A compact quad-band bandpass filter with separate signal paths

This article presents a compact microstrip quad-band bandpass filter. Each passband has separate signal path, thus, it is convenient to control the passband performance independently. The third passband consists of two electrically coupling half-wavelength resonators and its input/output coupling adopts the tapped line structure. Meantime, the third passband structure serves as one part of the input/output coupling structure of other three passbands and, for other three passbands, it not only accomplishes the input/output coupling but also provides the direct source-load coupling which can generate the transmission zeros resulting in high frequency selectivity. Other three passbands are all constructed by two magnetically coupling quarter-wavelength resonators. The four passband structures are novelly combined together so as to result in a compact size. A prototype filter with the center frequencies of 2.4/3.5/4.5/5.8 GHz is designed, fabricated, and measured. The measured results give a good agreement with the simulated ones, which verifies the effectiveness of the filter. (c) 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:2156-2160, 2015