What is EMC Immunity testing & what to look out for
Electromagnetic Compatibility (EMC) can be divided into two parts: Immunity and Emission. The essential requirements of the EMC directive state that equipment (including devices and fixed installations) must not cause interference (emission) and must not be susceptible to interference (immunity). To determine the immunity of equipment, several tests have been defined. Previously, we have covered pulse-shaped tests such as ESD, EFT, and Surge. In this post, we will look at immunity to electromagnetic fields.
Conducted or radiated?
Immunity is often divided into conducted and radiated immunity. This division, however, is artificial. In practice, we only deal with electromagnetic fields reaching the device through the air. So why do we speak of conducted immunity? This has a practical reason. For low frequencies, the wavelength is relatively long. We can understand this by looking at the simple formula between wavelength and frequency:
λ=f/ν
Here, λ (lambda) is the wavelength, ν the propagation speed of the wave through the medium, and f the frequency. For electromagnetic radiation, ν is equal to f (the speed of light), so the formula becomes:
λ=f/C
An example makes it clearer. We round the speed of light C to 300,000,000 m/s. It is slightly lower. For a frequency in the FM band of 100 MHz, we get:
λ= 100,000,000 1 per s / 300,000,000 m/s= 3 meters
For a frequency of 27 MHz, the wavelength is about 11 meters. An effective antenna has a length of ¼ λ so for 100 MHz, a quarter of 3 meters is 75 cm. This length is practically seen in the length of a conventional car radio antenna. At 27 MHz, the length of an effective antenna increases to 2.75 meters. For conducting EMC immunity tests, this means the transmitting antenna applied must be 2.75 meters long. This is not very practical. On the other hand, we can also consider the equipment under test (EUT). To be influenced by electromagnetic radiation, there must be an effective antenna in the device. In relatively small devices, this is rarely the case. The only effective antennas, i.e., sufficiently long antennas, will be the cables connected to the device.
How can the relatively low-frequency electromagnetic field lead to influencing the device? This can occur because the field induces an interference current in the cable, and this interference current can lead to AM detection in the device. It has been chosen to inject this interference current directly into the cable using a clamp or CDN (Coupling/Decoupling Network). Hence, the term conducted immunity is used.
Immunity Standards
Standards are divided into basic standards that describe the measurement method. For conducted immunity, this is the NEN/EN/IEC 61000-4-6, and for radiated immunity, it is the NEN/EN/IEC 61000-4-3. Generic and product standards refer to these basic standards for conducting the tests. Generic and product standards provide the values and set the limits that equipment must meet. For immunity, the generic standard for household environments is NEN/EN/IEC 61000-6-1, and for (heavy) industrial environments, the generic immunity standard is NEN/EN/IEC 61000-6-2. Some well-known product standards are those for Audio/Video (EN55020 -> EN55035), ITE equipment (EN55024), and laboratory equipment (EN61326-1). These product standards also specify how influence must be determined.
Immunity Measurement Setups
The basic standards prescribe the measurement methods. Radiated immunity is usually conducted in an anechoic chamber. For immunity, the purpose of such a chamber is to keep the generated electromagnetic radiation inside and to ensure that the radiation does not reflect off the walls, which would lead to unwanted resonance peaks. The setup, as shown next to it, consists of a high-frequency signal generator that generates the interference signal of a certain frequency. This signal goes via a coaxial cable to a high-frequency broadband power amplifier that amplifies the signal to the desired level.
The amplifier’s output usually goes through a so-called directional coupler to a broadband antenna. The purpose of the directional coupler is to split the forward power and reflected power so that it can be measured with two RF power meters. A broadband antenna such as a logarithmic periodic (LogPer) antenna is used so that antennas do not need to be constantly changed during measurements. Although this significantly shortens the measurement time, there is a disadvantage to such antennas. The gain of such antennas is low. When high field strengths are needed, horn antennas are often used. They have a limited bandwidth (often 1 octave) but much higher gain. This allows a higher field to be achieved with the same amplifier.
Homogeneous Electromagnetic Fields
The basic standard prescribes that a homogeneous electromagnetic field must be generated at the location of the EUT. This can be determined with a so-called 16-point calibration. The size of the plane of four-by-four points is determined by the distance from the antenna to the EUT and the size of the EUT. The EUT must be fully illuminated. A homogeneous plane of 1.5 m by 1.5 is very common. The distance between the antenna and the EUT can vary from 1 meter for automotive components to 10 meters for large objects such as vehicles.
In conducted immunity, the antenna is effectively replaced by a clamp or CDN. The setup with signal generator, amplifier, and directional coupler with power meters is the same as the setup for radiated immunity. For the test, at low test levels, there is no need to conduct it in a shielded room. Tests with higher currents, such as those in automotive tests, are conducted in a Faraday cage for health reasons.
Frequency Bands
For household and industrial environments, conducted immunity measurements are performed in the band from 150 kHz to 80 MHz. Radiated immunity ranges from 80 MHz to 2.7 GHz. This upper limit will soon be raised to 6 GHz.
In the automotive industry, the band for bulk current injection runs from 1 MHz to 400 MHz and radiated immunity from 80 MHz to 18 GHz.
Test Levels
As stated, the product or product group standards determine the levels. For radiated immunity, these levels are given in V/m. The standards provide values ranging from 3 V/m for household environments, 10 V/m for industrial environments, to ~30 V/m for vehicles (E-marking). Certain manufacturers, especially in the automotive industry, set much higher requirements. Requirements of 100 V/m to even 600 V/m are common. The unit used in conducted immunity is Vrms. For household use, 3 Vrms is used, and for industrial use, 10 Vrms. In the automotive industry, the entire cable harness is often tested simultaneously, known as bulk current injection. The unit used here is mA, and requirements range from 60 mA for legal requirements to 300 mA as manufacturer requirements.
RadiField Concept
Raditeq Instruments has an established concept. For the frequency band from 1 GHz to 6 GHz, three amplifiers, each with its own directional coupler, power meters, and antenna, are integrated into one system. The advantage of this innovation is that a specific electromagnetic field can be generated much more efficiently and substantially cheaper. With this new concept, significant losses in coaxial cables and connectors will also become a thing of the past. We will inform you further about this innovation and the technology behind it later. Check out the RadiField here