Compact low-loss wideband differential bandpass filters are presented in this letter. The proposed filters are constructed by a pair of multilayer differential microstrip-slotline transitions (MDMSTs) and microstrip line resonators. Wideband common-mode suppression and low insertion loss can be easily achieved by employing the MDMSTs. Wideband differential-mode passband and sharp notched band can be obtained by combining the microstrip resonators at the middle and the two sides with the pair of MDMSTs, respectively. The demonstrated filters with a compact size of 24 mm × 12 mm (0.56λg× 0.28 λg) exhibits a fractional bandwidth of 115% centered at 4.25 GHz (f0) with minimum insertion loss of 0.43 and 20 dB common-mode suppression from 1 to 10 GHz (2.35 f0).
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Shi et al. (2013) studied this question.