To prevent the occurrence of EMC problems the UK government adopted stringent laws, first back in 1992, forcing all manufacturers and importers of electronic goods to ensure that their products are electromagnetically compatible.

Business customers also often require that the products they buy are CE marked. This mark signifies that the goods meet all the relevant 'CE marking' directives in place in Europe, including the EMC Directive as written into UK law by Statutory Instruments. The legislation in place guides manufacturer's down a route of proving EMC compliance through EMC type Testing. It offers two basic methods to prove EMC Compliance:

EMC Testing to harmonised standards and make a declaration that the product complies (known as the EC Declaration of Conformity).

Agree an alternative or reduced test plan with a 'Competent Body', test and submit all relevant document to the body (known as the Technical Construction File).

Should a case come to court, the manufacture can defend themselves if they can show that they have taken "all reasonable precautions and due diligence". I.e. have checked the product, undertaken EMC Tests and made a concerted effort not to inadvertently break the law.

A summary about EMI - electromagnetic interference - the types of EMI and how they affect electronics equipment.

There are many forms of electromagnetic interference, EMI that can affect circuits and prevent them from working in the way that was intended. This EMI or radio frequency interference, RFI as it is sometimes called can arise in a number of ways, although in an ideal world it should not be present.

EMI - electromagnetic interference can arise from many sources, being either man made or natural. It can also have a variety of characteristics dependent upon its source and the nature of the mechanism giving rise to the interference.

By the very name of interference given to it, EMI is an unwanted signal at the signal receiver, and in general methods are sought to reduce the level of the interference.

Types of EMI - Electromagnetic Interference

EMI - Electromagnetic Interference can arise in many ways and from a number of sources. The different types of EMI can be categorized in a number of ways.

One way of categorizing the type of EMI is by the way it was created:

Man-made EMI:  

This type of EMI generally arises from other electronics circuits, although some EMI can arise from switching of large currents, etc.

Naturally occurring EMI:

This type of EMI can arise from many sources - cosmic noise as well as lightning and other atmospheric types of noise all contribute.

Another method of categorizing the type of EMI is by its duration:

Continuous interference:  

This type of EMI generally arises from a source such as a circuit that is emitting a continuous signal. However background noise, which is continuous, may be created in a number of ways, either manmade or naturally occurring.

Impulse noise:  

Again, this type of EMI may be man-made or naturally occurring. Lightning, ESD, and switching systems all contribute to impulse noise which is a form of EMI.

It is also possible to categorise the different types of EMI by their bandwidth.

Narrowband:  

Typically this form of EMI is likely to be a single carrier source - possibly generated by an oscillator of some form. Another form of narrowband EMI is the spurious signals caused by intermodulation and other forms of distortion in a transmitter such as a mobile phone of Wi-Fi router. These spurious signals will appear at different points in the spectrum and may cause interference to another user of the radio spectrum. As such these spurious signals must be kept within tight limits.

Broadband:  

There are many forms of broadband noise which can be experienced. It can arise from a great variety of sources. Man-made broadband interference can arise from sources such as arc welders where a spark is continuously generated. Naturally occurring broadband noise can be experienced from the Sun - it can cause sun-outs for satellite television systems when the Sun appears behind the satellite and noise can mask the wanted satellite signal. Fortunately these episodes only last for a few minutes.

EMI coupling mechanisms

There are many ways in which the electromagnetic interference can be coupled from the source to the receiver. Understanding which coupling method brings the interference to the receiver is a key to being able to address the problem.

Radiated:  

This type of EMI coupling is probably the most obvious. It is the type of EMI coupling that is normally experienced when the source and victim are separated by a large distance - typically more than a wavelength. The source radiates a signal which may be wanted or unwanted, and the victim receives it in a way that disrupts its performance.

Conducted:

Conducted emissions occur as the name implies when there is a conduction route along which the signals can travel. This may be along power cables or other interconnection cabling. The conduction may be in one of two modes:

Common mode:  

This type of EMI coupling occurs when the noise appears in the same phase on the two conductors, e.g. out and return for signals, or +ve and -ve for power cables.

Differential mode:  

This occurs when the noise is out of phase on the two conductors.

The filtering techniques required will vary according to the type of EMI coupling experienced. For common mode lines are filtered together. For differential mode, they may be filtered together.

Inductive coupling:  

What is normally termed inductive coupling can be one of two forms, namely capacitive coupling and magnetic induction.

Capacitive coupling:  

This occurs when a changing voltage from the source capacitively transfers a charge to the victim circuitry.

Magnetic coupling:

This type of EMI coupling exists when a varying magnetic field exists between the source and victim - typically two conductors may run close together (less than λ apart). This induces a current in the victim circuitry, thereby transferring the signal from source to victim.

By determining the form of coupling that exists and the way in which it is reaching the victim, it may prove to be that the most effective method of reducing the EMI is by putting measures in place to reduce the coupling and reduce the level of interference to an acceptable level.

Electromagnetic interference, EMI is present in all areas of electronics. By understanding the source, the coupling methods and the susceptibility of the victim, the level of interfere.

FCC is the abbreviation for the Federal Communications Commission. The FCC is responsible for rating personal computers and other equipment as either Class A or Class B. The ratings indicate how much radiation a personal computer emits: this rating process is often referred to as "certification." Almost all personal computers satisfy Class A requirements, which mean they are suitable for office use. Class B machines, which are suitable for anywhere, including the home, must pass more stringent tests. Class B indicates that the machine’s radio frequency (RF) emissions are so low that they do not interfere with other devices such as radios and TVs.

The FCC should rate computer components and systems before they are released for sale. The rating process is usually the responsibility of the manufacturer. For instance, before AOpen releases a new PC system for sale, the entire system is submitted to the FCC for rating. FCC rating (Class A or Class B) should be listed on the specification sheet or box of each product a manufacturer sells. Many customers, especially large corporate customers, will only accept FCC rated systems.

RoHS stands for Restriction of Hazardous Substances and regulates the use of certain hazardous substances in electronic equipment. The RoHS standard is fully implemented in Europe, with lesser restrictions applying in the US.

In particular, while RoHS mostly originated out of concern about the use of Lead in manufacturing, it addresses and restricts the use of not just Lead, but also Cadmium, Mercury, Hexavelent Chromium (a carcinogen), and Polybrominated biphenyls (flame retardants that may cause a variety of health issues) and Polybrominated diphenyl ethers (another flame retardant that may cause a variety of health issues).

The primary difficulty in meeting RoHS standards is that RoHS mandates that each component of a device fulfills the entire RoHS directive. In other words, if a single small subsystem is not in compliance, the entire device will not be considered in compliance. This can mean expensive redesign of almost an entire existing device.