Two novel balanced filters with wideband common mode suppression using dual-mode ring resonators are proposed in this paper. Two and four transmission zeros close to the passband are realized to improve the selectivity for the differential mode. In addition, over 20-dB common mode suppression can be realized from 0 GHz to 5f0(f0is the center frequency of the passband), due to the all-stop transmission characteristic of the open/shorted coupled lines. To verify the presented concepts, two prototypes ( εr=2.65, h=1.0 mm) with 3-dB fractional bandwidths of 22.9% and 21.9% for the differential mode are designed and fabricated. The theoretical and measured results agree well and show good in-band filtering performance and high selectivity.
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Feng et al. (2015) studied this question.