PCB-Level Common-Mode Noise Suppression

With the development of electronic information technology, the speed and integration level of electronic system become increasingly higher. As a result, it is more challenging to solve the EMC problems. Common-mode noise is the important source of EMI, and the noise suppression technologies are the research hotspots of EMC field. Both high speed circuits and RF circuits have the risk of common-mode noise interference. To be a master of engineering, I cooperated with Huawei company to study the noise interferences on differential pairs and RF power amplifier. Meanwhile an optimized common-mode filter for differential pairs and high impedance surface for power amplifier are proposed to suppress the noise.Common-mode noise carried by differential pairs may result in EMI, however, the existing noise suppression technology can't meet the demands of engineering well because of the high-density signal lines and high-frequency noises. PCB embedded common-mode filter (CMF) has been proposed a long time ago, but it is mainly applied to suppress the noise below 10GHz. Both 10GHz-CMF and 25GHz-CMF have been designed for differential pairs 5-7mil wide in this paper. The structure of CMF etched on the ground plane is proposed firstly, however, it is so sensitive to the layer offset of PCB that can't be applied to products. Then coplanar CMF which means filters and signal lines are on the same layer is proposed, and the bandwidth of the-10dB stop-band was increased 5 times through the optimization method. Moreover, the SI problems of the filter are solved so that it can meet the engineering demands.Common-mode noise inside the shielding box affects the gain of PA. A filed-circuit hybrid method is proposed to model the effect of the noise and to figure out how to solve the problem. High impedance surface attached on the shielding box is an effective way to suppress the noise, and the gain is reduced after attaching the HIS. In addition, the gain is also corrected after applying phase compensation technology.