Of the many adages in engineering that is also known as a long acronym, an important one to note is POSIWID, or “the purpose of a system is what it does.” Coined by Stafford Beer, the phrase is a reminder that complex systems often have unintended effects. Designers have to recognize the reality of outcomes, not just their intentions.

In electrical devices, the reality is that any new prototype has a hidden purpose — to act as an antenna receiving and emitting electromagnetic radiation. Even though it is not the desired outcome, an electronic device will generate its own set of unique frequencies. That is what it does.

Fortunately, wise engineers before us have already grappled with this side effect of electrical equipment. To join the community of deployed products, a new design must demonstrate good citizenship and neither interfere nor malfunction in the presence of other devices. Without electromagnetic compatibility (EMC), our world would be filled with static, failures, and safety hazards.

Basic Concepts of EMI

A schematic of a circuit tells us nothing about what kind of unintentional antenna the system will become once it is built. When operational, a magnetic field surrounds each component, cable, and via. These fields interact and combine to produce new frequencies, some with even greater intensity.

Circuit designs with similar components but in slightly different proximity might produce very different EMI effects. In one instance, destructive interference might cancel out adjacent sources, while constructive interference from two sources will amplify the resulting frequencies.

Random intermodulation can also cause higher frequencies to emerge through harmonic radiation or excite resonances elsewhere in the product that further shape the profile of emissions. Higher frequencies may require radio frequency interference (RFI) suppression should they fall in the range of wireless communications.

Electrostatic discharge (ESD) and transients also contribute to a product’s EMI challenges. ESD is a sudden and rapid transfer of electrostatic charge between two objects at different electrostatic potentials. It can occur when a charged object (such as a human body or a tool) comes into contact with the electronic device. ESD can cause damage to sensitive electronic circuits and components.

Transients, like bursts, surges, and interrupts, are brief, high-energy disturbances that can change the electrical behavior of the system.

When an electronic prototype reaches its EMI lab assessment, it may exhibit a complex tangle of EMI effects due to all the interactions between induced fields, harmonics, and pockets of static potential — none of which was according to plan.

Materials for EMI Reduction

One preemptive practice to minimize EMI is to incorporate materials and features that would make the device a poorer, less powerful antenna. Various conductive and dielectric materials act to interrupt the field by absorbing, reflecting, or otherwise reducing the radiation.

Signal Paths and Vias

The structures inside a printed circuit board (PCB) can lower their antenna-like effects by minimizing impedance. Dielectric materials with a low dissipation factor, controlled impedance materials, and copper weights are used for high-speed signal traces. Prepreg materials are used between copper layers to provide a well-controlled and uniform dielectric constant, making for more consistent impedance throughout the board.

Vias can act as antennas and radiate EMI if left unfilled. Therefore, vias are typically filled either with conductive or non-conductive materials to help reduce this radiation and maintain signal integrity. Techniques like via shielding, where vias are surrounded by grounded copper layers or fence structures, can contain electromagnetic fields.

Grounding

Continuous ground planes in a PCB, often made of a solid copper layer, provide a low-impedance return path for signals, reduce ground loops and work to minimize radiated EMI.

Grounding can extend to vias to ensure a continuous and low-impedance ground reference throughout the PCB. Strategically placed grounding vias can also be added near high-speed signals or components.

Shielding

Additionally, other materials and design techniques also play a role in EMI mitigation.
Shielding materials — copper or aluminum — are used for shielding enclosures, cables, and components to contain EMI. Absorbing materials with magnetic or dielectric properties can absorb electromagnetic waves, reducing reflections and EMI propagation.

These precautionary measures cannot guarantee a prototype with no EMI issues. Yet in many cases, efforts in shielding and grounding during the early design stage can keep the path towards compliance on track.

EMI Filtering Technologies

Besides building circuits in particular forms and adding layers of material, there are also the options to alter the circuit itself with the addition of filter components, specially designed to limit certain frequencies inside the current. Two options exist for this intervention, found inside and outside the device.

Inside the circuit

Another approach to EMI reduction is to integrate discrete components into the circuit design for the purpose of limiting the variety of frequencies in signal or power lines. Different categories of filter types attenuate a different range of frequencies.

The disadvantage to larger-scale board-mounted filter components is that the unpredictable noise can sometimes mismatch the chosen type of filter. Should the initial filter selection not achieve the right range of frequency suppression during testing, designers must go back to the drawing board and install a different filter component, one that more closely matches its unique unintentional-antenna properties.

Discrete filter parts may not have the same footprint, form factor, or mount, so this replacement may necessitate modification to the printed circuit board (PCB) or surrounding casing.

Outside the circuit

The alternative is not to alter the circuit inside the device but rather to extend the circuit with filtering capability outside the device. Technology developed by Quell does just that.

Micro-components are embedded into a silicone insert which slips over and connects to pins of the device’s power or signal cable at an input or output port. The configuration of the embedded micro-circuit targets specific frequency ranges.

The advantage of the filter insert solution is that it can be added to a prototype after a test failure without having to reengineer and rebuild the device. Since the inserts install in seconds without solder or tools, engineering teams can try out different filter types while still at the testing center and the product can be re-tested on the spot.

This outside-the-box method of filtering EMI — as used in EESeal® — works to mitigate both radiated and conducted emissions. There are also filter inserts that offer radio-frequency interference (RFI) suppression, enhanced grounding in the form of gaskets, and protection against electrostatic discharge (ESD) and transients through embedded diodes.

Anticipating the Unpredictable

While design intentions can be read in schematics, EMI side effects are often a black box, neither predictable nor transparent, since the causes of interference may be from a combination of sources inside the device.

With preventative measures and filtering technologies, devices can reduce conductance and emissions to acceptable levels and pass compliance testing. Even without a clear idea how unintentional radiation originates, engineers have several strategies of how to steer the effects towards a better outcome.

Read more about the micro-filtering technology of EESeal.