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Consumer electronic devices have exploded in popularity in recent years and they have become an integral part of our daily lives. For all these devices to work in harmony, the Federal Communications Commission (FCC) has put a great deal of work into creating rules that these devices must abide by. These rules and regulations primarily exist to ensure devices do not emit interference that may degrade or diminish the performance of surrounding devices. The increasingly complexity of these products poses greater challenges for the designers who create them, especially with regards to complying with the FCC’s regulations.
It is best to consider electromagnetic compatibility (EMC) as a critical design element in the early stages of a product development cycle. Oftentimes EMC is an afterthought where action is taken only when an issue arises, at which point it becomes costly to the project’s budget and/or schedule. When starting a new design, some key areas to focus on are:
High Level Design Architecture
Enclosure Material. It is important to consider what will be encasing the electronic assembly and protecting it from the outside world. Metal enclosures are the preferred choice, as they will act to block the energy radiated from the electronics they contain and they present an opportunity also to have a clean termination path for external cabling. Plastic enclosures do not offer nearly the benefits that a metal enclosure does, however special metalized coatings can be applied to aid in shielding.
Circuit Board(s) Placement. Circuit boards are ideally placed fully within the enclosure. Their placement should also accommodate connector shield termination directly to the enclosure/chassis.
User Interface Components. If user interface components (buttons, LEDs etc.) are to be used, the enclosure opening should be kept as small as possible. Round openings are favorable to long rectangular ones, as they are less likely to allow RF energy to escape.
Input/Outputs and Cabling. I/O and power cable should be positioned in a way so that they do not require excessive cable length to reach the circuit board once inside the enclosure. If bulkhead connectors are used, it is important that they have a low impedance path to ground.
Circuit Board Schematic/Layout
High speed signal routing. These traces are ideally kept as short as possible with a consistent return plane underneath for the entirety of the route. Keep lower speed signals away from high speed traces.
Clock placement/edge rates. Clocks/Crystals should be as close as possible to the ICs they are associated with. Clock signal edge rates can be slowed within acceptable margin to reduce the noise generated.
Ground structure. Ground structures should ideally consist of plane placed in a PCB layer beneath all signal lines. Ground plane should be consistent without and slots or cutouts.
I/O cable signal filtering. Differential and common mode filtering components can be added to signal lines will exit circuit board/enclosure via I/O cabling. Examples of this are common mode chokes, ferrite beads, passive and active filtering networks etc.
Connector placement. Connectors should ideally be placed near the edge of the circuit board, so that the connector shield can directly connected enclosure/chassis.
Antenna Placement. Antennas are ideally placed near the edge of the circuit board. If a metal enclosure is utilized, it is important to use a proper bulkhead connector with low impedance shield termination. RF traces should be kept clear of other low speed traces to avoid coupling signals and generating excessive harmonic radiation.
Keeping EMC in mind when making all electronics related design decisions is sometimes an arduous task, however it is becoming a necessity with the increasing speeds of high speed digital circuitry. The increased work upfront more often will make the FCC certification process go smoothly.
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