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A novel wideband and miniaturized common-mode noise suppression filter is proposed based on the concept of an effective negative-permittivity metamaterial (MM) transmission line (TL). The propagation properties for the odd and even modes in the proposed structure are derived from the TL theory and Bloch theorem. Two- and four-port equivalent-circuit models are developed to explain the common-mode suppression characteristics. The dispersion relation has a good agreement with the full-wave simulation and measurement result. Based on the low-temperature co-fired ceramic fabrication technology, miniaturized common-mode filters with four and eight cells are realized using the concept of the effective negative-permittivity MM. For the four-cell structure, the filter size is 0.16 ??g ?? 0.26 ??g with the corresponding real size of 3.2 mm ?? 5.12 mm. It is found that the common-mode noise can be reduced over 10 dB from 3.8 to 7.1 GHz with the fractional bandwidth of 60% in the frequency domain, and is reduced over 50% for voltage amplitude in the time domain. More importantly, the differential signal integrity, in terms of insertion loss and group delay in the frequency domain and eye diagrams in the time domain, is not degraded within the wide stopband. To our best knowledge, it is the first broadband common-mode filter designed for gigahertz differential signals based on the concept of MM TL with most compact size.