Radiocommunications Agency  EMC Awareness

Segregation and partitioning

What this technique is used for

Segregation, partitioning or zoning is a technique for reducing the interference coupling between different and mutually antagonistic parts of a product or system.

Most of the published information on segregation concerns systems and installations, but it is a very important technique in IC design, in PCB layout, in the design of individual electronic units, and in the design of cabinets containing a number of electrical and electronic units. It is also important for cable routing.

How this technique is used

In PCB layout , inside items of electronic equipment and inside electrical cabinets – noisy and sensitive items are segregated to reduce crosstalk and internal interference that cause signal integrity problems.

One of the simplest rules in EMC is the inverse relationship of coupling amplitude with distance. Both mutual inductance and mutual capacitance – stray impedances that are nearly always undesired, and sometimes hard for the designer to control – decrease in inverse proportion to the separation distance of the conductors involved.

In order to be able to segregate parts of a circuit, you have first of all to apply the principle of partitioning. This identifies those parts of the circuit that have to kept separate from other parts on EMC grounds, so that appropriate separation techniques can be implemented. Partitioning is based on classifying modules as very noisy, slightly noisy, indifferent, slightly sensitive or very sensitive.

Key issues in employing this technique

For low cost, partitioning should be done at the earliest stage

The first design step, before individual circuit elements are created, is to partition the system. A poorly partitioned, or non-partitioned system may have the interfaces between its component sub-systems ill-defined and the external ports will be dispersed around the periphery. This makes segregating the various modules, and 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 ports on opposite sides of the system is large, leading to high induced ground voltages in the presence of incoming interference. Conversely, noise voltages generated within the system are coupled out efficiently by the inherent dipole mechanism, leading to high radiated emissions.

Usually the only way to control emissions from and immunity of such a system is by placing an overall shield around it and filtering each interface, which is a very expensive solution. In many cases it will be difficult or impossible to maintain integrity of the shield and still permit proper operation.

Segregation can be achieved by separation or by filtering and screening

Partitioning separates the system into critical and non-critical modules from the point of view of EMC. Critical modules 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 modules are those whose signal levels, bandwidths and circuit functions are such that they are not susceptible to interference nor likely to cause it: non-clocked logic, linear power supplies, the operator interface and LF power amplifier stages are typical examples.

Critical sections can then be segregated from non-critical simply by separation distance, if this is feasible, or be enclosed in a shielded enclosure into and out of which all external connections are carefully controlled. This enclosure may encase the whole product or only a portion of it, depending on the nature of the circuits: a major design goal should be to minimize the number of controlled interfaces, and to concentrate them physically close together. The shield acts both as a barrier to radiated interference and as a reference point for ground return currents.