Symmetrical Spiral Inductor-Coupled Bandpass Filter with a Stepped-Down Metal Width

Abstract

A compact microstrip symmetrical rectangular spiral inductor-coupled bandpass filter using a variable stepped-down metal width is presented. The proposed filter is based on an electromagnetic coupling between two symmetrical spiral turns with variable metal widths and spacing. The asymmetrical port terminals are coupled to the main inductor–resonators as a new coupling scheme using two narrow tuning stubs. The coupling scheme of the ports with the stubs maintains the 50 Ω impedance matching with input and output devices. The two symmetrical spiral inductors in the design structure are precisely mounted and adjusted together with equal spacing and gaps, so that the combined configuration behaves as a single resonator as inductor–interdigital coupling. The structure of combining two such resonators enables strong inductive coupling and effective capacitive coupling between the two ports and generates a resonant frequency of 5.597 GHz. The measured insertion loss (S21) was −0.93 dB, and the measured return loss (S11) was −24.4 dB.