Filtering Components for Pesky EMI Problems

PCBs are the most important component in electronic systems, and they are also responsible for creating or suppressing EMI from electronics assemblies. Many of the EMI problems created in a PCB layout are solved by implementing best practices for designing a PCB stackup and routing traces. The remaining problems in some systems, such as mixed-signal systems, require an appropriate grounding strategy to ensure there are no radiating conductors in the PCB assembly.

When these methods are unable to fully address the complete range of EMI problems, what can be done to ensure a product can meet EMC requirements. There are additional methods that help address conducted EMI which might leave the product and lead to radiation, or that can suppress noise from entering the system via input cabling. These additional methods center around filtering and are often low-cost additions to the bill of materials for a product.

Additional Methods to Reduce EMI

If you look at the PCB design methods mentioned above, the EMI suppression solutions mentioned above all address radiated emissions from signals and components. The right PCB stackup, placement, and routing of traces can solve simple problems that lead to radiated emissions, but you may still have conducted emissions. Those conducted emissions might also create new radiated emissions, such as from common-mode noise on groups of conductors.

Thankfully, there are simple components that can be used to target and reduce susceptibility to EMI and emissions of EMI from an assembly, which can be purchased off the shelf or designed as simple add-on circuits.

Cable Chokes and Cores

Chokes and cores are two of the simplest components that can help reduce common-mode noise and differential-mode noise coming into a system or leaving a system. When placed on cables, they should be placed very close to the inlet/outlet from the enclosure so that any noise on the cable is eliminated.

Chokes on cables

Chokes on cables should be chosen carefully because they limit bandwidth on signals that might leave a cable. Because of this, chokes are not the best option for data line cables, especially when the data interface is standardized in terms of data rate or bandwidth. Instead, PCB chokes can be used for both data and power lines that might exit the PCB.

PCB Chokes

Choke components mounted to PCBs are different from cable chokes and they offer a more sophisticated approach to filtering noise. Chokes can be used on incoming or outgoing data lines, as well as power lines. They often appear as line filters on AC inputs in order to remove conducted noise from the grid and prevent it from reaching the power stage.

Chokes are available that suppress common-mode noise, which can be used as power input chokes or as power/signal output chokes. For power systems, these chokes need to be selected to handle the appropriate amount of current, while data line chokes need to have high enough cutoff that they do not distort output signals.

PCB mount chokes

EMI Filter Circuits and Modules

EMI filter circuits are one option for reducing conducted emissions which could later turn into radiated emissions. One of the most common places to see an EMI filter circuit is on the input of a power stage, such as with an AC input into a DC/DC converter.

One option for quickly adding a filter circuit to a PCB without having to go through design, simulation, and testing on your own is to use a filter module. Manufacturers like Murata offer these modules as PCB-mounted components, and some modules are designed to provide higher order filtering of both common-mode and differential-mode noise in a single package. An example with a low pass filter circuit diagram (LC topology) is shown below.

5th order LC filter module circuit from Murata (source: BNX023-01L datasheet)

Still Having Problems?

If you have done everything correctly and you are still having an EMI problem, what should you do? The problems with EMI that can’t be solved by stackup, routing, filtering, and grounding can be something more complicated, and it may be the case that the source of EMI in the PCB or assembly was mis-diagnosed. When this occurs, there are a few options for moving forward:

1. Board-level shielding: targets radiated emissions or EMI susceptibility seen as conducted emissions

2. Enclosure-level shielding: targets radiated emissions, typically from mating surfaces or openings in the enclosure

3. Absorbing materials: includes conformal coatings and seals on mating surfaces made from absorbing materials

One point to note is that these methods are essentially band-aids that only address radiated emissions, they do not address conducted emissions. Conducted emissions might start as radiated EMI (crosstalk) and so need to be addressed in the PCB layout and routing.

In some instances, you will need to investigate specific signals or circuits for sources of noise and ringing leading to radiated or conducted emissions. Start from the observation point (output/input lines, cabling, etc.) and work backwards. Circuit simulations can also help identify or diagnose a source of EMI so that it can be targeted with the above methods.

Whenever you need to evaluate an EMI filtering and suppression technique for your PCB, you can fully qualify circuit behavior with the complete set of circuit simulation features in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.

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