The Effect of Common and Differential Mode Noise on VCC and Ground towards Overall Radiated Emission performances

Abstract

Present day data communication equipments use essentially a combination of digital ICs (both LSI & VLSI), analogue and A/D and D/A converters. The requirement of small size and light weight imposes the space constraints also. This scenario gives a bigger challenge than before to a design engineers especially under today’s EMC compulsory requirement. As the global trend in cost saving and size reduction continues within the Information Technology and Telecommunication industries, there is an increasing requirement and emphasis on eliminating EMI at PCB level rather than a reliance on cable solutions. For a design engineer to successfully suppress EMI on the PCB they must firstly have a clear understanding of which areas that normally generate EMI problems to be addressed. They should also understand the issues behind of that area, why it is attracting most in EMI countermeasure. Among the areas and its issues that normally addressed are a) Issues arriving from fast rise and fall times in system clocks, b) Issues from VCC transients during semiconductor “wake up” and c) Common and differential mode noise on VCC and GND lines. Here, the authors of this paper try to give an example of the effect of common and differential mode noise on VCC and GND lines toward the final radiated emission results. We purposely develop a high speed circuit with a combination of digital ICs, optical/electrical converter, analogue and A/D and D/A converters. The circuit is working but with less attention for PCB grounding planning. As a result, we can prove that most of the failure frequencies for the radiated emission test are due to the VCC and ground noise.