Editor’s note: This question was asked in response to Interference Technology’s recent webinar by Keith Armstrong. To view the webinar, click here.
Question: In the case of a product which has a metal enclosure and shielded cables:
1. Is it recommended to connect the metal enclosure to the 0V reference plane?
2. For shielded cables, where to connect the shield of the cable –> to the metal enclosure, or to the 0V reference plane?
Answer: To answer both questions we need to understand how best to use metal (or metallized) enclosures as shields, and how to connect them to cable shields, filters, and PCB RF Reference Planes (I sometimes call them 0V Reference Planes too, because they are usually at the circuit’s 0V DC potential, but they don’t have to be at 0V so we should always try to call them RF Reference Planes to avoid missing good opportunities for improving cost-effectiveness by thinking they ought to be at ground or 0V potential).
This requires that we understand the “Skin Effect” – the way that AC currents flow closer to the surface of a conductor as the frequency increases.
I presented 47 slides during my short webinar, but the whole presentation has 58 slides – the slides numbered 50-58 inclusive briefly describing the Skin Effect, and showing how correct RF-bonding between metal enclosures, cable shields, shielded connectors and cable filters uses the skin effect to try to ensure that external surface currents (CM RF currents picked up by conductors from RF fields outside the metal box) flow only on the outside of the metal surfaces, giving the best RF immunity that is possible given the other aspects of the EMC design (e.g. the quality of the cable shield).
Also, these slides show how correct RF-bonding between metal enclosures, cable shields, shielded connectors, cable filters and PCB RF Reference Planes, uses the skin effect to try to ensure that internal surface currents (CM RF currents generated by unbalanced signals, and unbalanced stray couplings for circuits inside the box) flow only on the inside of the metal surfaces, giving good RF emissions.
Essentially, we provide short – low impedance – return paths for the CM currents, so they naturally ‘prefer’ to flow along them and improve our EMC. And we use skin effect to increase the impedances of the alternative current paths, that we don’t want CM currents to flow in, increasing the percentage of CM current that flows in the short loops that we want them to.
The figures in my slides 50-58 imply that it is possible to get 100% of the CM current to flow where we want them to. If we want 100dB of shielding (or filtering) this means we must ensure that 99.999% of the external CM currents stay outside the box, and 99.999% of the internal CM currents stay inside – but this is only achievable at significant cost and by paying great attention to every design detail. Most shielding and filtering in commercial and industrial products needs to achieve around 40dB or so, i.e. 99% diversion of CM currents.
I’ve used the terms “RF bonding” and RF Reference Plane in this answer, and they are briefly described in my answer to Q3 above.
It is important to note that the same EMC design techniques are effective when we don’t have a metal enclosure – providing we design our PCB’s RF Reference Plane well-enough. We treat the edge of the RF Reference Plane as if it were the wall of a metal enclosure.
It isn’t as good as having a well-shielded metal enclosure, but we have various PCB EMC design techniques that can make it almost as good, not least using board-level shielding using what are often called “tin cans” soldered to the PCB’s RF Reference Plane (although there are modern board-level shielding techniques using plated plastics, which are not tin cans).
-Keith Armstrong
