Partitioning

Partitioning separates the system into critical and non-critical sections from the point of view of EMC. Critical sections are those which contain radiating sources such as microprocessor logic or video circuitry, or which are particularly susceptible to imported interference: microprocessor circuitry and low-level analogue circuits.  Non-critical sections are those whose signal levels, bandwidths and circuit functions are such that they are not susceptible to interference nor capable of causing it: non-clocked logic, linear power supplies and power amplifier stages are typical examples. It can be helpful to segregate different digital functional sections, or those with different power rails.

A poorly partitioned, or non-partitioned system will have ill-defined interfaces between different component parts and the external connections (ports) will be dispersed around the periphery. This makes controlling the common-mode currents that will exist between the various external and internal interfaces very difficult. Dispersal of the ports means that the distances between them on opposite sides of the system is large, leading to high induced ground voltages in the presence of incoming interference and efficient coupling of internally generated emissions to the cables. All ports should be concentrated together.

Given a well-partitioned system, full shielding of an enclosure isn’t always necessary or even desirable. The “clean/dirty box” approach can be an effective compromise when different parts of the circuit need different degrees of shielding. Typically, it is not cost-effective to shield displays or keyboards even if the internal processing circuitry needs it. In cases where connection is made via terminal blocks, it is almost impossible to shield these successfully.

In these cases the shielding can be limited so that it only protects the most important parts of the circuit. But for this to be successful, every interface into and out of the shielded part must be properly filtered, at the point at which it traverses the shielding barrier.

If a shield is to perform its proper function, it must be used as a complement to filtering: you can't have one without the other. All interfaces that pass through the shield barrier, whatever signals or power they carry, must be filtered at the barrier unless they are carried in screened cables, in which case the cable screen must be bonded to the barrier. The filtering should be installed in such a way that it harmonizes with the barrier, so that the barrier prevents coupling between components on each side of the filter. Thus, series chokes should straddle the barrier, parallel capacitors should be grounded directly to the barrier with the lowest possible inductance: a good way of doing this is to use a ground plane on the PCB to mount the capacitors, and use gaskets, spring fingers or local mounting studs to make direct contact from this plane to the shield metal. Filter assemblies in metal cases should have their cases bolted metal-to-metal against the barrier; for extreme performance, you can add a conductive gasket around the periphery of the filter case.

Any deviation from this best practice will reduce the performance of the filter. Since the main effect of inadequate mounting is to increase stray capacitance across the filter or to increase the inductance of the connection to the ground reference, small deviations will be felt only at high frequencies; for instance the lack of a conductive gasket will be more noticeable above 100MHz; grounding via a strap rather than metal-to-metal will be most noticeable above 10MHz, and grounding via a long earth wire will render the filter ineffective above 1MHz.

Zoned cabinets

Where sensitive circuitry forms only a small part of the total within a cabinet it is preferable to enclose it in its own fully screened sub-unit, which is then housed in a cabinet that is not necessarily a screened enclosure, and is therefore cheaper and easier to maintain (right-hand diagram). This means that the "cabinet zone" is unscreened, and the "sensitive equipment zone" (screened) forms a separate volume within it. Zones can be nested if you need to establish further particularly sensitive or noisy local volumes; benign circuitry, such as relays or low-speed non-susceptible interfaces, need have no screening at all.

But where a number of circuits or modules require screening, housing them in a cabinet which is in itself a screened enclosure may afford a better solution (left-hand diagram). Where this is done a separate 'dirty' compartment needs to be provided for cable entries, or else all cables should be screened with their screens properly terminated to the outer enclosure. All screened cables entering the clean compartment are to have their screens grounded at entry, and all unscreened cables are to enter via filters.

To prevent interference currents circulating through the cabinet structure any ground terminal for external connection should be closely adjacent to the termination points of the screens. That is, cable entry panels should actually carry the ground stud, which is coupled via a short strap to the surrounding metalwork, such as a vehicle chassis or deck.