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.