No, these acronyms are not new state-sponsored 3-letter agencies, but they are sometimes considered as dreadful!

ElectroMagnetic Compatibility: ElectroMagnetic Interference, Radio-Freqency Interference and ElectroStatic Discharge, this is what these mean.

This subject is seldom covered during hobby electronic discussions, because it is not essential (things will "work" without them), it is difficult to understand and in this regard is like some kind of "black magic", and even if you already know it, this subject is just boresome;-)

So why bother?

Actually, our own experience shows us that EMC is really the difference between a working prototype on the bench and a reliable product in the wild!

Another serious motivation is that any device must be certified for compliance against EMC standards in order to be transported, sold, distributed or even used outside a lab.

This is why we decided to take EMC into consideration right from the start for our FunKey Zero project, as the goal is to prototype as many features as possible for the #Keymu - open source keychain-sized gaming console that we would really like to turn into a usable end-user product.

EMC

ElectroMagnetic Compatibility (EMC) is both a device characteristic and a branch of electrical engineering that deals with unintentional generation, propagation and reception of electromagnetic energy which may cause unwanted effects. The corresponding issues are emission (whether deliberate or accidental), coupling and susceptibility (or its opposite immunity).

Please note that from an EMC compliance test point of view, this means that a device is potentially both a source and a victim of interference.

EMI

On the other hand, ElectroMagnetic Interference (EMI) is the disturbance generated by an external source that affects an electrical circuit, i.e. it is the phenomenon itself, not the characteristic or the study of it.

As the equivalent electrical element between 2 coupled devices is a complex impedance that can be a resistor (R), a capacitor (C) or an inductor (L), or any combination thereof, the coupling can be linearly detailed as conduction, electrostatic coupling or electromagnetic induction, respectively.

Continuous Interference

A Continuous Wave (CW) interference arises where the source continuously emits in a given range of frequencies. This type of interference is divided into sub-categories according to the considered frequency range.

For lower frequencies (typically less than a wavelength), conduction coupling is predominant and may affect 2 conductors in phase (such as a disturbance that hits both conductors at the same time) or out of phase. In the first case, this phenomenon is called common-mode coupling, whereas the second is called differential-mode coupling. This distinction is important, as the first class of common-mode coupling may be more easily handled to greatly attenuate the disturbance when the 2 conductors carry opposite signals ("differential signals"), whereas the second class of differential-mode coupling is much more difficult to deal with. In this last case, only frequency filtering is effective.

RFI

For higher frequencies and when the source and victim are separated by a large distance (typically more than a wavelength), both devices act as radio antennas: the source emits or radiates an electromagnetic wave which propagates across the space in between and is picked up or received by the victim. In this case, coupling is then achieved by radiation.

Radio-Frequency Interference (RFI) is thus a special case of EMI, for which disturbance are electromagnetic waves whose frequencies lie in the range extending from around 20 kHz to 300 GHz (the upper limit constantly increases as technology pushes it higher!), roughly the frequencies used in radio communication.

Pulse or Transient Interference

An ElectroMagnetic Pulse (EMP) arises when the source emits a short-duration pulse of electromagnetic energy. This event may be repetitive, such as for artificial sources like electric motors, electric ignition systems or continuous switching digital electronic circuits like DC/DC voltage converters.

Sources of isolated or short series of EMP are: power line surges / pulses, Lightning ElectroMagnetic Pulses (LEMP) and Nuclear ElectroMagnetic Pulses (NEMP).

ESD

ElectroStatic Discharge (ESD) arises when differently-charged objects are are brought into direct contact or close together and when the dielectric between then breaks down, often creating a visible spark in the air. In this regard, ESD is a special case of EMP.

ESD voltage on any device creates a large surge current which will flow through ohmic drops and if these resistances are large enough then it will create huge internal voltages.

Inside semiconductor ICs in particular, this may lead to catastrophic damages like direct punch holes in the substrate, or any other more subtle damages due to Electric Over-Stress (EOS), i.e. whenever the component operates beyond its absolute maximum ratings.

In order to test the effectiveness and reliability of ESD protection circuitry, the JEDEC industry standards for ICs are defining several test models (sources: http://www.electronicdesign.com/power/what-s-difference-between-hbm-cdm-and-mm-test and http://www.vlsifacts.com/esd-models-and-their-comparison-esd-part-2/):

• Human Body Model (HBM). This model represents an ESD event between a human body and an electronic component. HBM model helps to simulate stress level developed by electronic component through human touch discharging the static charge through device to ground. Its model and pulse waveform are:

• Machine Model (MM). This model represents an event when a machine or an automatic handling unit touches the IC. This is highly likely when there is a metal to metal contact during production. Machine model is not used very often nowadays since fabrication is all automated and ESD-free. Its model and pulse waveform are:

• Charged Device Model (CDM). This model assumes the IC itself is charged and then touches any ground plane. The CDM also addresses the possibility of charge residing in the package and later discharge through a pin which is grounded.  Its model and pulse waveform are:

A combined pulse waveform superimposition of all the above models gives:

In addition, at the system (device) level, the IEC defines a System Level Model in IEC 61000-4-2 test, which has typically 8 times higher testing voltage than CDM, and 20 times higher peak current testing than HBM. This is a model which assumes the electronic components to be mounted on the PCB and requires the system to be powered up and operating. This is similar to the previous component level models but at much higher stress levels. The typical requirement is 4KV – 8KV (ouch!):

Now What?

Now that we know our enemies, what should we do to protect our device and make it compliant to regulations so we don't disturb other ones?

Let's review all the measures we can take!

Shield

If possible, we should put a metallic shield around the electronic circuit. This is not very convenient for prototyping, but it helps a lot both to protect against external EMI and to avoid leaking EMI.

If the shield is solid metal, and all the shielded connectors are bonded to it (with EMI fingers). All noise is shunted around this path, all noise currents due to self-capacitance, due to current riding along to other cables, whatever.  Everything inside is oblivious to all this happening, because it's a Faraday cage. The continuous, low impedance shield is an absolute requirement to achieve this.

Actually, a shield may be required by some regulations, like does the FCC Part 15.247 for RF electronic circuits.

Enclosure

If a metallic shield is not possible, at least the electronic device must be contained within an enclosure to protect it from direct contact ESD on most of its surface.

On one hand, it completely spoils this very nice tech-look, but on the other hand, it may save your day when you spill a beverage on it!

And you can always use a transparent casing;-)

Ground Plane

If you don't have a metallic enclosure, you're a bit more pressed for options.  A circuit board with solid ground plane is as good a substitute as you can get.

Having a solid ground plane helps a lot to reduce EMI emissions by reducing the distance between a possible perturbing signal and the ground. Having a continuous ground plane also helps to evacuate incoming EMI energy and avoid sensitive electronic chips.

Grounding

Collecting EMI energy with a continuous ground plane is one thing, but this energy must be evacuated, and a proper grounding is the only solution. However, this is not always possible, like for a portable device, for example.

ESD Protection

ESD may have very nasty effects on semiconductors, and even when protected by a shield or a simple non-metallic enclosure, they may still hit the electronic device by contact or close proximity through ALL externally accessible devices: buttons, sliders, connectors, jacks, LEDs, LCD displays, loudspeaker, antennas...

In order to prevent ESD, EACH SINGLE supply or signal trace connecting this device on the PCB must feature a Transient Voltage Suppressor (TVS) device, placed as close as possible to the externally accessible component pin and connected to a good ground plane:

If we try to avoid Quantum Physics altogether, it is sufficient to say that these TVS devices are a kind of avalanche diodes, that are like a small capacitance (a few pF) when a voltage lower than a given threshold is applied between its pins, and convert into a short when the voltage is higher.

The main characteristics of these TVS diodes are their capacitance value,  threshold voltage, maximum allowed pulse voltage and duration, trigger time, and if they allow monodirectional or bidirectional current flow through them (used when a signal may be negative relative to ground).

EMI Filtering

Event when using a metallic shield to avoid radiation, EMI can still find its way in/out by conduction through all connected wires: power supply cables when applicable, and through all data cables and connectors.

For the general differential-mode coupling case, the solution is to add electronic filters that will remove all unwanted noise outside the expected frequency range. They may be band-pass or low-pass filters, and you can find some integrated filters (eventually with combined ESD protection) to add to your circuit, Ferrite bead choke to clamp onto a cable, or you can make your own filter using discrete components.

The connections that requires such EMI filtering are : audio jacks, USB power, externally accessible UARTs and all otherwise accessible data lines.

Common-Mode Noise Suppression

As explained above, common-mode interference is when a disturbing source pollutes 2 conductors with the same perturbation. In the case where the 2 conductors carry 2 differential signals that are always opposite, the perturbation can be removed almost completely using "common-mode choke".

This is very effective for Ethernet, for example, which specifications require transformer coupling, usually with some common mode filtering to take the edge off, and a 1nF capacitor, from center tap to ground, to handle ESD. It is usually combined with Bob-Smith termination resistors for the cable side, and 50 ohm termination resistor for the PHY device side.

Unfortunately, this is not true for USB which has poor common mode rejection, even in true differential High Speed mode, and no, common-mode filtering won't help here -- the USB signaling method with pull-up / pull-down resistors is not designed to accommodate that, unfortunately).

USB Device Shield / Ground Connection

Warning: this is a highly controversial subject and may trigger a flame war!

If the above EMI recommendations are mostly accepted, including for Ethernet connection (well, except maybe for the USB common-mode...), it seems that for this subject, contradictory recommendations abound, each with its own unsupported claims. Even authoritative-sounding sources such as Intel, Texas Instruments, FTDI, Cypress Semiconductor and Atmel / Microchip seem to disagree on the correct way to handle the cable shield on USB devices:

1. The Hardware Book website recommends not to connect the cable shield to ground on the device at all
2. Intel (Section 5.4, p. 9) recommends to connect the shield directly to signal ground
3. TI (Section 2.2.4, p.3) recommends to connect the shield to signal ground through a ferrite bead
4. FTDI (Section 3.2, p. 11) recommends to connect the shield to signal ground through a capacitor
5. Cypress (Figure 7, p. 7) and Atmel, now Microchip (Section 3.3.3, p. 8) recommend a 1 Mohm resistor in parallel with a 4.7 nF capacitor

The only reason to follow opinion #1 would be to avoid ground loops by connecting the shield on both side to grounds at different levels, but because USB cables are so short, ground loops aren't really much of an issue.

Opinion #2 will actually work, even when exposed to EMI (and won't radiate itself).  The downside is, all that EMI is conducted over your board's ground, and will find its way out any cables attached to it.

In Opinion #3, the ferrite bead blocks the EMI RF noise but does pass DC allowing DC level between systems to be aligned. The problem is that this system does not deal with ESD, which has no path to ground. I have seen such 0603 ferrite beads melted by a distant lightning strike!

As for Opinion #4, if using a high voltage capacitor (> 500V), this may effectively shunt the EMI RF noise and ESD to ground, but it does not pass DC allowing DC level between systems to be aligned. It is not a problem though if the device is portable and not connected to any other ground.

Opinion #5 is the ultimate one, the resistor creates equipotential between system ground and chassis ground, and the (high-voltage) capacitor shunts the EMI RF noise and ESD to ground.

Conclusion

EMC issues are often underestimated and not well understood. It is however important to take them into consideration during the design phase, should the device be sold one day or the other.

If EMC is not required during the prototyping phase of a device on a workbench, compliance against EMC rules and standards is absolutely mandatory to be able to distribute it as an end-user product.

The few design rules above should be enough to cover most simple cases, and use as guidelines for more complex ones.