Electromagnetic interference (EMI), or radio frequency interference (RFI), is a type of electric or electronic emission that can degrade, impair or prevent electrical circuit performance. These emissions originate from both manmade and natural sources, and can be both wanted and unwanted. EMI and RFI can be either conducted, meaning they are sent along power and signal lines, or radiated, which means they propagate in free space. Because EMI and RFI can be damaging to many important applications, such as hospital machinery and military devices, it’s usually important to implement methods to avoid or reduce their impact.
One preventative method is the EMI/RFI filter. This passive electronic device suppresses conducted EMI/RFI emissions on any power or signal line, but does not suppress radiated emissions in free space. Filters can also be attached to specific devices to limit or suppress their EMI/RFI output, or to inhibit their susceptibility to EMI/RFI emissions from other devices. Because there are many different criteria to consider when selecting an EMI/RFI filter, it’s important to know the characteristics of both the device being protected and the filter selected.
Many filters allow only certain types of frequencies through, meaning a set range of EMI/RFI emissions will be transmitted to the device being protected. This is because certain EMI/RFI emissions, within a defined range, will not interfere with performance frequencies of different electric and electronic devices. For example, cable televisions operate at different frequencies than radio waves, so they do not interfere with one another. There are four primary frequency pass settings:
Low Pass filters allow the passage of low level frequencies below a certain cut off point.
High Pass filters allow the passage of high level frequencies above a certain cut off point.
Band Pass filters allow the passage of frequencies within a certain frequency range.
Band Stop or Band Reject filters allow the passage of frequencies outside a certain frequency range.
Insertion loss is a positive gain measurement that gauges the general effectiveness of an EMI/RFI filter in decibels (dB). This measurement is found by determining the ratio between the signal strength of an application when the filter is not installed (v1) and the signal strength of an application when the filter is installed (v2). In the past, the industry standard insertion loss figure was 50 Ohms; while this measurement is still common, recommended insertion loss figures are much less rigidly applied.
Filtering Power Lines
Power lines are generally subdivided into different phase sections, with neutral lines also attached. It is not enough to attach a filter to the power line and be done; rather, each phase requires its own filter, as does the neutral line.
When selecting an EMI/RFI filter, it’s also important to consider size and weight, especially if a given application has limited space. EMI/RFI filters are available in many sizes, but their range and effectiveness can be impaired if they are unable to fit comfortably in an application. Additionally, portable applications, or those that might be found on aircraft, have weight factors that may prohibit an application from being properly installed and used.