Interdigital-inductors based bandpass filter for wireless local area network applications

A planar structure of symmetric two rectangular spiral-inductors coupled bandpass filters (BPFs) using variable stepped-metal widths is proposed. The filter is based on an electromagnetic (EM) coupling between two symmetrical spiral turns with variable metal width and spacing. The asymmetrical port terminals are coupled to the main inductor-resonators with a new coupling scheme using two narrow tuning stubs. The coupling scheme of the ports with the stubs also maintains 50 Ohm impedance matching with input and output devices. The discontinuities of two ports with the main inductor resonators strongly prohibit flowing DC current and out-banding signals. The two symmetrical inductors are combined together to behave as a virtual planar transformer. The structure of combining two interdigital inductor resonators gives a strong inductive-coupling as well as an effective capacitive-coupling, and generates a resonant frequency of 5.6 GHz. The upper and lower stop-bands rejection levels exceeding more than -45 dB were measured. The return loss (S₁₁) was measured -24.4 dB and insertion loss (S₂₁) was of -0.93 dB. The proposed BPF has a 3 dB fractional bandwidth of 8.9%.