COMPACT DIFFERENTIAL BANDPASS FILTER WITH IMPROVED IN-BAND COMMON-MODE SUPPRESSION WITH LOADED T-SHAPED RESONATORS

Compact symmetrical four-ports difierential bandpass fllters with good common-mode suppressions are proposed in this work. The presented fllters are designed based on half-wavelength coupled resonators with compact size, good flltering responses for difierential- mode, and wide common-mode suppression range. To further improve the common-mode performances within the difierential-mode passband, T-shaped resonators are loaded at the center of the structure. It is noted that, the size of fllter does not become larger with loaded T-shaped resonators. Both these two fllters are centered at 1.8GHz for Global System Mobile Communication (GSM) with 7.8% fractional bandwidth. For difierential-mode, the insertion is less than i1:2dB in the 3-dB passband and the matching is better than i20dB. Good stopband characteristics are also obtained with more than i20dB out-of-band attenuation from dc to 1.6GHz in the lower stopband and from 2.0 to 4.8GHz in the upper stopband. For common- mode, better than i15dB suppression is achieved within dc to 6.2GHz and with the help of the loaded T-shaped resonators, the rejection in the difierential-mode passband is improved to be more than i40dB. Theory analysis, simulation, and measurement show good agreement with each other.