EMC and EMI control – How does it affect the Electronic Repairer?
I learned about Electromagnetic Interferences (EMI) the hard way when I was just 17! As a keen radio amateur – called Radio Ham – I built my own transmitter and exchanged messages with other enthusiasts around the world. Soon I found out that, when I was transmitting, my parent’s TV – and most of the neighbourhood’s TVs! – were totally jammed, loosing vertical and horizontal sync and displaying all sorts of patterns and lines over the picture. I was wondering how a transmitter built for frequencies around 14 MHz could be upsetting TV sets designed to receive frequencies above 100 MHz… I would soon learn about harmonics and antenna mismatch!
A Typical EMI Suppresor
The field of Electromagnetic Compatibility (EMC) was not very developed at that time and my parents somehow pioneered it by imposing a ban on my transmissions until midnight! Simple, efficient and all was fine until another phone call. A neighbour introduced himself as a Civil Engineer/Professor. He told me that he was teaching Structural Design (sounded like dark art to me!) at an evening college of engineering. Back home after midnight, his relaxing time was listening to classic music played on his state of the art sound system. Guess what he was hearing? My calls to the world, loud and clear on a background of Vivaldi… Again I wondered how a 14 MHz signal could be received and demodulated by a system designed for audio frequencies. I would soon learn about ground loops and detection by non-linear devices!
Yes, EMI can be annoying. It can be dangerous too, even deadly. Most of the texts on the subjects refer to the United Kingdom destroyer HMS Sheffield, sunk by an Exocet missile in 1982 during the Falkland war between UK and Argentina. The ship had a sophisticated missile detection system. However, the powerful radar was interfering with the ship’s radio communication system. When the captain wanted to talk to his bosses in the UK he had to temporarily disconnect the antimissile system. This is when it happened; 20 seamen lost their life.
We all know that electronic devices and mobile phones must be turned off in a plane during take-off and landing as they can interfere with the aircraft navigation system. We heard several stories about near crashes where CD players or other devices might have been involved.
An EMI situation involves a source and a victim. The source can be natural, for example lightening strikes and electrostatic discharges, or man-made like another piece of hardware such as electric motors, power supplies (mainly switch mode power supplies) or any type of electronic circuit. The victim is usually an electronic circuit or apparatus.
The coupling mechanism between source and victim, i.e. how the interferences travel, can be either by radiation (radio waves), by capacitive coupling, by inductive coupling or simply by conduction (travel along a wire or a cable). Often it is a combination of several, for example a power cable can conduct interferences and also act as an antenna to send or receive interferences.
Electromagnetic Interferences are regulated by standards. Those standards are specific to various applications, type of equipment and also differ depending on the regulating country. They define the maximum emission permitted and also the level of immunity or susceptibility for various equipment and situations. Usually they specify four different measurements:
– Radiated Emissions (source)
– Radiated Susceptibility (victim)
– Conducted Emissions (source)
– Conducted Susceptibility (victim)
Equipment or devices which do not comply with those standards are illegal and cannot be sold! To comply with the standards, designers use a number of methods including shielding, filtering and specific design techniques.
The subject of EMC and EMI control became increasingly important as the gap between maximum emission levels and maximum susceptibility decreased. As time goes, clock frequencies increase making them more likely to generate radio interferences and influence other circuits. On another hand, circuits become smaller and more sensitive making them more likely to be susceptible to interferences. Most technical schools today have introduced EMC as a subject in their curriculum.
Where does this affect the electronic repairer?
This is where we are all concerned. Designers will go all their way to limit both emissions and susceptibility by using shielding, filters and other “magic” to ensure that their circuit or piece of equipment meets the standards, and then can be sold on the market. Those “adds on” often need to be removed by the repairer to access components to be tested or replaced; they must be put back! For example, not putting a shield back after a repair might be tempting because it might appear that this doesn’t affect the operation of the circuit, so why bother?
Shielding of a TV tuner
Well, without the shield or filter the equipment becomes illegal and might affect other equipment (emission) or be affected by other equipment (susceptibility). A professional repairer will always replace everything the way it was before the repair, even if certain components appear unnecessary to the normal functioning of the circuit.
EMI shields can be complicated metallic shapes or just a piece of conductive tape or both. In all cases they must be put back after the repair is completed. They also include grids and those metallic parts that insure the contact between a cabinet and doors or panels, display shields and gaskets etc…
EMI filters or suppressors are built in the design as extra components but also appear as ferrite cores around the cables. We all came across those “lumps” at one or both ends of data cables.
EMC is a fascinating field; it can be compared with the environment protection, a fight against electronic pollution. It is the duty of the designer to insure that equipment or devices conform to the standards and the duty of the repairer to insure that the equipment or devices remain compliant.