PCB EMC Design


This article is not so much a schematic capture article, but for a lack of a better place to locate it on the site it will reside here. It is more or less an article on circuit design and how to keep EMC / EMI in mind during the design phase, so come certification testing there are little to no problems with emissions or susceptibility. Let’s get started.

EMC (Electromagnetic Compatibility) breaks down into two major categories. Emissions and Susceptibility. EMC Emissions are the radio waves that are transmitted by your board or product, through the air or along the power cord and back into the real world. Ever wonder why you’re asked to turn off your cell phones and all portable electronic devices before plane leaves the tarmac?

Any device that is powered will emit some amount of EM emissions. These radio waves can easily interfere with other electronics which is why its so important your pcb design to consider EMC in general. Susceptibility is the ability for your pcb board to work properly ‘under the influence’ of EMI. Remember back in the day when you were watching tv and your wife turned on the microwave to heat leftovers for dinner? The channel would get all fuzzy, the sound wouldn’t work right. Thats because the emissions of the microwave were interfere with the TV. This day and age the majority of these susceptibilities are handled at the pcb level. The following are some guidelines to curb the effects of EMC.

Board Stackup
During the pcb layout phase you have 2 option for the pcb stackup : Stripline and Microstrip. With a stripline configuration your power and ground planes are adjacent. Making the power and ground on adjacent layers greatly increases your interplane capacitance. This improves the high frequency decoupling and reduces noise on your power rails. The other option, microstrip, puts a pair of signal layers between the power and ground layers. This configuration reduces the noise coupling between traces (crosstalk) in different circuits. This is a great place for routing high speed traces. This configuration also has the benefit of creating a Faraday cage to keep any emissions to a minimum.

Decoupling Capacitors
Nearly every application note these days recommend decoupling caps be connected very close to the IC being used. That’s why nearly every schematic you see has loads and loads of caps. These capacitors provide a local storage tank of current for IC that are switching very quickly. Without the capacitor the current would have to flow much further ( a larger loop ) which will cause large voltage drops. Remember your traces have resistance and V = I X R. Without the decoupling caps the wires attached to Vcc and GND will transmit noise like a dipole antenna on your car stereo!

Ground Plane Design
This is a subject for another article, but a quick note here. Don’t remove the copper on the ground plane of your pcb. This generates voltage drops and radiated emissions unless its a very well thought out cut in the ground plane. Removing copper reduces the amount of capacitance as well. In general is a bad idea, so when ever possible leave the copper intact.