4.3 System Zones

Handling every EMC problem at once is a very complex task. It is therefore a good idea to split the system into smaller subsystems or zones, and handle these individually. The zones may, in some cases, only be different areas of the same PCB. The important part is to have control of what happens inside one zone, and how the zones interact. For each zone, the designer should have some idea about what kind of noise the zone may emit, and what kind of noise it may have to endure. All lines going in and out of a zone may require some kind of filter. It is also very important to be aware about how noise may be radiated from one zone to another. Local shielding of very noisy and/or very sensitive circuits may be necessary.

The split may be done in two ways or a combination of these:

• The zones may be put apart from each other to separate noisy circuits from sensitive ones. The typical example here is a line-powered system containing both analog and digital circuits, where the (switch mode) power supply, the digital circuits and the analog circuits are put on different areas of the PCB.
• The zones may be put inside each other. The noise going into and out of the innermost zone will then have to pass several layers of filters and/or shielding. The total noise reduction will then be much more efficient than what can be received by one layer. An example here is a particularly sensitive analog circuit, perhaps with its own shield, on the analog part of a PCB inside a shielded enclosure with filtered I/O connectors. Another example is a fast microcontroller with fast communication to a nearby memory, and slower communication to other parts of the system. Then the MCU and the memory can be defined as the inner zone – the noisiest part. All lines leaving this zone should then be filtered, making sure that none of them carry the highest-frequency noise further out. The next level of filters may then be on the edge of the “digital zone”, and perhaps also a third layer of filtering on the system I/O ports is used to reduce emitted noise even further. Three layers of filters may sound expensive, but three simple filters may cost much less than an advanced “one-filter-handles-all” solution.