In the growing world of the Internet of Things (IoT), all devices need to work together without interference. Each of them needs to work without disturbing others, Electromagnetic Compatibility, (EMC) and without being disturbed by them (Electromagnetic Immunity). These are functional requirements, as well as regulatory ones. Reducing the risk of EMC problems at an early stage is critical to avoiding extra costs and delays when launching an IoT product on the market. This article covers key aspects to consider when designing a new IoT board to get a good EMC/EMI performance.
Watch power integrity
IoT boards, like other types of boards, need a stable power source, distribution and absorption of peak currents. This is known as Power Integrity (PI). If it is not the case, the board will experience power integrity issues such as voltage drops or noise generation, which will led into ringing, jitter, unwanted emissions, among others.
It is common for IoT boards to have high-speed signals, so the fast switching provokes peaks in the current consumption, affecting the stability of the voltage rails. To design a good Power Distribution Network, designers need to dimension it to limit the voltage ripple so all the Integrated Circuits can tolerate the variations and keep the distribution impedance lower than the target impedance.
Avoid self-interferences
The energy coming from elements existing in the board, like digital clocks, high-speed data buses, or switched power mode power supplies, can reduce the sensitivity of the communication receivers. It is not just a functionality problem but also regulatory since manufacturers need to be compliant with certain tests related to receiver sensitivity and transmission power. Compliance is at stake when this sensitivity is affected, provoking extra costs and delaying the time to market. This is why keeping interferences provoked by the board elements is critical to increase the chances of passing the tests and start selling the IoT product.
A good technique to prevent self-interferences in an IoT board is carefully planning the board zoning. Define the areas prone to generating noise, like DC-DC converters, high-speed signals, or Radio Frequency blocks, and the most sensitive ones, like analog circuits or sensors. Separate them and use filtering techniques. Considering any cable or wiring harness connected to the board is also relevant since they can act as unintended antennas.
Shield high-speed areas
IoT boards have a very constrained space, which makes it very challenging to keep sensitive circuits far from noise generators. When the limits of separation are reached, shielding can help to contain the noise emissions within a specific area. A shield will attenuate the emissions in wide broadband, containing most of the noise within the PCB. The efficiency of a shield is measured by the Shielding Effectiveness (SE), which measures the attenuation (in dB) of the electromagnetic fields after traversing the shielding material.
Do not forget protections
Very often, IoT boards are connected to other boards, systems, external power sources, and even humans (think about a touchscreen). Each of those is a potential source of noise and interference. It is important that every interface is protected against external noise by using filters of different types, such as common-mode noise or differential-mode noise filters. Filters are two-fold: they protect the board from external noise, and they help contain the noise within the board. On top of that, the board could be damaged by overvoltage or transients, so power lines should be protected by using Transient Voltage Suppressors (TVS) or any other component to limit the amount of energy entering the board.
Test early, test often
EMC tests and any other Radio Frequency test, such as spurious testing, are expensive and time-consuming. Designers and testers often wait until the electronic product is manufactured to start testing it. However, the room for corrections by then is very small, and the negative impact on the product roadmap is huge. To avoid huge pains, the best thing to do is to start making tests such as conducted emissions, radiated emissions, or some radio tests with pre-compliance tools. Shifting the validation to the early stages of the product lifecycle reduces risks and helps control extra costs.
Author: Ignacio de Mendizabal, EMC Expert, mitai.ai
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