The Basis of Electromagnetic Interference (EMI): Electromagnetism
Electromagnetic interference. Those two words encompass a world in which once one enters, there is no return. Prior to dealing with electromagnetic interference (EMI) in a hands-on fashion, I was your typical, 60 Hz technician, believing a cable to be the equivalent of a short between two points. I saw things through the lens of millisecond periods. All of the action took place inside of the cables. Life was easy back then . . . back before I became bombarded with thoughts of transmission lines, ferrite cores, and radiation – oh, the horror of it all!
EMI, as I will forever refer to it, is unique in so many ways that it has created an entire industry and field of engineering of its own. Why? Because it is difficult to wrap your head around where high frequency currents are flowing, how they are coupling from one object to another, and why on Earth you are seeing them ten meters outside of a product’s grounded chassis. Black magic? Hardly – it just takes time and, most importantly, devotion to the subject to fully understand it and to attenuate the noise. After all, this is not a new phenomenon. As long as electrons have been moving, electromagnetic fields have existed. It wasn’t until the power grid became widespread, the advent of radio-technology, and the digital revolution that the subject went from marginal to regulatory.
So what is EMI, anyway? The best and easiest way I always use to describe EMI to either the layman or the engineering novice (of which I’m not far off) is an example from my childhood. Being the budding mind that I was, I tended to not have much homework growing up (somewhere along the way I lost my ability to retain information or accept it with more than a look of confusion.) I would come home from school, unload my backpack, run to the television, and watch whatever inane and useless cartoon I could find. Being a boy with his Nickelodeon, I was content . . . until my mother began work on our supper. When she would begin to make mashed potatoes, suddenly the television screen would have diagonal white lines appear on it, slowly moving vertically down the screen. I would deal with it, biting my lip and exhausting myself of frustration until she finished. Once the mashed potatoes were made, the mixer went off and my television resumed having a clear picture. It had been that infernal mixer, again!
This never did any damage to the television (or me, I think,) but it was interesting nonetheless that something so small, multiple rooms away, could possibly disrupt my television. This is the essence of EMI and the reason why it must be controlled and reduced via electromagnetic compatibility (EMC) testing and regulations.
Electricity is the movement of electrical charges. Taking this idea one step further, electrical charges create electromagnetic fields. This occurs because a difference in charges (potential) creates an electric field and the movement of charges (current) creates a magnetic field. These fields, alone, do not propagate very well and are typically absorbed by the source conductor or a close-by receiver (conductor, cable, piece of metal, etc.) However, when we have a magnetic field that is changing, it creates a changing electric field which then creates a magnetic field which then creates an electric field and so on. These magnetic and electric waves, caused by changing waves (as opposed to changing current or charge distributions in the conductor) do propagate well, and at a certain distance are referred to as the far-field of an electromagnetic field. This is also known as electromagnetic radiation.
Everything above direct current (DC) radiates. In fact, the old saying for this is “DC to Daylight” – there’s no copyright on that one so use it until your friends are tired of hearing it. However, not everything above DC radiates well because wavelengths and antenna theory comes into play. The wavelength of a signal is inversely proportional to the frequency of the signal, which is how fast or slow it completes a cycle. The smaller the frequency, the longer it takes the signal to complete a cycle, the greater distance the wave will travel before the cycle completes itself. Electricity moves at the speed of light (approximately 300 meters per second in air) which means a cable appears as a ‘short’ to lower frequencies. Using 60Hz as an example, the time period for one cycle would be .0167 seconds, or 16.7 milliseconds (mS.) For the sake of this discussion, let’s say that the electric charge travels through a conductor at the speed of light, which is roughly 300 meters per second. A 60Hz signal will have a full wavelength of five million meters (16.13 million feet for my American friends.) That’s a lot of copper.
Since most cables are somewhere below five million meters long, all cabling appears as a ‘short’ to 60Hz alternating current. Being that 60Hz is an extremely low frequency (ELF,) and that it takes miles upon miles of cable to act as an efficient antenna, it radiates poorly. Add to that, all of the energy in a 60Hz system is used for the purpose of powering a load whether that load is a power supply, a toaster, a vacuum cleaner, etc. It is not used for radiating/transmitting. Because of the antenna properties required for efficient transmission at extremely-low frequencies, radiated EMI emissions are not seriously considered until 10kHz (note: these ELF frequencies and associated cabling can still disrupt other cabling by means of its magnetic and electric fields; this is why power cabling and low-voltage sense wiring is often separated.)
What happens when we increase the frequency above 10 kHz? This is where things begin to get ugly. High frequencies are constantly switching which means two things: the electric (E) and magnetic (H) fields are constantly changing (which causes radiation) and the wavelengths are short enough that a cable may no longer be a short; it may be an antenna. Everyone knows that radio stations transmit their signals from an antenna of the appropriate size for their frequencies. In the same sense, electromagnetic waves are transmitted whenever the conductor ‘looks’ like an efficient antenna for a specific frequency. At 100 MHz, for example, the wavelength is three meters. A quarter wavelength at this frequency would be ¾ of a meter (a little under two and a half feet.) You can see how a short cable could easily transmit this frequency and you can also see why there are regulations concerning only specific frequency ranges. Low frequencies are hardly an issue. As for 10kHz to 100 GHz? EM fields not only radiate, they do it without remorse.
This is where the issue of electromagnetic interference arises. Computers, printed circuit boards (PCB or PWA,) processors, and communications all operate in this frequency range. Devices running off of 60Hz current such as a motor, typically do not care what EMI gets on the line because they are being driven by such a strong 60Hz source signal (though it can cause problems.) The microvolts of high frequency noise that these loads see are nothing. However, a circuit on a PCB is running at the same frequencies as the interference signals and is operating at low enough voltages that a handful of microvolts may disrupt it. A continuous EM signal can disrupt a processor’s communication to another component and cause devices to malfunction. It may cause a display to flicker. Worst yet, it may even cause a critical component to misfire, leading to severe problems. This is electromagnetic interference.
Notice, though, that everything with a moving charge produces electromagnetic fields (and therefore hast the potential to cause EMI,) whether they are effective or not. This means not only can manmade equipment such as processors and switching circuits cause EMI, but also everything from lightening to solar activity. EMI can be created from a continuous source such as FM radio or a ‘pulsed’ or transient source such as relay closing. So that we understand the severity of the subject material, electromagnetic force is, as Wikipedia states: “the interaction responsible for practically all the phenomena encountered in daily life, with the exception of gravity.” That is one lofty description and as we have seen, is associated with every electrical action on the planet (and outside of it, as well.)
Does this mean, then, that when we say we are attempting to attenuate EMI that we are attempting to remove one of the most fundamental interactions in the known universe? Not at all. Electromagnetic compatibility is directly associated with reducing the affects of EMI on a system and also ensuring that equipment does not produce EMI at a level that will disrupt other products. It is impossible to ‘undo’ the physics of electromagnetism but it is possible to circumvent any harmful effects that may come from it. Through shielding, reduced loop areas, capacitors, inductors, filtering, grounding, and (to be honest) guess-and-check methods, we can keep electromagnetic fields isolated by attenuating them. The name of the game is not eliminating electromagnetism, it is eliminating electromagnetic interference.