Single/dual-band BPFs with spurious band suppression by using non-uniform coupled resonators

Abstract

Transmission zero behavior of two coupled sections — interdigital and combline — is investigated. It is shown that the shifting of transmission zero of any coupled-section of a particular length depends on the width of various parts and the orientation of coupled-section. Mathematical formulation has been performed to show the effect of stepped discontinuity on the transmission zero. Further, this transmission zero allocation property is used in suppression of harmonics. The idea is implemented in two types of resonators — first in parallel-coupled resonators and second in open-loop resonators. Two parallel coupled resonator based bandpass filters (BPFs) with second harmonic suppression — one of second-order and the other of fourth-order — using different coupled-sections have been fabricated, and suppression up to −30 dB and −54 dB respectively has been achieved. A fourth-order open-loop resonators based BPF with suppression of undesired passbands up to 6.3fo has been fabricated. Further, the above property is also used to design a dual-band BPF with wide stopband without increasing the size of the filter.