09-09-2015 | By Mathias Goebel
Modern designs are becoming more susceptible to EMI
EMI (electromagnetic interference) comes from unintended electromagnetic radiation caused by changing currents in other parts of the same, or nearby circuits. This can affect signals, degrading, distorting or even destroying the signals present in transmission lines or in radio signals.
EMI may be radiated or conducted, and comes from most electrical or electromechanical devices, such as motors, as well as power lines, antennas and anything that carries an electrical signal such as PCB traces. In fact, any electronic device that is switching on and off will produce some kind of noise that may inadvertently interfere with other nearby electronics.
Although there are natural sources such as the sun, Northern Lights, and lightning, plus interference from nearby electronic systems and power lines, many sources of EMI are also found within the system itself. The power supply is often a culprit, since it typically contains high frequency switching components such as MOSFETs, which produce a lot of electrical noise.
The integrated circuits used in today’s high-tech electronic systems are evolving to use lower voltages and higher operating frequencies. This unfortunately makes them much more susceptible to EMI, which means the acceptable amount of noise in electronic systems is falling.
EMI should be considered from the start
EMI can do more than just distort signals – in extreme cases, it can cause complete system failure. Manufacturers are responsible for making sure that electronic devices are not susceptible to EMI, but also to make sure their devices are not producing noise that could interfere with other systems’ EMC (electromagnetic compatibility). The best approach is to consider EMI from the very beginning of the design, because it can be extremely time consuming and expensive to fix unforeseen EMI problems once a design is underway, or has even been completed and has failed testing.
Initial assessment of the system should reveal which parts of the design are potential noise sources and which parts are particularly sensitive to EMI. Potential noise sources, including power supplies, should be located as far as possible from any sensitive ICs. It’s important to consider this at the start, before design options become limited further down the line.
Grounding is not trivial
Potential sources of EMI should be identified early on and blocked, that is, low-impedance paths to ground should be provided for any EMI currents. They should be as short as possible, since even a few centimetres can make a significant difference for signals more than a few kilohertz.
On PCBs, ground planes are used. Any EMI susceptible tracks must be run above the ground plane, avoiding any areas where there are breaks in the ground plane. Traces placed too close to the edge of the ground plane can also cause problems due to fringing effects.
If grounding is neglected and loops are allowed to form, voltages can be induced due to magnetic coupling, and/or signals can be radiated, both of which cause EMI issues.
Board level shielding also depends on proper ground design
If, even with proper grounding and layout, EMI is still a problem then shielding can be used to reduce or eliminate radiated noise from parts of the system. A typical implementation might be a metal can or grate placed over the offending component or subsystem. Again, because the ground plane of the PCB is effectively one side of the can, ground plane design can affect how well the shield works. Any vias (vertical interconnect accesses) or traces entering the shielded area are potential areas where noise could escape.
Other factors affecting how well shields work include near field effects (which occur when the shield is too close to the source) and resonances (at high frequencies, resonances can induce strong oscillations, adding to the EMI problem).
Screened enclosures are effective but expensive (and often ugly)
System level shielding is implemented via screened enclosures, typically conductive boxes that are connected to ground, which can be an effective way to prevent radiated noise from penetrating or escaping the system. However, this option can be expensive. High quality screened enclosures will have as few joints/seams as possible as these are weak points where EMI can be radiated. EMI filters can be added to any cables or interfaces to the enclosure, and the cables themselves can be screened too.
Non-conductive enclosures with a conductive coating can also be used; they can be more aesthetically pleasing than the ‘big metal box’ which represents most screened enclosures, though they are not usually as effective.