How to reduce RF emissions [Christian rosu]

Thiết kế EMC và phương pháp giảm nhiễu!

Long I/O and power cables usually act as good antennas, picking up noise from the outside world and conducting this into the system. For unshielded systems, long PCB tracks may also act as antennas. Once inside the system, the noise may be coupled into other, more sensitive signal lines. It is therefore vital that the amount of RF energy allowed into the system be kept as low as possible, even if the input lines themselves are not connected to any sensitive circuit. This can be done by adding one or more of the following:
* Series inductors or ferrite beads will reduce the amount of HF noise that reaches the microcontroller pin. They will have high impedance for HF, while having low impedance for low-frequency signals.
* Decoupling capacitors on the input lines will short the HF noise to ground. The capacitors should have low ESR (equivalent series resistance). This is more important than high capacitance values. In combination with resistors or inductors, the capacitors will form low-pass filters. If the system is shielded, the capacitors should be connected directly to the shield. This will prevent the noise from entering the system at all. Special feed-through capacitors are designed for this purpose, but these may be expensive.
* Special EMC filters combining inductors and capacitors in the same package are now delivered from many manufacturers in many different shapes and component values.