To achieve greater attenuation, a second or higher-order filter as shown in Figure 3 consisting of two reactive components or more is required. Generally, these single component filters are not very useful as their attenuation only increases at a rate of 6 dB/octave or 20 dB/decade. Capacitors shunt noise current away from a load while inductors block or reduce the noise. The simplest type is called a first-order filter consisting of just a single reactive component. Examples of Various Filter Packages and a Typical Filter Configuration(Courtesy of Schaffner Company)Įssentially, an EMI filter is made up of two basic types of components–capacitors and inductors. A power line or mains EMI filter is placed at the power entry point of the equipment that it is being installed into to prevent noise from exiting or entering the equipment. The measure of a filter’s ability to reduce a given signal level is insertion loss or attenuation. The cut-off frequency is the frequency between the signal’s passband and the reject bands at 3 dB attenuation below the acceptance line. An ideal EMI filter will reduce the amplitude of all frequency signals greater than the filter cut-off frequency. Contractual requirements imply or specify filters.Įssentially, an AC power or mains EMI filter is a low pass filter that blocks the flow of “noise” while passing the desired input which can be DC or 50/60/400 Hertz power frequency. Even for military/aerospace equipment, they must be protected from failure due to EMI noise and security requirements may call for filters to protect classified data. Designers often forget that an EMI filter can assist in meeting immunity and fast transients requirements and radiated emissions as well. One reason is that regulatory agency requirements dictate that conducted and radiated emissions be constrained below specified limits, but the unit must also pass immunity/transient requirements.
However, for this article, we will focus on the AC/Mains EMI ( Electro Magnetic Interference) power type filter. The general term filter, of course, can also be used for a device on control and signal type lines. Filters accept an electrical signal at its input and deliver a different or modified signal at its output depending upon the filters internal configuration. The term “signal” can be a communication or power type signal. It is a device usually composed of discrete components which can be placed between circuits, networks, or equipment/systems to either emphasize, de-emphasize or control the frequency components of a desired or undesired signal. To start at the very beginning, what is an electrical filter? An electrical filter can be passive, active, analog, or digital.