A Basic Introduction To Electromagnetic Compatibility (EMC)
By Robin Mitchell
Creating projects for hobby reasons rarely calls on electromagnetic compatibility (EMC), but it is not unheard of when a hobby project becomes a commercial product. However, strict rules and regulations surround electronic devices on the market mainly involving EMC considerations, so in this article, we will look into what EMC is and some small tips on how to improve the EMC performance of a circuit.
What is EMC?
EMC stands for electromagnetic compatibility and it is concerned with circuit EM emissions and susceptibility to EM emissions. All circuits that switch or change voltage polarity emit EM radiation which can potentially be detected by a receiver. A classic example of EM emission is an RF transmitter circuit that generates a high-frequency oscillation which in turn creates photons with a frequency equal to the frequency of the oscillator (these EM emissions can then be detected by a radio receiver). This example of EM emission demonstrates intentional EM emission (since the goal is to send information over radio waves). However, many circuits containing microcontrollers and/or switching circuits produce EM emissions which can cause serious problems with other nearby circuits. Therefore, these emissions must be reduced as much as possible for a circuit to pass FCC and CE regulations (which is required when selling a product).
All EM emission related problems come down to three main areas:
- The Emitter – Something must be creating EM emissions (possibly at unacceptable levels)
- The Medium – Something must be carrying the EM emissions
- The Receiver – Something must be receiving EM emissions and be failing as a result
EMC issues can be solved by removing one of the three areas above and some techniques apply to more than one area shown above. Typically, it is easier to remove emissions and mediums than it is to remove receivers since most electronic circuits that exist on the market comply with receiver regulations (circuits inside products must not be susceptible to standard emissions from sources such as WiFi, Mobile phones, and other radio equipment).
Removing Sources Of EM Emission
Method 1 – Smooth Switching Circuits
Switching circuits and circuits that involve data/clock signals can cause serious problems when it comes to emission. The issue does not usually come from the switching frequency itself but from the rise and fall times from the signals. A sine wave signal in a wire will mostly emit EM radiation whose frequency is equal to the frequency of the sine wave. A square wave, however, is made up of a singular sine wave and an infinite number of sine waves at odd harmonic frequencies. Therefore, square waves have a tendency to emit radio waves not just on the frequency of the switching circuit but at many other frequencies as well. This is why spectrum analyzers are so important when trying to comply with EMC regulations (showing detected signals at different frequencies).
Square waves are made up of many sine waves
One method for reducing EM radiation from switching circuits is by increasing the rise and fall times. This is easily done with a capacitor and series resistor. One example of such smoothing can be found on many USB and other serial communication lines. Typical capacitor values range from 10pF to 47pF for such circuits. However, care must be taken when smoothing square waves as circuits based on CMOS technology will respond unexpectedly during intermittent voltages. Series resistors help to limit current and therefore help with the smoothing of square waves.
Method 2 – Length of Trace/Wire
A radio that has poor reception can significantly be improved by increasing the length of the aerial. The opposite applies to circuits that are prone to EM emissions. Traces and wires that carry signals which may be prone to EM emissions should be kept as short as possible. This is one of the reasons why high-frequency ICs such as microcontrollers and memory chips are placed so close to each other. One other trick to prevent stray emissions is to keep the trace as straight as possible. Bends and corners in traces can create reflections and help with EM emissions so removing these if possible is usually a good idea. However, if you need to choose between short traces and few bends, always choose the length of the trace and use vias to jump over traces in the way.
Preventing Access To Medium
The word “medium” here usually refers to the space between the emission source and the receiver. Therefore, if any emitted signals can be prevented from leaving, then no devices can be interfered with. However, it is easier to first try and reduce emissions in the first place.
Method 1 – Ground Planes and 4 Layers
Ground planes are large spans of copper on a PCB that are connected to a common reference (typically ground). Ground and power planes have a very good habit of absorbing stray EM emissions. So if EMC is a real concern for a PCB, 4 layers can be used with the top and bottom layer being power and ground. All signal connections are made using the two internal layers and by doing this, any emissions made by those traces are readily absorbed by the ground and power planes.
Method 2 – Stitching Via
PCBs that only have two layers cannot use an entire layer for power planes. Therefore, one technique to help guard against EM emissions is to split up the PCB layout into individual modules (such as power management, analog, and digital circuitry), and then place a “cage” of vias connected to ground. This creates a Faraday cage that helps to trap EM emissions and therefore improves the EMC of a PCB design.
See the isolated module that is on a higher layer of PCB
Method 3 – Metal Enclosures
The last resort for trying to improve the EMC performance of a PCB is to enclose the circuit in a metal cage or box (very similar to the ones found on top of radio modules). This enclosure is usually tied to ground which helps to prevent emissions leave the product and emissions coming in from outside EM sources. Again like the stitching via, these cages act as Faraday cages and help to direct the EM radiation to ground where it dissipates.
A module with a metal shield to protect against EM emissions
EMC is critical for any product that is going to market. No matter who you are or what it is you make, the chances are there will be laws and regulations that will hinder your circuit. Considering EMC early on during the design stage can drastically reduce the time needed to produce a functioning design and can even reduce the number of times needed to send the device to an EMC testing site which can cost a fortune.