Compact Differential Bandpass Filter with Improved In-band Common-mode Suppression with Loaded T-shaped Resonators

Compact symmetrical four-ports differential bandpass filters with good common-mode suppressions are proposed in this work. The presented filters are designed based on half-wavelength coupled resonators with compact size, good filtering responses for differentialmode, and wide common-mode suppression range. To further improve the common-mode performances within the differential-mode passband, T-shaped resonators are loaded at the center of the structure. It is noted that, the size of filter does not become larger with loaded T-shaped resonators. Both these two filters are centered at 1.8 GHz for Global System Mobile Communication (GSM) with 7.8% fractional bandwidth. For differential-mode, the insertion is less than −1.2 dB in the 3-dB passband and the matching is better than −20 dB. Good stopband characteristics are also obtained with more than −20 dB out-of-band attenuation from dc to 1.6GHz in the lower stopband and from 2.0 to 4.8 GHz in the upper stopband. For commonmode, better than −15 dB suppression is achieved within dc to 6.2 GHz and with the help of the loaded T-shaped resonators, the rejection in the differential-mode passband is improved to be more than −40 dB. Theory analysis, simulation, and measurement show good agreement with each other. Received 27 July 2013, Accepted 21 August 2013, Scheduled 28 August 2013 * Corresponding author: Guo Yang ([email protected]).