to 7.5GHz while the 1-dB passband range is from DC to 0.67 GHz. The advantages of this proposed low-pass fllter are avoiding any lumped elements and compact layout structure.