Common-mode noise mitigation using ring resonator patterned ground plane for WiMAX and TD-LTE applications

This paper proposes a novel miniature bandstop filter for common-mode noise suppression in WiMAX and TD-LTE devices using ring resonator patterned defected ground structure (DGS). The proposed filter consists of two layers; coupled microstrip lines on the top layer, ring resonator based DGS on the bottom layer and the two layers are separated by FR4 dielectric substrate. Due to asymmetry in the PCB layout, the differential signal is converted to common-mode signal. The common-mode signal is considered as a noise in differential signaling systems and it must be suppressed to preserve the differential signal integrity. The filter provides differential insertion loss of less than −2 dB for frequencies up to 5 GHz. A peak insertion loss of −34 dB is observed for the common-mode signal at 3.5 GHz and a −20 dB stop bandwidth of 580 MHz from 3.14 to 3.72 GHz. The common-mode to differential mode conversion is below −70 dB for frequencies up to 5 GHz. The common-mode noise suppression is improved by 72% in the time domain. The fractional bandwidth is 17%. The size of the filter is 0.41λg × 0.2λg. From the eye-diagram, the values of MEO and MEW are 0.93 V, 0.58 ns. The proposed filter is fabricated and the measured results have good correlation with the simulated results.