Today we are surrounded by a wide variety of high frequency electromagnetic fields. Radio, television and mobile communication technologies transmit a multitude of high frequency signals. These fields are useful for modern communication, but can adversely affect other electronic devices. Conductor tracks and wires of the devices act as antennas and the coupled electromagnetic energy can, depending on the field strength and circuit, negatively alter the performance characteristics of a device or directly destroy components. For example, we all know the “crack” of radios, when a mobile phone is nearby. In case of power supplies, the negative impact of the electromagnetic field could express itself in form of a drop in the output voltage.
To prevent this from happening, the EMC standard IEC 61000-4-3 regulates tests regarding the immunity of devices against high frequency electromagnetic fields. This article describes the general regulations of the standard, gives an overview on electromagnetic fields with their effects and names some measures to increase the immunity of electronic devices.
What are high frequency electromagnetic fields
High frequency electromagnetic fields are located in the electromagnetic spectrum in the frequency range between 100 kilohertz (kHz) and 300 gigahertz (GHz). They are generally radiated from an antenna and can transmit energy and information over long distances. Due to the wide range of possible uses of high-frequency electromagnetic fields, especially for modern communication today, (e.g. radio, television, mobile communications, cordless cellphones, WLAN and Bluetooth applications) people are surrounded by a multitude of different transmission devices that operate with different transmission powers and frequencies. The frequency and wavelength of electromagnetic fields are linked by the propagation velocity (in free space this is the speed of light c) and describe the wave character of the fields. At high frequencies f, the wavelengths λ (lambda) are small and correspondingly larger at low frequencies. When propagated in free space, the wavelengths are between 3 kilometers and 1 millimeter.
Table: Frequency bands and wavelengths
Unit of measurement of the electric field strength
The intensity or strength of the fields is indicated either in the form of the electric field strength (unit: volts per meter, V / m), or the magnetic field strength (unit: amps per meter, A / m), or in the form of power flux density (unit: watts per square meter, W / m2).
Propagation of high-frequency electromagnetic fields
As the distance from a transmitter increases, the field strength decreases rapidly. In free space, the power flux density decreases with the square of the distance, which means, with the double distance the flux density decreases to a quarter. Because many antennas radiate with certain preferred directions due to their design, the intensity at locations in the vicinity of a transmitter can be very different, despite identical distances to the source. As a rule, it is not possible to deduce the field strengths at a particular location alone from the distance. High-frequency electromagnetic fields can also be reflected or absorbed by objects that are in the direction of propagation. Which mechanism predominates depends, among other things, on the material properties of the respective object. Therefore, the propagation of high-frequency fields in the real environment often differs significantly from the simple case given above; the propagation in free space.
Effect on humans
Humans contain many electrically charged particles and polar molecules. Although polar molecules, such as the water molecule, are electrically neutral as a whole, they carry a negative charge at one end and a positive charge at the other. Electric and magnetic fields exert a force on electrically charged or polar particles to move. In a high-frequency electromagnetic field, the particles move very fast in time with the frequency. They rub together and heat is created. If the fields are very strong, entire cells can move due to the force effect. They align themselves in the field or migrate. Such non-thermal effects cannot be triggered by fields of radio applications, since their field strength is not sufficient for this.
Decisive for the biological effect of high-frequency fields is the energy absorbed by the body. The basis for this is the Specific Absorption Rate (SAR, unit of measure: Watts per kilogram, W / kg). It indicates the power (energy per time) absorbed per kilogram of tissue. If the body is only heated locally, the blood can dissipate the extra heat. If the whole body is heated, the skin is supplied with more blood and the heat is released by evaporation on the skin surface (sweating). Health effects can be expected if certain thresholds are exceeded and the body's thermoregulation is disturbed.
Effect on electronic equipment
Most electronic devices can be influenced by electromagnetic fields. These fields are often generated by other mobile devices in the environment (e.g. hand-held walkie-talkies, cell phones and laptops) or by stationary radio transmitters and television broadcast transmitters, radio equipment and various industrial sources. Conductor tracks and wires of the devices act as antennas and the coupled electromagnetic energy can, depending on the field strength and circuit, directly destroy components or negatively alter the performance characteristics of a device. For example, in case of power supplies this could mean a drop in the output voltage, for radios it is the known “crack” when a mobile phone is nearby.
Immunity of electronic devices
Immunity against electromagnetic fields is tested on devices according to the IEC61000-4-3 standard. An interference level with special field strengths is transmitted with the aid of an antenna and a homogeneous field is generated around the device to be tested. The device under test is positioned in a pre-calibrated area and the required disturbance, usually an amplitude modulated signal, gets sent out.
The frequency is then increased in a stepwise method, also considering a dwell time, and the reaction is observed. In order to meet the homogeneity conditions of the test room, it is necessary to perform the test in a full absorber chamber.
Picture: FRIWO’s full absorber chamber at headquarters in Ostbevern, Germany
Typical test severity levels are specified in the IEC 61000-4-3 standard or in a special product standard. Many product standards, such as EN55024 for IT equipment or EN55014-2 for electronic household appliances, assume test severity level 2 as 3 V / m. The medical device standard IEC60601-1-2 requires the much stricter test severity 3 at 10 V / m for medical devices in the home environment, as well as the standard IEC61000-6-4 for devices in industrial environment.
Test field strength
Table: Test levels acc. IEC61000-4-3
In addition to the test severity, the frequency range in which the device is tested is also very important. Most product standards are consistently tested in the frequency range from 80MHz to 1GHz. However, there are some product standards that require a continuous test range from 80MHz to 2.7GHz or even 3GHz due to the high transmission power of mobile devices and the WLAN technology used in the household.
Graph: Test levels at frequencies 
In addition, there are tests for radiated high frequency in special frequency bands, e.g. in the medical device standard IEC60601-1-2 (385MHz - 5.7GHz), with significantly higher field strengths (up to 28V / m) and another type of modulation. This is intended to provide a better simulation of the influence of modern means of communication on the device.
Graph: Test levels acc. IEC60601-1-2
Measures to increase the immunity of electronic devices
In terms of circuitry, there are different approaches to make devices more robust against the influence of electromagnetic fields.
The simplest way is shielding, to prevent that electromagnetic energy is coupled into the circuit, e.g. by a metal housing. Using this method, the protection of all inputs and outputs with a corresponding input filter should be not forgotten.
The layout of a circuit is very important. It should be designed in a way, that circuit traces are as small as possible to reduce the antenna effect.
The ground connection of components in a circuit should be as low-impedance as possible and star-shaped. The relevant literature often warns about ground loops, but FRIWO’s experience shows that when a ground loop leads to a lower resistance connection and is placed around a circuit part, a strong shielding effect is visible. Therefore ground loops do not generally have negative effects.
Supply inputs of ICs (VCC) should always be connected to a small SMD capacitor (e.g. 100nF) which has to be placed very close to the pin. For very sensitive inputs, it is also helpful to place even a smaller 100pF SMD capacitor next to the 100nF. The capacitors form a low-pass filter, which protects the circuit from high frequencies. Control inputs can be very sensitive (e.g. in case of operational amplifiers), which means you need to take care that the position of a filter capacitor does not lead to positive feedback and the system becomes unstable.
In general, the increase of a circuit’s immunity can be very time-consuming and require completely new techniques, especially at high field strengths.
High-frequency electromagnetic field immunity of FRIWO products
The German power supply specialist FRIWO can rely on immunity knowhow out of more than 45 years of research & development. Thanks to an own full absorber chamber at FRIWO’s headquarters in Ostbevern, devices can be suppressed quickly and within shortened change times. This counts not only for customer-specific projects with high immunity requirements, but also in the standard platform portfolios. With the FOX external power supply series and the new open frame family HERC, FRIWO offers extremely efficient power supply units that are also suitable for harsh environments: the devices fulfill the strict test severity level 3 (10V / m) of EN61000-4-3.
They also meet current energy efficiency standards such as DOE Level VI from the US Department of Energy, even if this standard doesn’t apply to Europe yet or excludes open frame power supplies from the regulations in general. In addition to high efficiency, especially the minimum standby losses of the devices are decisive to meet the requirements.
In addition to FRIWO’s direct sales and distribution partners, the units can also be ordered in small and very small quantities on the internet at www.friwo-shop.com – 24 hours a day, 7 days a week.
 German Federal Office for Radiation Protection
 Test report from FRIWO