Millimeter-Wave Low-Loss and Sharp Roll-Off Rectangular Micro-Coaxial Line Lowpass Filters Using Micro-Structure Electrochemical Fabrication (MEFAB) Process

Abstract

This article presents millimeter-wave (mmW) low-loss and sharp roll-off rectangular micro-coaxial line (R $\mu $ CL) lowpass filters (LPFs) fabricated by the micro-structure electrochemical fabrication (MEFAB) process. The double stepped-impedance open stubs loaded resonator (DSIOSLR) is proposed to design R $\mu $ CL LPFs. It consists of a pair of high-impedance lines with a pair of stepped-impedance open stubs (SIOSs) loaded at their center. The DSIOSLR, synthesized from a third-order Chebyshev lowpass prototype, achieves sharp roll-off and high stopband suppression due to the transmission zero (TZ) generated by SIOSs. Higher-order Chebyshev LPFs can be realized by cascading multiple DSIOSLRs. To validate the proposed method, five cascaded DSIOSLRs, six cascaded DSIOSLRs, and size_reduced DC-21 GHz R $\mu $ CL LPFs are designed and fabricated. The simulated and measured results show good agreement. In the frequency range from dc to 20 GHz, the maximum insertion losses for the three R $\mu $ CL LPFs are 0.62, 0.7, and 0.54 dB, respectively, and the corresponding normalized roll-off ratios are 252, 373, and 273 dB, respectively. For on-chip measurement and interconnection, an R $\mu $ CL-to-coplanar waveguide grounded (CPWG) transition with a metal block for support enhancement is also proposed. As an example, a back-to-back R $\mu $ CL-to-CPWG transition is fabricated, exhibiting a low loss of 0.2 dB from dc to 21 GHz and 0.5 dB up to 50 GHz.