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Tips for conquering PCB electromagnetic issues

Posted: 11 Aug 2015     Print Version  Bookmark and Share

Keywords:Electromagnetic compatibility  EMC  electromagnetic interference  EMI  PCB 

Electromagnetic compatibility (EMC) and related electromagnetic interference (EMI) are two culprits system design engineers have historically kept a sharp eye on, among their myriad of other problematic areas. In particular, both are hampering PCB layout and design engineers, especially today as board designs and component packaging continue shrinking and OEMs demand higher-speed systems.

EMC is associated with the generation, propagation, and reception of electro-magnetic energy—not exactly a welcomed character in a PCB design. That energy results due to a mixed combination of energy producers and care must be taken to make sure such signals are compatible and do not interfere with each other when it is necessary that different circuits, traces, vias, and PCB materials operate in unison.

EMI, on the other hand, are the unwanted, damaging effects EMC or the undesirable energy generates. In such an electromagnetic environment, the PCB designer's goal is to assure that various energy elements are reduced to maintain minimal interfering effects.

Here are some tips on how to avoid these problems in your printed circuit board design.

Tip #1: Take it to ground
Designing a PCB's ground plane is the most important part of reducing EMI. The first step is to increase as much as possible the PCB's ground area within the board's total area, which reduces emissions, crosstalk, and noise. Care should be taken to connect each component to a ground point or plane. If that's not done, the neutralizing effect of a solid ground plane provides isn't being fully utilised.

A highly complex PCB design is characterized by several regulated voltages. Ideally, every reference voltage should possess its corresponding ground plane. However, an excessive number of ground planes could increase the PCB fabrication cost, making it overly expensive. A compromise measure is to split the ground planes at three to five different places, accommodating multiple ground sections in one ground layer. This keeps the cost of board fabrication cost in check while reducing EMI and EMC.

A low impedance ground system plays a big role in minimising EMC. In a multi-layer PCB it's best to have a solid ground plane versus copper thieving or hashed ground planes since it offers both a lower level of impedance with the current path serving as the backward source at its optimum level.

Figure 1: To deal with EMC problems in a multi-layer PCB, it is best to have a solid ground plane versus copper thieving or hashed ground planes.

The length of signal returns to ground is highly important, as well. The amount of time a signal takes to and from the source must be compatible; otherwise, an antenna-like phenomenon is created that radiates energy as part of EMI. Also, traces transmitting current to and from the source should have paths as short as possible. If source and return path aren't about equal in length, ground bounce, another form of EMI, is produced.

Figure 2: If the amount of time a signal takes to and from its source is not carefully synchronised, an antenna-like phenomenon is created that radiates energy as part of EMI.

Tip #2: Compartmentalise EMI
Because there are different segments to EMI, a good EMC design rule is to focus on analogue and digital circuitry separately. Analogue circuits carry relatively high amperage or high current and should be kept away from high-speed traces or switching signals. If at all possible, they should be guarded with a ground signal. On a multi-layer PCB, routing analogue traces there is should be a ground plane between them and the switching or high-speed traces should be on a different plane. Therefore, these signals, which have different characteristics, stay separately.

Sometimes a low pass filter can be included to eliminate high frequency noise coupled from surrounding traces. The filter suppresses the noise, allowing a stable current to continue on the return patch. It's important to keep the ground planes for the analogue and digital signals separate. Separation is critical due to the unique characteristics of analogue and digital circuitry. The digital signal should have a digital ground, and the analogue signal should be terminated at an analogue ground.

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