Low EMI design of microprocessor based PCB-a case study

Microprocessor and microcontroller based circuits normally use high frequency crystals. In general, there will be a number of data lines on such a printed circuit board. It is observed that if special care is not taken in the layout of the printed circuit board (PCB), it is most likely to generate electromagnetic interference. It is very common to have EMI-hot zones on such a PCB; some of the ground islands may become very noisy and this has been observed in the authors' case study of the microprocessor based PCB. In such a case, because of the generation of common mode current, the circuit will give intermittent errors like those they have experienced in their case study and it may fail in the long run as the ICs are vulnerable to EMI. This paper deals in detail with the generation of common mode current as a source of EMI and its effects. The analysis of the generation of the common mode current is also covered in the paper. The protection measures to contain EMI in the high frequency PCB are also analysed for the reduction of EMI effects during the design of the PCB. In the authors' case, the system consists of two sub-systems, viz, the display unit and the control unit. Particularly, the EMI emission pattern from the hot zones is presented graphically. EMI emissions both conducted and radiated before and after application of control measures have been taken in accordance with the FCC Class B Operation.