A generalized coupled-line circuit is introduced to construct a wide-stopband low-pass filter in this paper. This circuit configuration 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 different circuit parameters are discussed, and the corresponding design curves are given. For theoretical verifications, four typical numerical examples are designed, calculated and illustrated. Furthermore, two single-stage low-pass filters (LPF 1 and LPF 2) with the 1-dB cutoff frequencies of 0.72 GHz and 1.45 GHz are fabricated and measured. The implemented LPF2 has 19-dB stopband rejection in the range of 2.05 to 6.36 GHz. 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 filter (LPF 3) with extended stopband. The measured 16-dB stopband of LPF 3 is up to 7.5 GHz while the 1-dB passband range is from DC to 0.67 GHz. The advantages of this proposed low-pass filter are avoiding any lumped elements and compact layout structure.
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