4.8 Shielding

In some cases it is not possible to get the noise levels of a system low enough without adding a shield. In other applications a shield may be used because it is easier to use a shield than to achieve low noise levels by other means.

Depending on the application, the shield may cover the whole system or only the parts of the system that need it most. If the zone system is used in the design, it is easy to determine which zone(s) that need to be shielded.

In either case, the shield must be completely closed. A shield is like a pressurized container: almost good enough is as bad as nothing at all. As described earlier, all lines entering or leaving a zone need to be filtered. A single line that is not filtered will act like a single hole in a bucket of water. It will cause a leak.

A semi-closed shield, connected to ground, may still reduce noise. It will act as a ground plane, reducing the size of the loop antennas.

A common rule of thumb says that the maximum dimension of any mechanical slit or hole in the shield should be less than 1/10th of the minimum wavelength of the noise. In a system where the maximum significant noise frequency is 200MHz, this wavelength is 150cm, and the slits should be less than 15cm. But such a hole will still cause some reduction of the effectiveness of the shield. A hole that does not affect the effectiveness of the shield has to be less than 1% of the minimum wavelength, in this case 1.5cm.

It may turn out that a 100% effective shield is not required, though. The filters on the I/O and power lines are usually more important. In many applications, where high-frequency noise (>30MHz) is dominant, it may not even be necessary to use a metal shield. A conductive layer on the inside of a plastic housing will, in some cases, be sufficient.