For wideband common-mode noise suppression in high-speed differential signals, a low-cost compact filter is proposed and designed by etching two coupled C-slotlines on the ground plane. It is found that the bandwidth of the common-mode stopband over −10dB is from 2.4GHz to 6.35GHz with no degradation of the differential-mode insertion loss and group delay within the wide common-mode stopband. In addition, an equivalent circuit model is developed and provides a quickly prediction of the common-mode stopband. The results show a good consistency between the simulations and measurements.