Meeting EMI/EMC Standards in Your PCB Designs
Your next device might make it to a GTEM test cell for EMC testing
What if you set two cell phones next to each other and suddenly neither of them worked properly? Thankfully, this doesn’t happen because designers and manufacturers made serious efforts to ensure these devices comply with EMC standards on conducted and radiated EMI. Any device should comply with EMC standards before it makes it to the marketplace.
While this sounds complicated, you have a number of simple design strategies to help your next device pass EMC tests. Making yourself aware of the various EMC standards organizations and their specifications is a good place to start.
EMC/EMI Standards for PCB Design
The U.S. Federal Communications Commission established some of the earliest EMC standards in 1979. The European Community later defined their own EMC standards, which formed the basis for future European Union standards.
Other standards organizations, including IEC, ISO, SAE, IEEE, and CISPR, also defined a number of standards geared towards specific applications or industries. While IEC and CISPR standards are enforced in Europe, IEEE standards are more popular in the U.S. In particular, the IEEE standards forms the basis for antenna calibration tests.
The U.S. military defines its own MIL-STD EMC requirements, which are among the most stringent standards worldwide.
Complying with EMC Standards
Companies caught releasing non-compliant devices or products might receive a warning, or be fined substantial sums of money. Failure to meet EMC requirements also poses a safety concern and damages a company’s reputation. Designing with an eye towards EMC can help ensure you won’t receive civil penalties once your device comes off the fabrication line. Designers take steps to comply with EMC standards by considering EMI from two perspectives:
Designing for EMI immunity: Design a device to withstand unwanted EMI from nearby devices. This typically starts with the right stackup and routing strategy.
Suppressing conducted and radiated EMI: Design a device to minimize the radiation it emits. The layer stack, grounding strategy, and component placement all play a role here.
Radiated EMC measurement with a near field probe
Some Strategies for Complying with EMC Standards
There are some basic design practices every designer should use to ensure their boards pass even basic EMC checks.
Stackup, Power, and Grounding
An EMC compliance strategy starts with your layer stack. Designing your board with a low inductance ground system has the greatest effect of minimizing EMI susceptibility. With multilayer boards, you should place a ground plane directly below signal layers in order to minimize loop inductance.
Interference in low-level signals leads to lower signal-to-noise ratios. So, it is a good idea to route these signals on an interior layer. If you have enough layers in your stack, place these traces between two ground planes, and then place your power plane below the bottom-most ground plane. Placing the power plane close to the ground plane provides strong capacitive coupling. Any noise or conducted EMI in the power plane will easily bleed into the nearby ground plane rather than interfering with signals.
Be careful when routing signals from an interior layer up to a surface layer as you will need to maintain tight coupling. You can maintain coupling to a reference plane by placing a nearby parallel via between the ground plane and the surface layer.
Judicious use of shielding is another strategy to provide your board immunity to radiated EMI. This also suppresses EMI radiated away from your board. If you are working with a wireless device, you can just place the antenna outside the shielding so it can still send and receive signals.
The simplest solution is to just use grounded shielding which will form a Faraday cage around sensitive components and traces. Not all designs and components will accommodate this solution. So, you might require a more elaborate shielding method. If you use a uniform ground plane in the interior of your board, a grounded via fence around the edge of your board provides similar protection.
Shielding can for suppressing radiated EMI
Mixed-signal Layout and Routing
Wireless devices that manipulate digital data are inherently mixed-signal devices. Thus, you should separate digital, low-frequency analog, and RF analog sections of the board into different regions. These sections should have their own dedicated areas in the ground layer, but the ground layer should be kept continuous. Place the final return to ground where return signals from one section will not travel under another.
Last but not least, it is a good idea to use bypass/decoupling capacitor as high pass filters. Connect these between the power pin of an active component and a grounded via to pass any residual noise back to ground. This provides filtration in two ways. Any ripple or switching noise will have a short return to ground with small loop inductance, reducing its effects on active components as well as emitted radiation. Noise induced by EMI will also have a short return back to ground, providing the same benefits.
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