Basics Practice Reviews

EMC – What is "electromagnetic compatibility" and where does it affect us? | Basics

The topic of EMC keeps popping up in connection with electronics of all kinds. This small article is intended to roughly explain what this topic is all about. Since this topic is so extensive that there are already several books on it, I will keep it very superficial. Accordingly, I will only put the connections into a simplified way, since everything else goes beyond any reasonable framework for an article for an IT site. So it is definitely not a scientific work, it is just a very rough explanation. Again, pictures of Igor'sLAB were used. Thank you for your permission.

What is EMC?

The abbreviation EMV stands for electro-magnetic compatibility. This ultimately means that the device itself does not cause so much electromagnetism that it interferes with other devices or the like, and is not disturbed by devices that comply with the regulations. This sounds much simpler than it is. The EMC always depends on the frequency. There are frequency ranges where EMC emissions are undesirable. Therefore, regulations have been adopted which specify the frequency range in which how many EMC emissions are allowed. Accordingly, the regulations are almost always designed in such a way that it is relatively easy to see in which frequency range how much is allowed. Usually these are shown as graphs.

Here's an example of what something like this might look like:

Figure 1- Example Curve for EMC Limits

The frequency is usually displayed logarithmically. Graphs are not uncommon for each frequency range. In places, maximum values for the peaks and maximum values for the average are mentioned.  In any case, it can be assumed that too many emissions in the frequency range of Tetra radio, mobile phone, Wi-Fi or the like can cause problems and trouble with authorities relatively quickly.

Electromagnetism Basic Knowledge

In order to understand what the EMC is actually about, some basic knowledge about electromagnetism is needed. This knowledge is often taught in schools and then forgotten again. In fact, this topic is constantly being discussed. Basically, a conductor flowing through current generates a magnetic field around it. For optical illustration, it has been agreed on field lines, which show the orientation of the magnetic field.

Figure 2: Field lines on a current-flowing conductor

In principle, the magnetic field is all the stronger, even closer to the current-flowing conductor. Voltage and current also influence the strength of the magnetic field. At the same time, the magnetic field can be restricted with a grounded metal housing. If you now pack a 2nd, not current-flowing ladder next to it, relatively little happens at first. If the conductor or magnetic field changes or moves, the voltage in the conductor is affected. A voltage is induced from the changing magnetic field into the 2nd conductor.

Of course, the whole thing must also be taken into account with printed circuit boards. Considering that modern printed circuit boards consist of several copper layers, you have to consider not only what is next to the track, but also what is above and below. Here we take a look at a 4-layer printed circuit board:

Figure 3: Field lines on a current-flowing trace

The field drawn here shows only a single position, for example. In reality, this must be considered over the entire length of the track. And the next thing to remember is that the other traces can also interfere. And because it's so funny, these magnetic fields can also affect other circuit boards nearby.

However, these properties can also have a positive effect. For generators (Figure 4) and transformers (Figure 5), these properties are exploited very extensively. At the anchor winding of the generator, for example, direct current is applied. The rotation also rotates the magnetic field and induces the voltage curve shown at the bottom right in the outer coils. Usually generators are equipped with a metal housing for shielding

Figure 4: Schemes of a Generator

In the case of a transformer, the property is also exploited that one can "lead" a magnetic field with an iron core through the magnetic field-generating winding. Thus, the magnetic field of e.g. the left coil to the right coil and induces a voltage. The ratio of the voltage is directly proportional to the number of coils.

Figure 5: Transformer Schematics


Signal building basic knowledge

Now we come to the somewhat in-depth knowledge, which is also part of the study of electrical engineering. The voltage curve of an optimal bus signal would look something like this in the direct comparison to the more realistic image in practice:

Figure 6: Ideal bus signal
Figure 7: More realistic bus signal

And even that is still very optimistic. For high-speed signals, such as PCIe often looks much worse. If you want to get the frequency spectrum of the signal, for example, the Fourier analysis is an offer. The signal is sent to the 1. Harmonic shaft and the other harmonics. If you add all these waves together, you get the original voltage curve of the signal again.

Figure 8: Decomposition of the Fourier-based bus signal

Each of the waves has its own frequency and voltage level. If you put them in a diagram now, you get the following picture:

Figure 9: Frequency spectrum of the bus signal

This picture shows quite nicely the spectrum of the example bus signal around the base frequency in the middle. If you draw a Hull curve here, you can see quite nice a frequency range in which the bus signal works. And these frequencies are also in which the magnetic field changes around the conductors of the bus.


Emc Electro-magnetic compatibility
Rgb Red-Green-Blue (Additive Color Blend)
Common used LED colors with which as many colors as possible can be mixed
Tetra radio Digital government radio. Is used as the successor to the analogue
Radio system with 2m and 4m wavelength seen

Universal Serial Bus; All-purpose bus for connecting peripherals

Pwm Pulse width modulation; Control of the e.g. the performance
controlled by a pulse-by-pulse control.
Duty cycle Ratio between on-and-off time at PWM
Wireless Wireless Local Area Network


Peripheral Component Interconnect Express; High-speed bus
Often used in 16-fold version to connect graphics cards
Active speakers Loudspeaker with integrated (usually) analog amplifier
Oszi abbreviation for oscilloscopes;
A device that measures the course of voltages and can visually display them.