Synthesis Design of Wideband Bandpass Filter With High Selectivity and Harmonic Suppression

Abstract

A novel wideband quasi-Chebyshev bandpass filter (BPF) with steep skirt selectivity and extended upper stopband is presented. It is comprised of two microstrip-to-microstrip vertical transitions with nonuniform slotline sections, which cascaded directly by a nonuniform microstrip connecting line. In terms of odd-/even-mode analysis, the in-band six poles which make up the wideband bandwidth can be explicitly derived. Moreover, the Chebyshev equal-ripple filtering responses with prescribed specifications can be conveniently achieved in desired passband by adopting synthesis theory. In order to suppress the second and third spurious harmonics, a modified circuit model based on synthesis method is further proposed. Two additional transmission zeros (TZs) TZ3, TZ4 can be generated at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3f_{0}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5f_{0}$ </tex-math></inline-formula> respectively by adjusting impedance ratio of slotline stubs properly under the condition of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\theta _{\mathrm {s1}} = \theta /4$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\theta _{\mathrm {s2}} = \theta /2$ </tex-math></inline-formula> , which widen the upper stopband bandwidth greatly and enhance harmonic rejection level effectively. Meanwhile, combing with the inherent periodic finite-frequency TZ5, TZ6 and TZ7, the upper stopband could be extended to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6.22f_{0}$ </tex-math></inline-formula> with rejection level better than 14.6 dB. Besides, a pair of finite-frequency TZ1 and TZ2 caused by source-load coupling occur at both sides of passband to sharpen the cutoff slope of skirts remarkably. For verification, a prototype is fabricated and measured, the experimental results agree well with the theoretical predictions, which verify the validity of proposed method.