Star-connected shorted coupled-lines bandpass filter

Abstract

This paper presents a microwave bandpass filter topology based on star-connected shorted quarter-wavelength coupled-line sections. It results in a combination of shorted half-wavelength resonators, leading to a medium band response. The arrangement of the coupled-line sections in the form of a star helps limiting the overall longer length of the filter, independent of its degree of order. As the resonators are found to be very close to each other, cross coupling occurs amongst them causing transmission zeros to appear at both sides of the passband, which improve the selectivity of the filter. Two bandpass filters having 3rd- and 4th-order are designed based on the concept, fabricated and measured, using microstrip technology on RO3210 substrate with thickness, h = 1.27 mm, dielectric constant, e r = 10.2 and loss tangent, tan δ = 3×10−3. Centered at 1 GHz, the measurement results show very good coherence with those from simulation.