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EMC Filters/EMI Filters / RFI Filters Frequently Asked Technical Questions

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What is electromagnetic interference?
Electromagnetic interference, EMI, is any undesirable electromagnetic emission or any electrical or electronic disturbance, man-made or natural, which causes an undesirable response, malfunctioning or degradation in the performance of electrical equipment. 
 
What is radio frequency interference?
Radio frequency interference, RFI, is any undesirable electrical energy with content within the frequency range dedicated to radio frequency transmission. Conducted RFI is most often found in the low frequency range of several kHz to 30MHz. Radiated RFI is most often found in the frequency range from 30MHz to 10GHz. 
 
How does interference propagate?
EMI or RFI propagate through conduction over signal and power lines and through radiation in free space. 
 
What are some common sources of conducted interference?
Typical sources of conducted interference include switching power supplies, ac motors, and microprocessors. In short, just about any electrical and electronic device has the potential to generate conducted and radiated interference. 
 
What are some common sources of radiated interference?
The most common offender in the radiation of EMI is the electrical power cord of the electronic device itself. Since the powercord can act as an antenna, conducted EMI can also become radiated interference. 
 
What is common mode (CM) noise?
Common mode interference, a.k.a. asymmetrical interference, is a noise signal which is found in phase on both the line and neutral conductors with respect to ground.  Common mode noise also typically has equal amplitude on both line and neutral conductors.  . 
 
What is differential mode noise?
Differential mode interference, a.k.a. symmetrical interference, is a noise signal which exists between the line and neutral conductors. 
 
Why is EMI regulated?
EMI is regulated to allow today's sensitive equipment to function properly without suffering degradation in performance due to interference generated by other electronic devices. The EMI spectrum is a limited natural resource that must be maintained to allow reliable radio frequency communications. The successful regulation of EMI interference will allow future electronic devices to operate as defined, in the intended environment, without suffering any degradation in performance due to interference, and without disrupting the performance of other equipment. 
 
What are the applicable US requirements?
Conducted and radiated emissions for computing devices are regulated by Part15, Subpart J of the US Federal Communications Commission, (FCC). The limits for conducted and radiated emissions are based upon the end application of the device.
    Class A:    Commercial, Industrial and Business Applications
    Class B:    Residential Applications
 
What standards apply to EMI and filters in the EU?
The EMC Directive 89/336/EEC mandates that all electronic equipment must comply with the applicable EN specification for EMI. Some typical EN specifications follow:
Electronic Equipment Spec.
Industrial, scientific and medical equipment EN55011
Broadcast receivers and associated equipment EN55013
Electrical motor-operated and thermal appliances for household and similar purposes, electrical tools and similar apparatus EN55014
Electrical lighting and similar apparatus EN55015
Information technology equipment  EN55022

What is an EMI filter?
An EMI filter is a passive electronic device used to suppress conducted interference present on any power or signal line. It may be used to suppress the interference generated by the device itself as well as to suppress the interference generated by other equipment to improve the immunity of a device to the EMI signals present within its electromagnetic environment. Most EMI filters include components to suppress both common and differential mode interference. Filters can also be designed with added devices to provide transient voltage and surge protection as well as battery backup. 
 
How does an EMI filter work?
An EMI filter has a high reactive component to its impedance.  That means the filter looks like a much higher resistance to higher frequency signals.  This high impedances attenuates or reduced the strength of these signals so they will have less of an effect on other devices. 
 
What criteria are used to select the proper EMI filter?
The proper selection of an EMI filter encompasses the entire electromechanical configuration of the filter. The filter's mechanical footprint including mounting and terminations may impact the effectiveness of the filter in your system. Primary considerations also include leakage current, insertion loss, rated voltage and current, and agency approvals required for proper usage in the end application. We do not anticipate the standard catalog products to fulfill every application and welcome the opportunity to customize any product to best suit your needs. For additional information, please reference our electronic or print catalogs. 
 
How does the 50 Ohm data compare with actual performance in my system?
The 50 Ohm insertion loss data which appears in our catalog represents an industry standard to allow comparison of different manufacturers' products against a known standard. Since filters are source and load impedance sensitive and the actual load impedance varies greatly from system to system, often a test is the easiest way to determine the actual performance within a specific system.. 
 
How are EMI tests performed?
EMI tests are performed using passive networks called LISNs, Line Impedance Stabilization Network, connected in series with the power lines to the equipment. The LISN establishes a consistent impedance to allow for repeatability of test results. Conducted emissions are measured via an RF connection to a port on the LISN.  Radiated emissions may are measured using an antenna or current detecting probe. 
 
What aspects of filter design and performance are regulated by safety agencies?
Safety agencies define limits for the maximum leakage current, operating temperature rise, and the mechanical design to protect end users. 
 
What is Hi-pot testing?
Hi-pot or High Potential testing is also known as dielectric-withstandings testing.  It consists of applying a known potential or voltage between either the two lines or a line and the case to expose any electrical deficiencies in the construction of an EMI filter. This test is performed to expose insufficient insulation.  As part of our testing, all JMK products are tested for dielectric withstandings (Hi-Pot) between the conduction lines (DM) and between the conductors and the case (CM). 
 
What is leakage current?
When a filters uses capacitors between the conductors and ground, small amounts of current "leak" through the filter to ground. This current can creates an electric shock hazard if the system is improperly grounded, therefore, this current is limited by the international safety agencies to prevent a danger to personal safety.
 
What is insertion loss?
Insertion loss is quite simply the effectiveness of an EMI filter. Insertion loss is defined as the ratio of voltages across the load with over the voltage without the filter inserted in the circuit.  Filter performance is dependant on the input and output impedance seen by the filter, insertion loss data is reported for a filter in a 50 Ohm system.  While this provides a standard for comparison, it does not reflect the actual performance of the filter within your system. 
Since filter performance is specific to the attached system, JMK mainstains a test facility to test the individual needs of our customer's power supply to select or design an appropriate filter.  Normally such testing is offered free of charge.
 
How are the various insertion loss limits measured?
See JMK's Insertion Loss Page. 
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