So you probably heard of the term EMI thrown around when designing your PCB. If you haven’t heard the term than it is imperative you read this article because it could save you a lot of time, money and headaches.
What is EMI?
EMI or Electro-Magnetic Interference is the disruption of operation of an electronic device when it is in the vicinity of an electromagnetic field (EM field) in the radio frequency (RF) spectrum that is caused by another electronic device [1-3].
All of us encounter EMI in our everyday life and it gets quite irritating. Common examples you may have experienced are:
- Disturbance in the audio/video signals on radio/TV due an aircraft flying at a low altitude.
- Noise on microphones from a cell phone handshaking with communication tower to process a call (Most of us have experience this, and have told people to please switch off their cellphones)
- A welding machine or a kitchen mixer/grinder generating undesired noise on the radio or through your computer speakers.
Now if you are designing a circuit or controller that relies on a high signal integrity, you may encounter a significant amount of noise in your signals if you do not design your PCB with Electromagnetic Considerations (EMC). This mostly affects microcontroller circuits with operating frequencies above 30 Mhz . Therefore if you want to achieve the highest signal performance or if your signal is already prone to other forms of noise, it is best to minimize the probability of your PCB acting like an undesired Radio Frequency (RF) Receiver/transmitter.
Basic EMI Rules
- Eliminate EMI at the source – Slow edges or signals well below the 30Mhz range do not contain harmonics in the EMI band .
- Build a solid Faraday cage around the product – The Faraday Cage that surrounds a product is intended to contain any RF energy that escapes from your circuits [1-3].
- Design a low ripple Power Distribution System (PDS) – Big topic to cover – Will write an Article on designing an effective PDS to minimize ground ripple .
- Make sure ground planes of PCBs are not segmented – Do not split ground planes, even in cases such as splitting an analog and ground plane. A split ground plane usually behaves a dipole antenna .
- Attach logic ground to the Faraday cage at only one place – Connecting logic ground to a Faraday cage in more than one place often turns the cage into an antenna .
- Attach logic ground to the Faraday cage where the unshielded wires exit. This should be done with a plane capacitor (capacitor created from the power and ground plane on the PCB) 
- Place low pass filters on unshielded wires exiting the Faraday cage .
- Place shields around wires exiting the Faraday cage that have useful signals in the EMI measurement range. A twisted paid or coaxial cable is also acceptable .
- Reduce all the magnetic loops of the Printed Circuit Board. You can also Decrease the surface of the magnetic loops, as well as shorten the loops around the oscillators and crystal .
- Choose Differential Signaling links between boards and boxes and avoid parallel interfaces where possible. The benefits of differential signally is discussed in this Article in-depth [1-3].
- Surface-Mount Devices vs Through-Hole Components Surface-mount devices (SMD) are better than leaded devices in dealing with RF energy because of the reduced inductance’s and closer component placements available .
These are just some of the main EMI guidelines you should follow. Now ofcourse there are more EMC guidelines to follow and to learn, so you should do some further research of ways to reduce EMI. This is to ensure that you understand thoroughly which EMI rule is applicable to your design. Also be very careful of the many invalid EMI rules that you read on the internet. Make sure that they are backed up by citation or scientific experiments.
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Citations  PCB Design Guidelines For Reduced EMI – Texas Instruments http://www.ti.com/lit/an/szza009/szza009.pdf  Top 10 EMC Design Considerations, Cypress Perform - http://www.cypress.com/file/106951/download  3 DAY SIGNAL INTEGRITY AND HIGH SPEED SYSTEM DESIGN, Speeding Edge – Lee Ritchey www.speedingedge.com