Electromagnetic compatibility (EMC) design is rooted in electromagnetic fields (EMF): How they occur, how they interact, and how they can be mitigated so as not to cause electromagnetic interference (EMI) with other electronics nearby. So what exactly are EMFs, and what do you need to know about them?
If you enjoy watching spooky television shows, then you’ve likely heard ghost hunters claim they can hear signs of spiritual activity in EMF readers. The reality is a bit more mundane—“EMF” really just means “radiation” or “energy”—yet the implications can be just as great. Whether it’s immunity or emissions, transients or steady state, conducted or radiated, make no mistake: Your product can create disruptive electromagnetic fields and in turn, be affected by other products’ emissions too.
What are Electromagnetic Fields (EMF)?
As the name suggests, electromagnetic fields are made up of electric fields and magnetic fields, each mutually orthogonal to the other, created by electron flow in conductors. Magnetic fields encompass conductors (circuitry) with current flowing through them, while electric field lines emanate from the conductors like spokes from a wheel axle.
Near-Field EMF Vs. Far-Field EMF
Electromagnetic fields emanating from a product change their relationship with each other as they produce transverse electromagnetic waves and propagate further. EMFs close to the conductors generating them are considered near fields.
“Near” might seem a casual term, but it is quite specific, as there is a “point of no return” where near-field EMFs transition to far-field electromagnetic radiation.
Consider a current loop with low impedance and high current. This product creates a strong magnetic field and a weaker electric field. As the distance from the current loop increases, the strength of that magnetic field decreases.
“Yes, naturally,” you might say; but this is more than a product of distance. Over distance, the magnetic field transforms into a transverse electromagnetic wave where the ratio between the electric field and the magnetic field transitions from the circuit impedance (which can be anything) to the wave impedance of 377Ω. This ratio is always the same, and the electric field can increase to adhere to this ratio.
Source: https://www.researchgate.net/publication/358043946
Let’s say we switch around the product so the current loop in our example has high impedance and low current. The EMF will follow this same inverse law until the fields balance out to the same ratio with a wave impedance of 377Ω. Interestingly, whether impedance starts high or low, this equalization only happens within a sixth of the wavelength of the frequency of interest.
Beyond this point, the electromagnetic behavior is no longer that of a near field, but of a transverse wave—that is, it becomes far-field EMF. Transverse waves are often of more interest in EMC radiated emissions testing as they carry the net effect of the various near-field emissions.
NOTE: The radiation field type we primarily deal with in EMC is non-ionizing, from DC to the wavelength of visual light. Be aware of the other type, known as ionizing radiation, that can lead to cellular and DNA damage, such as UV, X-ray, and nuclear radiation—but those are topics for someone else’s blog!
EMF and Electromagnetic Interference (EMI)
Well, then what has this to do with anything? A lot.
EMI can be the result of unwanted external EMF. Someone else’s product/system radiation can align with your product’s more sensitive frequencies or the orientation of coupling mechanisms (traces/cables) in your product can be vulnerable to certain types of interference. Place these products next to each other on the coffee table and one or both of them may stop working.
That’s why EMC labs measure the levels of electromagnetic emission for product categories as well as the design’s electromagnetic immunity (sensitivity) to standard EMF levels. This sets and enforces a reasonable expectation that all the electronics in the world will play nicely in the same sandbox.
EMC design is all about creating products that satisfy EMC requirements out of the gate without adding costly or complex mitigation components. Then, we test that product in isolation to establish how immune and quiet the design is before releasing it into the wild.
Fighting for margin to exceed EMC requirements is often tempered by the cost in time and effort to improve a design that is legally considered good enough, but the effort can have a dual payoff: Often the very treatment used to reduce emissions can also strengthen your immunity, and vice versa. Products designed to simply meet requirements will not fare as well as those that exceed requirements by a 10dB or 20dB margin, but they are entitled to life in the electromagnetic world, just the same.
