Radiated emission

Subject classification: this is a physics resource.

Overview

in the field of EMC, the term Radiated Emissions refers to the unintentional release of electromagnetic energy from an electronic device or apparatus. Any electronic device may generate Electromagnetic fields that unintentionally propagate away from the device’s structure. In general, Radiated Emissions are usually associated with non-intentional radiators, but intentional radiators can also have unwanted emissions at frequencies outside their intended transmission frequency band. As was discussed in the EMC Regulations module, the allowable Radiated Emissions from electronic devices and apparatus are regulated by various organizations and agencies. Electronic devices that have significant amounts of radiated emissions may interfere with their normal operation or the normal operation of other devices in close proximity. For these reasons, it is important to understand the concepts behind the origins of radiated emissions so that fundamental design techniques can be used to minimize the emissions. This module will investigate the origins of radiated emissions, and discuss methods to predict, measure, and decrease the radiated emissions from an electronic device. Differential Mode (DM) and Common Mode (CM) currents will be introduced, and their roles in radiated emissions will be investigated. Using current probes to measure the differential and common mode currents on current carrying wires will be discussed. Finally, a discussion will be given on the role of circuit geometry and device structure on the radiated emissions from an electronic device.

Radiated emission find their origin in the flow of radio frequent current on interconnections and cables. We may distinguish radiated emissions from current flowing in a loop (creating magnetic field H) or from flowing on a (straight) wire (electrical field E). It's important to understand the concept that a changing (RF) magnetic field does create a new electric field and that a changing (RF) electric field creates a (new) magnetic field. The combination of a magnetic field and its corresponding electrical field is what we commonly call radio waves. The ratio of electric field and magnetic field at a given point is called wave impedance (Z=E/H). At a given distance from a magnetic field source, and correspondingly at a given distance from an electrical field source, we cannot distinguish their origin anymore,and the ratio of the electric and magnetic field have reached an equilibrium. This point is where a so-called close field changes into far field. The distance of this point from the source is frequency dependent and is approximately equal to the wavelength corresponding to the frequency of the source, divided by 2PI (6.28). For a 100 MHz interference signal the change from close to far field happens at 0.5 m approximately. In the close field range a EM-field is predominantly magnetic or electric. In the far field the impedance (ratio E/H) of the EM waves is always 377 Ohm.

Electrical field is measured in SI units as V/m (Volts per Meter). That means that a field of 1 V/m can be created by a potential of 1 Volt over a distance of 1 meter. (imagine a large capacitor). Magnetic field is measured in SI units in A/m. (Ampere per meter). This means that a field of 1 A/m will be created at a distance of 1/2pi (circumference = 1m) meter from an infinitely long wire carrying a 1 Ampere current.

Traditionally radiated emission is measured with an antenna sensible to the electrical field only. By limiting our measurment to the far field, we now also know the magnetic field component. In most tests we do not even mention the magnetic field component anymore, but nevertheless, its there.